Effects of short-term pulsatile and continuous insulin delivery on glucagon secretion and insulin secretion and action

In six normal nonobese subjects, hyperinsulinemic euglycemic clamps were performed during paired sequential two-hour intravenous (IV) insulin infusions separated by an hour washout period. Each infusion was either 32 mU/kg/h of continuous insulin (CI) or 75% of this dose as 40-second pulses delivered every 13 minutes (PI). Six studies were performed with each of the following sequences in random order: PI-CI, CI-PI, and CI-CI. Based on the initial infusions, the insulin-dependent fractional glucose disappearance rate (X) during pulsatile insulin delivery (3.0 +/- 0.4 min-1 X 10(2), n = 6) was 73% of that of the continuous infusions (4.1 +/- 0.3 min-1 X 10(2), n = 12). This ratio was similar to that of the measured time-averaged plasma insulin areas (PI = 24.7 +/- 3.8 v CI = 31.4 +/- 3.5 mU/L). There was an average 23% enhancement of insulin's hypoglycemic effect during the second 12 CI infusions compared with the 12 initial CI infusions (X = 5.1 +/- 0.5 v 4.1 +/- 0.3 min-1 X 10(2), P less than .05). There was no significant difference between the enhancing effects of PI and CI infusions on insulin action in the subsequent CI's (X = 4.9 +/- 0.9 for PI-CI v X = 5.3 +/- 0.2 min-1 X 10(2) for CI-CI). First infusion PI significantly (P less than .05) decreased plasma C-peptide levels (0.34 +/- 0.05 to 0.20 +/- 0.06 mumol/L), whereas CI did not (0.33 +/- 0.02 to 0.32 +/- 0.07).(ABSTRACT TRUNCATED AT 250 WORDS)     

Effect of nerve blockade on pulsatile insulin and glucagon secretion in vitro.

We recently reported that the secretion of insulin and glucagon by isolated murine islets is pulsatile and suggested that the pacemaker controlling these hormone oscillations is present in the islet. In the present study, we tested the hypothesis of an intrinsic islet neural controlling mechanism for the observed hormone pulsatility. Nerve blockade was attempted by infusion of tetrodotoxin (TTX) on a background of combined adrenergic and cholinergic blockade with atropine, propranolol, and phentolamine, (ATX). Because TTX acts by blocking Na+ channels, we also studied the effects of other cationic channel manipulations on the amplitude and frequency of the oscillations. The normal frequency and amplitude of glucagon and insulin oscillations were not affected by ATX. In contrast, TTX infusion increased the amplitude of insulin (198.6 +/- 20.9 vs. 507.2 +/- 62.8 pg/min, p less than 0.05, n = 4) and shortened the period from 5.03 +/- 0.26 to 3.33 +/- 0.0 min without affecting glucagon cycles. Whereas the Ca2+ ionophore A23187 had no effect on either hormone oscillation, the ATP-sensitive K+ channel blocker glyburide only increased the amplitude of insulin and decreased the amplitude of glucagon, without altering the frequencies. These data suggest that an intrinsic autonomously functioning islet nervous system is the pacemaker for the insulin oscillations and that the control of glucagon cycles differs from that of insulin.     

Effect of insulin sensitivity on pulsatile insulin secretion

OBJECTIVE: The aim of the study was to determine whether derangements in insulin pulsatility are related to the presence of insulin resistance or whether these changes occur only in non-insulin-dependent diabetes mellitus (NIDDM). DESIGN AND METHODS: The study included 26 obese, 11 NIDDM and 10 control subjects. The obese group was divided into a low insulin (plasma insulin <20 mU/l, OLI, 14 subjects) and a high insulin (OHI, 12 subjects) group. For pulsatility analysis blood was sampled every 2 min for 90 min. Pulsatility analysis was carried out using the PulsDetekt program. The insulin secretion randomness was quantified using interpulse interval deviation (IpID) and approximate entropy (ApEn). ApEn and ApEn normalized by s.d. of the individual insulin time series (nApEn) were calculated. Lower values of ApEn and IpID indicate more regular secretion. Homeostasis model assessment (HOMA) was used to quantify insulin sensitivity. RESULTS: Insulin pulses were significantly less regular in the OHI and the NIDDM groups compared with the control and the OLI groups (control: ApEn 0.54+/-0.16, nApEn 0.69+/-0.19, IpID 2.53+/-0.99; OLI: ApEn 0.64+/-0.12, nApEn 0. 79+/-0.15, IpID 2.92+/-1.09; OHI: ApEn 0.88+/-0.07, nApEn 0.92+/-0. 07, IpID 3.95+/-0.84; NIDDM: ApEn 0.92+/-0.16, nApEn 0.99+/-0.09, IpID 4.41+/-0.53; means +/- s.d.). There was no difference in the pulse regularity between the OHI and the NIDDM groups. CONCLUSIONS: Decrease in insulin sensitivity was correlated with the reduction of insulin secretion regularity. Therefore irregular insulin secretion is related to a reduction in insulin sensitivity, and it is not unique to NIDDM.     

Control of pulsatile insulin secretion in man

Plasma insulin and glucose concentrations were examined in man in a basal state from central venous samples taken at 1-min intervals for up to 2.5 h. Normal subjects have insulin oscillations of mean period 14 min (significant autocorrelation, p less than 0.0001) with changes in concentration of 40% over 7 min. The pulsation frequency was stable through cholinergic, endorphin, alpha-adrenergic or beta-adrenergic blockade, or small perturbations with glucose or insulin. Stimulation of insulin secretion by intravenous glucose, tolbutamide or sodium salicylate increased the amplitude of the insulin oscillations while the frequency remained stable. Patients with a truncal vagotomy or after Whipple's operation had longer-term oscillations of 33 and 37 min periodicity (autocorrelation: p less than 0.0001), with insulin-associated glucose swings four times larger than those of normal subjects. Type 2 (non-insulin-dependent) diabetic patients had a similarly increased insulin-associated glucose swing of six times that seen in normal subjects. The hypothesis is proposed that the 14-min cycle of insulin production is controlled by a 'pacemaker' which assists glucose homeostasis. The longer 33-37-min oscillations, seen in those with denervation, may arise from a limit-cycle of the feedback loop between insulin from the B cells and glucose from the liver. The vagus may provide hierarchical control of insulin release.     

The in vivo regulation of pulsatile insulin secretion

The presence of oscillations in peripheral insulin concentrations has sparked a number of studies evaluating the impact of the insulin release pattern on the action of insulin on target organs. These have convincingly shown that equal amounts of insulin presented to target organs have improved action when delivered in a pulsatile manner. In addition, impaired (not absent) pulsatility of insulin secretion has been demonstrated in Type II (non-insulin-dependent) diabetes mellitus, suggesting a possible mechanism to explain impaired insulin action in Type II diabetes. Whereas the regulation of overall insulin secretion has been described in detail, the mechanisms by which this regulation affects the pulsatile insulin secretory pattern, and the relative and absolute contribution of changes in the characteristics of pulsatile insulin release have not been reviewed previously. This review will focus on the importance of the secretory bursts to overall insulin release, and on how insulin secretion is adjusted by changes in these secretory bursts. Detection and quantification of secretory bursts depend on methods, and the methodology involved in studies dealing with pulsatile insulin secretion is described. Finally, data suggest that impaired pulsatile insulin secretion is an early marker for beta-cell dysfunction in Type II diabetes, and the role of early detection of impaired pulsatility to predict diabetes or to examine mechanisms to cause beta-cell dysfunction is mentioned. [Diabetologia (2002) 45: 3–20] Received: 9 November 2000 and in revised form: 26 July 2001     

Effects of gut hormone on insulin and glucagon secretion

Summary To examine the possible influences of gastrointestinal hormones upon the secretion of the hormones of islets of Langerhans, highly purified preparations of gastrin, secretin and pancreozymin were injected endoportally in anesthetized dogs. All three hormones were found to cause an immediate rise in the concentration of insulin in the pancreaticoduodenal vein. The effect of gastrin on insulin release was quantitatively trivial, while that of secretin was more substantial and of longer duration; however, pancreozymin appeared to be the most potent insulin stimulator and, in addition, caused a parallel rise in pancreatic glucagon secretion. Furthermore, pancreozymin was shown to augment both the insulin and the glucagon response to hyperaminoacidemia. Intraduodenal administration of amino acids, known to be the most potent stimulator of endogenous pancreozymin, was found to elicit a greater and more rapid release of insulin and glucagon than the intravenous administration of amino acids, suggesting that endogenous pancreozymin plays a physiologic role in augmenting the islet cell hormone response to ingested amino acids. The physiologic augmenter of the insular response to ingested glucose remains unidentified, however.Supported by USPHS, Grant AM-02700-08.     

Effect of fasting on alanine-stimulated insulin and glucagon secretion

The effect of a 5-day fast on glucose- and alanine-stimulated insulin and glucagon secretion was examined in the dog. As predicted from investigation of other species, prolonged fasting markedly inhibited glucogenic insulin secretion. In contrast, insulin and glucagon secretion after alanine challenge were unaltered by the prolonged fast. These data support the hypothesis that the physiologic response to this amino acid with fasting may be partially responsible for the regulation of insulin secretion in this state.     

Effect of clofibrate on arginine-induced insulin and glucagon secretion

The influence of clofibrate therapy on insulin and glucagon secretion was examined in the rat. Following arginine stimulation, serum insulin and glucagon levels rose significantly, resulting an an $\text{I}\text{G}$ molar ratio of 1.0 ± 0.3. In contrast, clofibrate therapy completely suppressed the arginine-stimulated insulin secretion, but potentiated the simultaneous glucagon response. The resulting $\text{I}\text{G}$ molar ratio fell to 0.45 ± 0.06, consistent with a change in the bihormonal status in the direction of increased catabolism. These effects on hormonal balance may mediate in part the hypolipemic action of clofibrate that simultaneously occurs.     

Effect of weight loss on the pulsatile insulin secretion

The aim of the study was to assess whether pulsatile insulin secretion is variable in the same individual and is related to changes in insulin sensitivity. Insulin sensitivity and pulsatility were measured before and after weight reduction in nine obese subjects. A pulsatility analysis was done using the PulsDetekt program. Blood was sampled every 2 min over a period of 90 min. The secretion randomness was quantified using approximate entropy (ApEn), and ApEn normalized by SD of the insulin time series (nApEn). Lower values indicate more regular secretion. Insulin sensitivity was measured using the homeostasis model assessment. Data are presented as median, minimum-maximum. After weight loss insulin sensitivity was increased (12.16, 7.60-76.70 vs. 38.96, 19.88-74.96%), the number of insulin pulses was reduced (11, 8-16 vs. 9,6-12), and they were more regular (ApEn, 0.92, 0.53-133 vs. 0.69,0,40-1.27; nApEn, 1.07, 0.74-1.33 vs. 0.97, 0.54-1.42). Before and after the weight loss there was a correlation between ApEn and nApEn and insulin sensitivity. Therefore, insulin secretion regularity is variable in the same individual and is related to insulin sensitivity.     

Modulation by verapamil of insulin and glucagon secretion in man

Summary The present investigation was designed to evaluate the effect of acute and protracted verapamil administration on insulin and glucagon secretion in man. For this purpose, 14 normal subjects received two consecutive glucose pulses (5 g i.v. in less than 20 sec or 20 g i.v. in less than 1 min, 7 subjects for each group), 70 or 90 min apart, before and during an infusion of verapamil (160 g/min). Seven additional normal subjects received two consecutive arginine pulses (5 g i.v.), 70 min apart. In 14 inpatients with coronary heart disease, we investigated the effect of protracted verapamil administration. Seven of these subjects underwent two oral glucose tolerance tests (100 g) and the other 7 two arginine tests (30 g) before and after a 10-day treatment with verapamil, 240 mg/die p.o. divided into three doses; the last dose, 80 mg, was given orally 1 h before the performance of the post-treatment test. Verapamil significantly inhibited the acute insulin response (AIR, mean change from 3–10 min) to glucose (5 g), as well as the AIR and AGR (acute glucagon response) to arginine (5 g). By contrast, verapamil failed to alter significantly the AIR to the higher glucose pulse. There was no significant change of oral glucose tolerance after verapamil, nor was there a change in insulin response to oral glucose. By contrast, insulin and glucagon responses to arginine infusion were significantly reduced by the drug.     

Effect of bombesin infused intrapancreatically on glucagon and insulin secretion

Synthetic bombesin was infused at a dose of 20 pmoles/kg/min for 10 min into the cranial pancreaticoduodenal artery of anesthetized dogs. Plasma immunoreactive glucagon concentrations in the cranial pancreaticoduodenal vein as well as in the femoral artery were concurrently and slowly elevated. However, the net release of glucagon from the pancreas did not increase significantly during infusion of bombesin. Plasma immunoreactive insulin concentrations in the pancreatic vein were transiently raised, and a delayed rise was noted in arterial plasma IRI. Net release of insulin was significantly augmented during infusion of the tetradecapeptide. Plasma glucose levels did not change after bombesin. These results indicate that the gastrointestinal tetradecapeptide may stimulate secretion of both insulin and gut glucagonlike immunoreactivity in the dog.     

Insulin modulation of glucagon secretion: The role of insulin and other factors in the regulation of glucagon secretion

Glucagon plays a critical counter-regulatory role to insulin to maintain optimal glucose homeostasis. Glucagon secretion from pancreatic α-cells is regulated by glycemia, neural input, and secretion from neighboring β-cells. Recently, we provided direct genetic evidence of a critical role for insulin signaling in the regulation of glucagon secretion in vivo. Pancreatic α-cell targeted disruption of insulin receptor expression in mice resulted in glucose intolerance, hyperglycemia and hyperglucagonemia coupled with an abnormal glucagon response to hypoglycemia. Furthermore, streptozotocin treated mice exhibited paradoxically increased plasma glucagon suggesting a dominant role for insulin in the regulation of glucagon secretion compared with glucose. In fact, normalization of hyperglycemia by phrolidzin treatment decreased plasma glucagon levels suggesting a stimulatory effect of glucose on glucagon secretion and also revealed the significance of insulin in hyperglycemic states. Together these studies provide novel insights into intra-islet regulatory pathways in the modulation of glucagon secretion and provide potential opportunities to develop therapeutic approaches for the correction of α-cell dysfunction in diabetes.