Colchicine and insulin secretion in man

The present study is aimed at investigating the effect of acute and chronic colchicine administration on insulin secretion in humans. Acute insulin response to glucose (0.33 g/kg) was significantly decreased by colchicine (3 mg i.v.). In fact, this response (mean change 2-10 min insulin) was 44 +/- 8 microunits/ml before and 32 +/- 6 microunits/ml after colchicine administration (P less than 0.01). As a consequence of this, glucose disappearance rates were reduced (P less than 0.05). Infusion of lysine acetylsalicylate (LAS), an inhibitor of endogenous PG synthesis, completely reversed the inhibitory effect of colchicine upon insulin secretion and also augmented acute insulin response to glucose (response before colchicine + LAS = 45 +/- 8 microunits/ml; response after colchicine + LAS = 51 +/- 9 microunits/ml, P less than 0.05). This effect was associated with an increase in glucose disappearance rates (P less than 0.05). The 10-day treatment with colchicine (2 mg daily) caused a significant suppression of insulin secretion induced by oral glucose (100 g) and significantly increased the plasma glucose concentrations following the test (P less than 0.05). These findings demonstrate that (1) both acute and chronic colchicine administration inhibit glucose-induced insulin secretion and deteriorate glucose tolerance in humans, and (2) LAS completely reverses these negative effects of colchicine. An increased synthesis of endogenous PGE, which are known to inhibit insulin secretion in humans, might account for the inhibiting effect of colchicine on insulin secretion.     

Insulin response to glucose in hypermetabolic burn patients.

Fifty-four intravenous glucose tolerance tests were performed in 12 normal individuals and 21 thermally injured patients. In the 17 hypermetabolic burn patients studied between the 6th and 16th days postinjury, fasting blood glucose was elevated (111 +/- 7 mg/100 ml, mean +/- SE compared to 85 +/- 3 in controls, P less than 0.001), but the instantaneous proportionality constant for glucose disappearance (k) was similar to that obtained in normal individuals (5.27 +/- 0.51, 100/min vs 4.01 +/- 0.58 in normals, NS). Fasting serum insulin concentrations were comparable in the 12 normals and 17 hypermetabolic burn patients (22 +/- 3muU/ml in normals vs 22 +/- 2), as was fasting insulin corrected for fasting glucose (24 +/- 3 in normals vs 21 +/- 3, NS), initial insulin response (0-10 min delta insulin, 58 +/- 13 in normals vs 67 +/- 10, NS) or total insulin response corrected per unit glycemic stimulus (insulinogenic index, 0.48 +/- 0.10 in normals vs 0.52 +/- 0.07, NS). With time following injury, the proportionality constant for glucose disappearance and insulin response decreased, and these alterations were related to the posttraumatic weight loss. In the 5 convalescent patients studied between the 37th and 90th days postinjury, glucose and insulin dynamics appeared similar to those observed in starved man. In these burn patients, hypermetabolism and negative nitrogen balance occurred in association with a normal insulin response to glucose. Increased hepatic gluconeogenesis appears to be characteristic of the catabolic response to this stress, directed by increased glucagon and catecholamines, not a decrease in fasting insulin or dampened insulin response.     

Beta-endorphin-induced inhibition and stimulation of insulin secretion in normal humans is glucose dependent

This study evaluated the effect of human beta-endorphin on pancreatic hormone levels and their responses to nutrient challenges in normal subjects. Infusion of 0.5 mg/h beta-endorphin caused a significant rise in plasma glucose concentrations preceded by a significant increase in peripheral glucagon levels. No changes occurred in the plasma concentrations of insulin and C-peptide. Acute insulin and C-peptide responses to intravenous pulses of different glucose amounts (0.33 g/kg and 5 g) and arginine (3 g) were significantly reduced by beta-endorphin infusion (P less than .01). This effect was associated with a significant reduction of the glucose disappearance rates, suggesting that the inhibition of insulin was of biological relevance. beta-Endorphin also inhibited glucose suppression of glucagon levels and augmented the glucagon response to arginine. To verify whether the modification of prestimulus glucose level could be important in these hormonal responses to beta-endorphin, basal plasma glucose concentrations were raised by a primed (0.5 g/kg) continuous (20 mg kg-1.min-1) glucose infusion. After stabilization of plasma glucose levels (350 +/- 34 mg/dl, t = 120 min), beta-endorphin infusion caused an immediate and marked increase in plasma insulin level (peak response 61 +/- 9 microU/ml, P less than .01), which remained elevated even after the discontinuation of opioid infusion. Moreover, the acute insulin response to a glucose pulse (0.33 g/kg i.v.) given during beta-endorphin infusion during hyperglycemia was significantly higher than the response obtained during euglycemia (171 +/- 32 vs. 41 +/- 7 microU/ml, P less than .01).(ABSTRACT TRUNCATED AT 250 WORDS)     

The effect of insulin treatment on insulin secretion and insulin action in type II diabetes mellitus

We have studied the effects of 3 wk of continuous subcutaneous insulin infusion (CSII) on endogenous insulin secretion and action in a group of 14 type II diabetic subjects with a mean (+/-SEM) fasting glucose level of 286 +/- 17 mg/dl. Normal basal and postprandial glucose levels were achieved during insulin therapy at the expense of marked peripheral hyperinsulinemia. During the week of posttreatment evaluation, the subjects maintained a mean fasting glucose level of 155 +/- 11 mg/dl off insulin therapy, indicating a persistent improvement in carbohydrate homeostasis. Adipocyte insulin binding and in vivo insulin dose-response curves for glucose disposal using the euglycemic clamp technique were measured before and after therapy to assess the effect on receptor and postreceptor insulin action. Adipocyte insulin binding did not change. The insulin dose-response curve for overall glucose disposal remained right-shifted compared with age-matched controls, but the mean maximal glucose disposal rate increased by 74% from 160 +/- 14 to 278 +/- 18 mg/m2/min (P less than 0.0005). The effect of insulin treatment on basal hepatic glucose output was also assessed; the mean rate was initially elevated at 159 +/- 8 mg/m2/min but fell to 90 +/- 5 mg/m2/min in the posttreatment period (P less than 0.001), a value similar to that in control subjects. Endogenous insulin secretion was assessed in detail and found to be improved after exogenous insulin therapy. Mean 24-h integrated serum insulin and C-peptide concentrations were increased from 21,377 +/- 2766 to 35,584 +/- 4549 microU/ml/min (P less than 0.01) and from 1653 +/- 215 to 2112 +/- 188 pmol/ml/min (P less than 0.05), respectively, despite lower glycemia. Second-phase insulin response to an intravenous (i.v.) glucose challenge was enhanced from 170 +/- 53 to 1022 +/- 376 microU/ml/min (P less than 0.025), although first-phase response remained minimal. Finally, the mean insulin and C-peptide responses to an i.v. glucagon pulse were unchanged in the posttreatment period, but when glucose levels were increased by exogenous glucose infusion to approximate the levels observed before therapy and the glucagon pulse repeated, responses were markedly enhanced. Simple and multivariate correlation analysis showed that only measures of basal hepatic glucose output and the magnitude of the postbinding defect in the untreated state could be related to the respective fasting glucose levels in individual subjects.(ABSTRACT TRUNCATED AT 400 WORDS)     

Putative hypothalamic glucoreceptors play no essential role in the response to moderate hypoglycemia

The response to insulin-induced hypoglycemia includes increased plasma levels of glucagon, epinephrine, norepinephrine, cortisol, and growth hormone. This integrated response is thought to be mediated via sympathetic afferent pathways emanating from the ventromedial hypothalamus. However, the precise loci of the receptors that sense glucopenia are not known. In this study, we investigated the importance of putative forebrain glucoreceptors to the systemic response to hypoglycemia. Three protocols were performed. Protocol 1: the systemic response was observed in conscious dogs to hypoglycemia induced by infusion of insulin at a high rate (150 mU/min). Protocol 2: the effect of concomitant bilateral, intracarotid glucose infusion on the response to intravenous insulin was examined. Intracarotid glucose infusion rates were chosen to yield central euglycemia in the face of systemic hypoglycemia. Protocol 3: as a control for protocol 2, glucose was infused at low rates into the systemic circulation, yielding hypoglycemia in both central and systemic blood. When insulin was infused at 150 mU/min, without glucose replacement (protocol 1; N = 6), plasma insulin increased from 14 +/- 3 to 335 +/- 35 microU/ml at 60 min (P less than 0.001). Glucose fell from basal (104 +/- 3 mg/dl) to 38 +/- 3 mg/dl (P less than 0.001). Significant increases were observed in epinephrine (basal, B: 63 +/- 8; steady state, SS: 1762 +/- 582 pg/ml; P less than 0.007), norepinephrine (B: 209 +/- 33, SS: 650 +/- 133 pg/ml; P less than 0.01), and glucagon (B: 256 +/- 35, SS: 467 +/- 35 pg/ml; P less than 0.03). In addition, endogenous glucose production (Ra) increased from 72 +/- 4 to 108 +/- 9 mg/min (P less than 0.02) despite frank hyperinsulinemia. Infusion glucose into the carotid arteries at 204 +/- 10 mg/min (protocol 2; N = 7) during a 4-h systemic insulin infusion was sufficient to prevent jugular hypoglycemia [jugular glucose, Gj, B: 100 +/- 3, SS (90-160 min): 101 +/- 3 mg/dl; P greater than 0.70], but not peripheral hypoglycemia (Gp, B: 102 +/- 2, SS: 55 +/- 3 mg/dl; P less than 0.001). Despite carotid glucose replacement, counterregulatory responses were still observed in epinephrine (B: 98 +/- 14, SS: 466 +/- 127 pg/ml; P less than 0.04) and norepinephrine (B: 213 +/- 19, SS: 474 +/- 133 pg/ml; P less than 0.05).(ABSTRACT TRUNCATED AT 400 WORDS)     

Effects of cyclosporin on insulin and C-peptide secretion in healthy beagles

Plasma glucose, C-peptide, and insulin responses to intravenous glucose (intravenous glucose tolerance test [IVGTT], 0.5 g/kg), glucagon (1 mg i.v.), and oral glucose (oral glucose tolerance test [OGTT], 1 g/kg) were assessed in six normal beagles before, during, and 1 and 4 mo after the administration of cyclosporin A (CsA) in doses previously shown to be required for uniform prevention of canine islet-allograft rejection (20 mg/kg; mean trough radioimmunoassay serum levels greater than or equal to 500 ng/ml). Insulin secretion in response to intravenous glucose and glucagon was significantly inhibited during the administration of CsA (areas under insulin-response curves, pmol.min-1.L-1; IVGTT, pre-CsA, 11,127 +/- 1285; during CsA, 5954 +/- 1147, P less than .05; glucagon tolerance test, pre-CsA, 18,617 +/- 2807; during CsA, 4401 +/- 486, P less than .05 vs. pretreatment levels). These secretory defects persisted 4 mo after CsA was discontinued (IVGTT, 4358 +/- 659; glucagon tolerance test, 10,567 +/- 2479, P less than .05). C-peptide responses paralleled these changes. Plasma glucose disposal in response to these secretagogues, however, returned to normal 1 mo after discontinuation of CsA. In contrast to the findings for IVGTT and glucagon, insulin-response curves to OGTT were not statistically different during CsA administration. We conclude that, although glucose disappearance rates are normal after discontinuation of the CsA administration, CsA causes irreversible impairment in islet secretory responses detectable with IVGTT and glucagon but not with OGTT. These results suggest that short-term CsA in doses required to prevent islet-allograft rejection in dogs can result in permanent loss of functionally competent beta-cells.     

Markedly reduced beta-cell function does not result in insulin resistance in islet autografted dogs.

Autotransplantation of islets of Langerhans has resulted in long-term normoglycemia in pancreatectomized dogs. This canine model is useful in evaluating both the progress of islet transplantation and the effect of a reduced islet mass upon the determinants of glucose tolerance: i.e., insulin secretion, insulin sensitivity, and glucose effectiveness. To determine the effect of a reduced islet mass on these factors, we measured the acute insulin response to arginine (AIRa) and glucose (AIRg), the slope of glycemic potentiation of AIRa (SP), insulin sensitivity (Sl), and glucose effectiveness (SG) in control (CN), diabetic (DM), and pancreatectomized dogs rendered normoglycemic with transplanted autografts of islets of Langerhans (TX). Normal fasting plasma glucose (FPG) (TX 4.7 +/- 0.2 mM; CN 4.9 +/- 0.1 mM; P greater than 0.05) was maintained despite a markedly reduced insulin secretion in TX (AIRa 24%, AIRg 15%, and SP 11% of CN). All measures of insulin secretion were significantly correlated (SP vs. AIRg, r = 0.80, P less than 0.0001; AIRa vs. AIRg, r = 0.92, P less than 0.0001) across all animals, but none of the measures of secretion were significantly correlated with either the number of islets transplanted or time posttransplant (P greater than 0.10). Insulin sensitivity was normal in islet autografted dogs (TX: 136 +/- 12 min-1/(nmol/ml); CN: 101 +/- 11 min-1/(nmol/ml), P greater than 0.05) but SG was reduced (TX: 1.93 +/- 0.28 x 100 min-1; CN: 3.53 +/- 0.35 x 100 min-1, P less than 0.05), as determined by the minimal-model method. In diabetic animals (FPG = 16.1 +/- 1.3 mM), insulin secretion was negligible by all measures (P greater than 0.05), and was associated with insulin resistance (Sl = 28 +/- 8 min-1/(nmol/ml)) and reduced SG (1.72 +/- 0.11 x 100 min-1). These studies indicate that across a range of insulin secretion in dogs, the secretagogues arginine and glucose provide similar estimates of beta-cell function. This markedly reduced beta-cell function does not result in insulin resistance when fasting normoglycemia is maintained, but is associated with a decrease in glucose action at basal insulin.     

Pancreastatin inhibits insulin secretion and stimulates glucagon secretion in mice

Recently a new peptide, pancreastatin, was isolated from porcine pancreatic extracts. It contains 49 amino acids and shows a structural similarity to chromogranin A, which occurs in secretory granules of the endocrine pancreas. Furthermore, pancreastatin has been found to inhibit glucose-induced insulin secretion in the perfused rat pancreas. However, its effects under in vivo conditions have never been studied. We have therefore investigated the effects of this peptide on insulin and glucagon secretion in vivo in the mouse. We found that an intravenous injection of pancreastatin (4.0 nmol/kg) lowered basal plasma insulin concentration at 6 min from 55 +/- 8 microU/ml in control mice to 21 +/- 7 microU/ml (P less than .01). The peptide also inhibited the plasma insulin response to both glucose (P less than .01) and the cholinergic agonist carbachol (P less than .001). Furthermore, 2 min after injection of pancreastatin, plasma glucagon concentration had increased to 301 +/- 19 pg/ml compared to 190 +/- 12 pg/ml in control mice (P less than .001). The peptide did not, however, affect the carbachol-induced plasma glucagon response. In addition, pancreastatin induced a transient hyperglycemia. Combined adrenergic blockade by means of a pretreatment of phentolamine and propranolol did not prevent pancreastatin from exerting its effects on plasma insulin levels, whereas the increase in plasma glucagon levels was abolished. Thus, in the mouse, the newly discovered intrapancreatic peptide pancreastatin 1) lowers baseline plasma insulin levels, 2) inhibits glucose- and cholinergically induced insulin secretion, 3) stimulates baseline glucagon secretion, and 4) induces hyperglycemia.     

Alpha-adrenergic blockade improves glucose-potentiated insulin secretion in non-insulin-dependent diabetes mellitus

The impairment of glucose-potentiated insulin secretion present in non-insulin-dependent diabetes mellitus (NIDDM) can be approximated in normal subjects by an epinephrine infusion. Therefore, we sought to determine the role of the endogenous sympathetic nervous system in glucose-potentiated insulin secretion in both NIDDM (n = 6) and normal (n = 6) subjects. Glucose-potentiated insulin secretion was calculated as the slope of the curve relating increasing ambient glucose levels to the acute insulin response to an intravenous pulse of 5 g of L-arginine. Glucose-potentiated insulin secretion was determined on separate days during alpha-, beta-, and combined alpha- plus beta-adrenergic blockade and compared with a saline control. In normal subjects, there was no effect of alpha-, beta-, or alpha- plus beta-blockade on the slope of glucose potentiation. In NIDDM, the initially decreased slope of glucose potentiation (0.25 +/- 0.06 microU X ml-1 X mg-1 X dl, mean +/- SE; P less than .01) was not affected by beta-blockade but increased during alpha-blockade (0.91 +/- 0.22 microU X ml-1 X mg-1 X dl; P less than .05). However, this improvement was abolished by combined alpha- plus beta-blockade (0.32 +/- 0.07 microU X ml-1 X mg-1 X dl). Plasma norepinephrine was increased above basal levels in both normal (+260 +/- 89 pg/ml) and NIDDM (+438 +/- 162 pg/ml) subjects during alpha-blockade (P less than .05 for both). This increase in plasma norepinephrine strongly suggests that there is an increase in synaptic cleft norepinephrine concentration.(ABSTRACT TRUNCATED AT 250 WORDS)     

Contribution of growth hormone-releasing hormone and somatostatin to decreased growth hormone secretion in elderly men.

OBJECTIVE: The pathophysiology of the decline in circulating growth hormone (GH) concentrations that may occur with ageing remains elusive. We have investigated the potential contributions of decreased endogenous GH-releasing hormone (GHRH) and increased somatostatin secretion to this phenomenon. DESIGN AND METHODS: The strategy used was to stimulate GH secretion in 8 young (20-24 years old, body mass index (BMI) 22.8 +/- 2.8 kg/m2) and 8 elderly (68-82 years old, BMI 23.4 +/- 1.6 kg/m2) male subjects on separate occasions by means of: (i) intravenous bolus 0.5 microgram/kg D-Ala2 GHRH(1-29)-NH2 alone; (ii) 0.5 microgram/kg GHRH after pre-treatment with two oral doses of 50 mg atenolol (to inhibit somatostatin secretion); (iii) 1.25 mg oral bromocriptine alone (to increase endogenous GHRH and/or inhibit somatostatin); (iv) 50 mg oral atenolol plus 1.25 mg oral bromocriptine; and (v) 0.5 microgram/kg GHRH after pre-treatment with 1.25 mg oral bromocriptine. RESULTS: The elderly men had a significantly lower peak and area under curve (AUC) GH response to intravenous GHRH when compared with 8 young men (peak 3.1 +/- 1.0 ng/ml v. 21.6 +/- 5.0 ng/ml, AUC 205 +/- 56 ng/ml/min v. 1,315 +/- 295 ng/ml/min, P < 0.05). Pre-treatment with atenolol before GHRH administration produced no significant increase in peak and AUC GH response in both groups, which remained lower in the elderly men than in their young counterparts (peak 5.5 +/- 1.8 ng/ml v. 29.3 +/- 7.0 ng/ml, AUC 327 +/- 90 ng/ml/min v. 2,017 +/- 590 ng/ml/min, P < 0.05). Bromocriptine alone did not cause a significant rise in GH concentration in either elderly or young subjects (peak 3.1 +/- 1.1 v. 8.8 +/- 3.2 ng/ml, P > 0.05). When atenolol was administered before bromocriptine, both groups responded but the elderly subjects had a significantly greater peak and AUC response (peak 3.6 +/- 0.7 v. 10.7 +/- 2.1 ng/ml; AUC 191 +/- 39 v. 533 +/- 125 ng/ml/min, P < 0.05). Bromocriptine given before GHRH failed to potentiate GHRH action on GH release in either group. Of 5 elderly men who underwent further evaluation of GH secretory ability, 2 subjects had GH levels > 10 ng/ml, either basally or after intravenous GHRH. The remaining 3 had an initially impaired GH response to bolus intravenous GHRH. After 100 micrograms GHRH subcutaneously twice daily for up to 2 weeks the GH responses to intravenous bolus GHRH (0.5 microgram/kg) were reassessed. One exhibited a normal response (> 10 ng/ml) after 1 week of daily GHRH treatment, another had a near-normal response after 2 weeks (9.7 ng/ml), while the third still had an impaired response by the end of the 2-week treatment period (3.2 ng/ml). CONCLUSIONS: The restoration of endogenous GH secretion in these elderly subjects by means of GHRH priming, and the failure of manipulation of somatostatinergic tone to restore a normal GH response to GHRH suggests that somatotroph atrophy due to a reduction in endogenous GHRH secretion is the principal cause of the diminished GH secretion with ageing.     

Glycaemic responses to different carbohydrate foods in healthy and diabetic blacks in Soweto.

Diabetes mellitus is uncommon in rural southern African blacks. With urbanisation and lifestyle changes, incidence rises to that in western populations. To assess associated changes in carbohydrate metabolism, glycaemic responses to glucose, refined maize, refined rice and bread were studied in 8 healthy and 8 non-insulin-dependent diabetic urban blacks. Additionally, in the healthy group serum insulin responses were measured. In the healthy, maize (the staple food of blacks) elicited the highest glucose response (207 mmol/l/min) and bread the lowest (107 mmol/l/min). The glycaemic indices of maize and glucose were similar. Serum insulin responses to maize were significantly lower than that of bread at 90 minutes (maize 66 muU/ml; bread 93 muU/ml; P = 0.02). In diabetics, maize and glucose elicited similar glycaemia (928 mmol/l/min and 921 mmol/l/min respectively). The high glucose response to maize could relate to its processing and physical form. The low insulin secretion could be due to inadequate stimulation by the 'entero-insular' axis. Moreover, variability in glucose insulin responses could stem from ethnic or genetic reasons. In the dietary management of black diabetics, refined maize should be replaced by other cereals.     

Effects of long-term optimization and short-term deterioration of glycemic control on glucose counterregulation in type I diabetes mellitus

To assess the effects of glycemic control on glucose counterregulation, rates of plasma glucose recovery from hypoglycemia and counterregulatory hormonal responses were studied in 18 C-peptide-negative patients with insulin-dependent diabetes mellitus (IDDM) before and after either improvement, no change, or deterioration in glycemic control. Hypoglycemia was induced by an i.v. insulin infusion (30 mU/m2 X min for 1 h) after maintenance of euglycemia overnight with i.v. insulin. In 13 patients with long duration of IDDM (9 +/- 0.5 yr, mean +/- SEM) and initially poor glycemic control (mean diurnal blood glucose, MBG 199 +/- 8 mg/dl, ketoamine-HbA1 12.4 +/- 0.2%; nondiabetic subjects 104 +/- 4 mg/dl and 6.8 +/- 0.09%, respectively), rates of plasma glucose recovery from hypoglycemia (0.30 +/- 0.01 versus 0.60 +/- 0.01 mg/dl X min in nondiabetic subjects, P less than 0.001) and plasma glucagon (AUC 0.56 +/- 0.09 versus 6.3 +/- 0.50 ng/ml X 150 min in nondiabetic subjects, P less than 0.01) and epinephrine (AUC 16.9 +/- 0.2 versus 25.7 +/- 0.2 ng/ml X 150 min in nondiabetic subjects, P less than 0.001) responses to hypoglycemia were impaired. Intensive therapy (three daily injections of insulin) instituted in 7 out of 13 IDDM patients for up to 9 mo improved MBG (124 +/- 6 mg/dl, P less than 0.01) and ketoamine-HbA1 (7.9 +/- 0.02%, P less than 0.01) but not rates of plasma glucose recovery (0.31 +/- 0.01 mg/dl X min) and plasma glucagon (AUC 0.69 +/- 0.07 ng/ml X 150 min) and epinephrine (AUC 14.9 +/- 0.17 ng/ml X 150 min) responses.(ABSTRACT TRUNCATED AT 250 WORDS)     

Regulation of alpha-cell function by the beta-cell in isolated human and rat islets deprived of glucose: the "switch-off" hypothesis

The "switch-off" hypothesis to explain beta-cell regulation of alpha-cell function during hypoglycemia has not been assessed previously in isolated islets, largely because they characteristically do not respond to glucose deprivation by secreting glucagon. We examined this hypothesis using normal human and Wistar rat islets, as well as islets from streptozotocin (STZ)-administered beta-cell-deficient Wistar rats. As expected, islets perifused with glucose and 3-isobutryl-1-methylxanthine did not respond to glucose deprivation by increasing glucagon secretion. However, if normal rat islets were first perifused with 16.7 mmol/l glucose to increase endogenous insulin secretion, followed by discontinuation of the glucose perifusate, a glucagon response to glucose deprivation was observed (peak change within 10 min after switch off = 61 +/- 15 pg/ml [mean +/- SE], n = 6, P < 0.01). A glucagon response from normal human islets using the same experimental design was also observed. A glucagon response (peak change within 7 min after switch off = 31 +/- 1 pg/ml, n = 3, P < 0.01) was observed from beta-cell-depleted, STZ-induced diabetic rats whose islets still secreted small amounts of insulin. However, when these islets were first perifused with both exogenous insulin and 16.7 mmol/l glucose, followed by switching off both the insulin and glucose perifusate, a significantly larger (P < 0.05) glucagon response was observed (peak change within 7 min after switch off = 71 +/- 11 pg/ml, n = 4, P < 0.01). This response was not observed if the insulin perifusion was not switched off when the islets were deprived of glucose or when insulin was switched off without glucose deprivation. These data uniquely demonstrate that both normal, isolated islets and islets from STZ-administered rats can respond to glucose deprivation by releasing glucagon if they are first provided with increased endogenous or exogenous insulin. These results fully support the beta-cell switch-off hypothesis as a key mechanism for the alpha-cell response to hypoglycemia.     

Abnormal glucose counterregulation in insulin-dependent diabetes mellitus. Interaction of anti-insulin antibodies and impaired glucagon and epinephrine secretion

To evaluate the roles of counterregulatory hormones and insulin antibodies in the impairment of plasma glucose recovery from hypoglycemia in diabetes mellitus, and to assess the relationship between the glucagon response and duration of the disease, 21 insulin-dependent diabetic patients and 10 nondiabetic subjects were studied. The diabetics consisted of 5 patients with recent onset of diabetes (less than 1 mo); 11 with 2.6 +/- 0.3 (mean +/- SEM) yr duration of diabetes, 5 of whom had insulin antibodies; and 5 patients with long-term diabetes (21 +/- 3 yr), insulin antibodies, and autonomic neuropathy. During insulin-induced hypoglycemia (28 mU/m2 X min for 60 min) in patients with recent-onset diabetes, plasma free insulin, glucose, and counterregulatory hormone concentrations did not differ from those of nondiabetic subjects. In patients with insulin antibodies, the disappearance of insulin after insulin infusion was delayed, and both restitution of normoglycemia and plasma glucagon response were blunted compared with patients without antibodies. When glucagon was infused (80-130 ng/m2 X min) during hypoglycemia in diabetics with impaired glucagon responses in order to simulate normal glucagon responses, plasma glucose recovery was normalized in patients without antibodies but not in those with antibodies. In patients with long-standing diabetes, restitution of normoglycemia was further impaired and this was associated with an absent plasma glucagon response and a diminished plasma epinephrine response. Plasma glucagon responses to hypoglycemia were inversely correlated to the duration of diabetes (r = -0.943; P less than 0.0005). It is concluded that impaired A-cell secretion is the predominant mechanism for the delayed glucose recovery after hypoglycemia in diabetic patients without insulin antibodies and normal epinephrine responses. Slowed disappearance of insulin due to the presence of insulin antibodies further delays the restoration of normoglycemia. Patients with long-standing diabetes and autonomic neuropathy exhibit decreased epinephrine secretion, which leads to an additional retardation of glucose recovery. Since plasma glucagon and epinephrine responses to hypoglycemia were normal at the onset of diabetes but diminished in long-term diabetes, it appears that the impaired glucagon and epinephrine responses to hypoglycemia are acquired defects that develop subsequent to B-cell failure.     

Effects of physical training and diet therapy on carbohydrate metabolism in patients with glucose intolerance and non-insulin-dependent diabetes mellitus

The effects of 12 wk of physical training in addition to hypocaloric diet (DPT group, N = 10) on body composition, carbohydrate (CHO) tolerance, and insulin secretion and action were compared with the effects of diet therapy alone (D group, N = 8) in CHO-intolerant and non-insulin-dependent diabetic subjects. Fat mass, fat-free mass (FFM), mean fasting plasma glucose, serum C-peptide, and insulin concentrations decreased similarly in both groups. The mean plasma glucose response to a mixed meal decreased approximately 20% in both treatment groups, and, after i.v. glucose, decreased 12% in the D group (P less than 0.05), but did not change in the DPT group (NS between groups). The acute serum insulin response (0-6 min) after IG increased significantly in the DPT group only (NS between groups). The mean basal endogenous glucose production (BEGP) decreased 17% (P less than 0.025) in the DPT group and by 31% (P less than 0.01) in the D group (NS between groups). Hepatic sensitivity to insulin, estimated by BEGP suppression during the euglycemic clamp, increased significantly by 25% in both groups. Total glucose disposal during the euglycemic clamp increased from 3.51 +/- 0.04 milligrams of glucose per kilogram of fat-free mass per minute (mg/kg-FFM/min) to 4.45 +/- 0.54 mg/kg-FFM/min (P less than 0.05) in the DPT group, but no change occurred in the D group (NS between groups).(ABSTRACT TRUNCATED AT 250 WORDS)     

Insulin secretion and clearance. Comparison after oral and intravenous glucose

Insulin secretion and clearance in response to the administration of oral and intravenous glucose was investigated in nine normal men. C-peptide metabolic kinetics were calculated by analysis of individual C-peptide decay curves after the bolus injection of biosynthetic human C-peptide. Glucose was administered to the subjects on three occasions: as a 75-g oral dose, a 75-g i.v. infusion, and an intravenous glucose infusion at a variable rate adjusted to mimic the peripheral glucose levels obtained after the oral glucose load (matching experiment). Glucose, insulin, and C-peptide concentrations were measured for the subsequent 5 h. The glucose level after the oral glucose load (115.9 +/- 2.6 mg/dl, mean +/- SE) closely approximated that after the matching experiment (120.5 +/- 2.5 mg/dl) but was significantly lower than after 75 g i.v. glucose (127.7 +/- 3.4 mg/dl, P less than .05). Analysis of the areas under the peripheral concentration curves (60-360 min) showed that the responses of both insulin (52.7 +/- 5.6 and 46.5 +/- 4.5 pmol.ml-1.300 min-1) and C-peptide (252.7 +/- 27.5 and 267.0 +/- 21.6 pmol.ml-1.300 min-1) were not significantly different after the oral and 75-g i.v. glucose studies, respectively, whereas in the matching experiment, both the insulin (26.1 +/- 3.9 pmol.ml-1.300 min-1) and C-peptide (178.0 +/- 18.9 pmol.ml-1.300 min-1) responses were lower (P less than .05) than in the other two studies. Insulin secretory rates were derived from peripheral C-peptide concentrations with an open two-compartment model and individually derived model parameters. (ABSTRACT TRUNCATED AT 250 WORDS)     

Importance of glucose per se to intravenous glucose tolerance. Comparison of the minimal-model prediction with direct measurements

Glucose disappearance after an oral or intravenous challenge is a function of the effects of both endogenously secreted insulin and of glucose itself. We previously introduced the term "glucose effectiveness," or SG, defined as the ability of glucose per se to enhance its own disappearance independent of an increment in plasma insulin. The present investigation, performed in conscious dogs, was undertaken to quantify this glucose effect by minimal-model-based analysis of insulin and glucose dynamics after a frequently sampled intravenous glucose tolerance test (FSIGT). The values from the standard FSIGT were then compared with direct measurements obtained from experiments in which the dynamic insulin response to glucose was suppressed with somatostatin (SRIF). In addition, we examined SG values from the modified FSIGT protocol, which involves both glucose and tolbutamide injections. Protocol l (N = 9): FSIGTs were performed and the glucose and insulin data were analyzed by computer. KG was 2.65 +/- 0.28 min-1, S1 was 4.09 +/- 0.34 X 10(4) min-1/(microU/ml), and SG was 0.033 +/- 0.004 min-1. Protocol II (N = 6): FSIGTs were performed on animals in which SRIF was infused (0.8 micrograms/min X kg) to obliterate the dynamic insulin response to glucose injection. Before the FSIGt, insulin and glucagon were infused intraportally to reattain basal glycemia. Without dynamic insulin, KG was reduced to 0.96 +/- 0.18 min-1 (P less than 0.0001). However, SG, estimated from the exponential rate of fall of plasma glucose in the absence of dynamic insulin, was similar to the standard FSIGTs: 0.025 +/- 0.004 (P greater than 0.25). Protocol III (N = 6): modified FSIGTs were performed using glucose and tolbutamide injections for a better estimate of model parameters. Model parameters Sl and SG, and the KG were not different from standard FSIGTs (P greater than 0.3). In fact, the value of SG (0.028 +/- 0.003 min-1) was nearly identical to the direct measure from protocol II. Therefore, the effect of glucose per se on glucose decline, estimated by modeling the standard and modified FSIGTs, was confirmed by a direct measurement with the endogenous insulin response suppressed with SRIF. Also, the time course of the insulin effect to enhance net glucose disappearance from plasma [Ieff(t)] was calculated from the data of protocol II, and was the same as the time course predicted by the model. These studies demonstrate the ability of the computer modeling approach to separate insulin-dependent and glucose-dependent glucose disappearance, and represent a direct confirmation of the minimal model.(ABSTRACT TRUNCATED AT 400 WORDS)     

Pertussis toxin-sensitive G protein mediation of PGE2 inhibition of cAMP metabolism and phasic glucose-induced insulin secretion in HIT cells

Although prostaglandin E2 (PGE2) is known to inhibit glucose-induced insulin secretion, it is uncertain whether PGE2 actions on the beta-cell are direct, whether they are equipotent for both phases of hormone secretion, and whether the same mechanism of action prevails throughout. Study of the HIT cell, a clonal line of pancreatic beta-cells, provides answers to these questions because perifusion with glucose and 3-isobutyl-1-methylxanthine stimulates biphasic insulin secretion. Perifusion with PGE2 decreased both the first and second phases of glucose-induced insulin release to 47 +/- 4% of controls. Pretreatment with pertussis toxin partly prevented PGE2 inhibition to 80 +/- 4% of controls for first phase and 79 +/- 4% of controls for second phase. To evaluate whether the partial prevention of PGE2 inhibition seen with pertussis toxin pretreatment was caused by Gi heterotrimer association between the preincubation period and the end of perifusion, PGE2 actions were also examined during continuous treatment with pertussis toxin. Under these conditions, PGE2 inhibition of both phases was totally prevented. However, no difference was observed in membrane protein ADP ribosylation when cells were examined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis after pretreatment or continuous treatment with pertussis toxin. Cyclic AMP (cAMP) accumulation was inhibited by PGE2 (from 3263 +/- 153 to 1549 +/- 158 fmol/10(6) cells) but less so after pretreatment with pertussis toxin (correlation between insulin release and cAMP accumulation during perifusion; n = 18, r = .85, P less than .001). Thus, PGE2 equally inhibits both phases of glucose-induced insulin secretion and cAMP generation through a pertussis toxin-sensitive G protein-mediated direct effect on the pancreatic beta-cell.     

Incretin effect due to increased secretion and decreased clearance of insulin in normal humans

To assess the contribution of changes in insulin secretion and clearance to the incretin effect (greater insulinemia after oral than after intravenous glucose), 10 healthy subjects were studied after oral glucose (1 g/kg body wt) and again when glucose was infused intravenously at rates to match arterialized plasma glucose concentrations after oral glucose. Although basal and integrated plasma glucose did not differ between oral and intravenous glucose, integrated responses of insulin (3.3 +/- 0.5 vs. 1.8 +/- 0.4 mU ml-1.240 min-1, P less than .001), C-peptide (456.5 +/- 58.5 vs. 327.9 +/- 46.3 ng.ml-1.240 min-1, P = .002), gastric inhibitory polypeptide, (16.8 +/- 3.5 vs. -2.8 +/- 1.0 micrograms.ml-1.240 min-1, P less than .001), and insulin secretion (6.6 +/- 1.1 vs. 4.7 +/- 0.7 U.240 min-1, P = .003) were greater with oral than intravenous glucose. However, insulin clearance, whether calculated as the molar ratio of integrated C-peptide to integrated insulin responses (6.9 +/- 0.7 vs. 14.2 +/- 3.8, P = .005) or from the formula insulin clearance equals insulin secretion divided by integrated insulin responses (1.1 +/- 0.2 vs. 2.5 +/- 0.7 L.min-1.m-2, respectively, P = .002), was less for oral than for intravenous glucose. Therefore, the incretin effect is mediated both by increased secretion and decreased clearance of insulin.     

Effect of glyburide on glycemic control, insulin requirement, and glucose metabolism in insulin-treated diabetic patients

Glycemic control and glucose metabolism were examined in 5 patients with insulin-dependent diabetes mellitus (IDDM) and 8 insulin-treated non-insulin-dependent diabetes mellitus (NIDDM) patients before and after 2 mo of therapy with glyburide (20 mg/day). Glycemic control was assessed by daily insulin requirement, 24-h plasma glucose profile, glucosuria, and glycosylated hemoglobin. Insulin secretion was evaluated by glucagon stimulation of C-peptide secretion, and insulin sensitivity was determined by a two-step euglycemic insulin clamp (1 and 10 mU X kg-1. X min-1) performed with indirect calorimetry and [3-3H]glucose. In the IDDM patients, the addition of glyburide produced no change in daily insulin dose (54 +/- 8 vs. 53 +/- 7 U/day), mean 24-h glucose level (177 +/- 20 vs. 174 +/- 29 mg/dl), glucosuria (20 +/- 6 vs. 35 +/- 12 g/day) or glycosylated hemoglobin (10.1 +/- 1.0 vs. 9.5 +/- 0.7%). Furthermore, there was no improvement in basal hepatic glucose production (2.1 +/- 0.2 vs. 2.4 +/- 0.1 mg X kg-1 X min-1), suppression of hepatic glucose production by low- and high-dose insulin infusion, or in any measure of total, oxidative, or nonoxidative glucose metabolism in the basal state or during insulin infusion. C-peptide levels were undetectable (less than 0.01 pmol/ml) in the basal state and after glucagon infusion and remained undetectable after glyburide therapy. In contrast to the IDDM patients, the insulin-treated NIDDM subjects exhibited significant reductions in daily insulin requirement (72 +/- 6 vs. 58 +/- 9 U/day), mean 24-h plasma glucose concentration (153 +/- 10 vs. 131 +/- 5 mg/dl), glucosuria (14 +/- 5 vs. 4 +/- 1 g/day), and glycosylated hemoglobin (10.3 +/- 0.7 vs. 8.0 +/- 0.4%) after glyburide treatment (all P less than or equal to .05). However, there was no change in basal hepatic glucose production (1.7 +/- 0.1 vs. 1.7 +/- 0.1 mg X kg-1 X min-1), suppression of hepatic glucose production by insulin, or insulin sensitivity during the two-step insulin-clamp study. Both basal (0.14 +/- 0.05 vs. 0.32 +/- 0.05 pmol/ml, P less than .05) and glucagon-stimulated (0.24 +/- 0.07 vs. 0.44 +/- 0.09 pmol/ml) C-peptide levels rose after 2 mo of glyburide therapy and both were correlated with the decrease in insulin requirement (basal: r = .65, P = .08; glucagon stimulated: r = .93, P less than .001).(ABSTRACT TRUNCATED AT 400 WORDS)