Effect of a 3-day fast on glucose storage and oxidation in obese hyperinsulinemic diabetics

Glucose disposal was measured for a 3-hr period after a 100-g oral glucose load by means of a new adaptation of continuous indirect calorimetry in 6 obese hyperinsulinemic diabetics and 5 nonobese normal subjects who served as the control. While total glucose oxidized during the 3-hr test was not significantly lower in the obese diabetic group (31 ± 3 g) than in the control group (37 ± 3 g), a marked impairment of net glucose storage was observed in the former group (26 ± 7 g) in comparison to the control group (64 ± 3 g; p < 0.001). This marked decrease in net glucose storage suggests that a limited capacity for glucose storage might play a major role in glucose intolerance in these cases of obese hyperinsulinemic diabetes. In the obese diabetic group, after a 3-day fast supplemented with protein, the plasma glucose values dropped significantly both in the fasting state and in response to the glucose load. This was accompanied by a marked improvement of glucose storage (52 ± 9 versus 26 ± 7 g before the fast; p < 0.001), a decrease in glucose urinary loss (5 ± 1 g versus 14 ± 4 g prior to fast), but a marked impairment in glucose oxidation (13 ± 1 g versus 31 ± 3 g before fast; p < 0.001). In the control group, a moderate impairment of glucose tolerance was observed, probably related to the important decrease in glucose oxidation (12 ± 3 g versus 37 ± 3 g prior to fast), in spite of the increase in glucose storage (82 ± 4 g versus 64 ± 3 g prior to fast). These observations suggest that glucose intolerance observed in obese hyperinsulinemic diabetics in the postabsorptive state might result at least in part from deficiency in net glucose storage capacity. The marked lowering of the plasma glucose tolerance curve in the same subjects after a 3-day period of fast is probably a consequence of the overall effect of a decrease in the glucose pool and an increase in net glucose storage in spite of a decrease in glucose oxidation and in urinary glucose loss. It does not exclude, however, other factors, such as changes in tissue insulin sensitivity.     

Effect of long chain triglyceride infusion on glucose metabolism in man

The effect of long chain triglyceride infusions (Intralipid 20%, 1 ml/min) on total body glucose uptake, glucose oxidation and glucose storage was examined in 25 healthy young volunteers by employing the euglycemic insulin clamp technique in combination with indirect calorimetry. Insulin was infused at three different rates (0.5, 2 and 4 mU/kg min) to achieve steady state hyperinsulinemic plateaus of 60 ± 4, 170 ± 10 and 420 ± 15 μU/ml. Prior to Intralipid infusion, the mean basal plasma free fatty acid concentration of all subjects was 385 ± 8 μmole/l. Following 90 min Intralipid infusion, the mean plasma free fatty acid level was increased to 760 ± 20 μmole/l (p < 0.001). At each insulin dose level, hyperlipidemia caused a significant reduction in total glucose uptake (5.9–3.5, 9.9–7.1, 11.1–8.8 mg/kg min, all p < 0.001. The decrease in total body glucose uptake was reflected by a decrease in both total glucose oxidation (2.4–1.6, 3.4–2.2, 3.7–2.8 mg/kg min, all p < 0.001) and glucose storage (3.6–1.9, 6.5–4.9, 7.4–5.9 mg/kg min, all p < 0.001). Basal glucose oxidation (1.3 ± 0.1 mg/kg min) fell by about 30% following 90 min of Intralipid infusion (0.9 ± 0.1 mg/kg min). Six additional subjects were studied with a lower infusion rate of Intralipid (0.5 ml/min). In these studies, insulin was infused at two different doses (0.5 and 2 mU/kg min) to achieve steady state plasma levels of 62 ± 2 and 171 ± 4 μU/ml. Intralipid caused again a significant reduction in total body glucose uptake during both the low (5.9 to 4.5 mg/kg min, p < 0.001) and the high (9.9–8.7 mg/kg min, p < 0.01) insulin clamp studies. This decrease in total glucose uptake was again the combined effect of an inhibition of both glucose storate (p < 0.05) and glucose oxidation (p < 0.001). In both high and low dose Intralipid infusion protocols, a strong inverse correlation was noted between the plasma free fatty acid concentration during the insulin clamp study and total body glucose uptake (r = 0.92, p < 0.001), glucose oxidation (r = 0.95, p < 0.001), and glucose storage (r = 0.90, p < 0.01). These results indicate that the inhibitory effect of free fatty acids on glucose utilization involves the biochemical pathways regulating both glucose oxidation and glycogen synthesis.     

Comparative study of isomalt and sucrose by means of continuous indirect calorimetry

Isomalt (Palatinit) an equimolar mixture of alpha-D-glucopyranosido-1,6-sorbitol and alpha-D-glucopyranosido-1,6-mannitol, was compared to sucrose in a prospective double-blind controlled crossover study. The acute effects of oral ingestion of 30-g loads of isomalt or sucrose on plasma glucose, insulin, free fatty acids (FFA), lactic acid, and carbohydrate (CHO) and lipid oxidation were studied over six hours by means of continuous indirect calorimetry in ten healthy normal-weight subjects. Unlike sucrose, whose ingestion was followed by significant changes in plasma glucose, insulin, and lactic acid during the first 60 minutes of the test, no significant changes in these parameters were observed following the administration of isomalt. The increase in CHO oxidation occurring between 30 and 150 minutes was significantly lower (P less than 0.01) following isomalt than after sucrose. Conversely, the decrease in lipid oxidation was significantly less (P less than 0.01) after isomalt in comparison to sucrose. It is concluded that the rise in CHO oxidation and in plasma glucose and insulin levels is markedly reduced when sucrose is replaced by an equal weight of isomalt. In contrast to other sugar substitutes, no increase in plasma lactic acid was observed after isomalt administration.     

Study on lipid metabolism in obesity diabetes

Knowing the relationship between obesity and diabetes, the purpose of our work was to study the alterations in lipid metabolism as measured by continuous indirect calorimetry in the course of a 100-g oral glucose-tolerance test in groups of obese patients without and with diabetes, respectively. Seventy-nine obese patients participated in the study. They were divided into four groups according to the degree of carbohydrate intolerance: group A, normal glucose tolerance; group B, impaired glucose tolerance; group C, diabetes with hyperinsulinemic response to the load; group D, diabetes with impaired insulin response. All four groups of patients presented an increase in lipid oxidation, both in the fasting state and during the three-hour glucose tolerance test, when compared to the control group. The lipid oxidation rate was roughly parallel to plasma free fatty acid (FFA) levels. The contribution of lipids to energy expenditure was higher in obese as compared to control subjects. These observations suggest that the larger part taken by lipids in the energy metabolism of both nondiabetic and diabetic obese humans is a consequence of their increased fat stores and that the resulting decrease in carbohydrate metabolism may lead, as a late consequence, to alterations in glucose tolerance. The latter may result in delayed glucose storage and oxidation in the obese patient.     

Relationship between glucose oxidation and glucose tolerance in man

The study was performed to determine the influence of peripheral glucose utilization on glucose tolerance. Glucose oxidation was measured in a group of 6 normal subjects by means of continuous indirect calorimetry during a 100 g oral glucose tolerance test for 3 hr, comparing the control state with experimental inhibition or stimulation of glucose oxidation. Suprabasal oxidation, corresponding to oxidation in response to the load, mainly by insulin-dependent tissue, was obtained by subtracting basal oxidation (essentially by non-insulin dependent tissues) from total oxidation. Suprabasal oxidation of glucose was inhibited by a neutral fat infusion, and stimulated by means of dichloracetate. In the control test, from the 100 g glucose administered, 18 g participated to suprabasal oxidation during the 3 hr of the test. A neutral fat infusion, started 2 hr before the glucose load and lasting throughout the test, decreased suprabasal oxidation to 7.5 g, i.e. to 42% of the control value. With the fat infusion, a larger fraction of the energy consumption was shown to originate from lipid oxidation (37% versus 25% in controls, p < 0.05) at the expense of carbohydrate (CHO) oxidation (44% versus 60% in controls, p < 0.05). However, these major changes in peripheral glucose oxidation were accompanied by only a moderate decrease in glucose tolerance. Dichloracetate administered prior to the test increased suprabasal oxidation to 25 g glucose oxidized in the 3 hours following the glucose load, i.e. an increment of 39% above the control value. A larger fraction of energy consumption was derived from carbohydrates (77% versus 60% in controls, p < 0.05). However, no significant change was observed in glucose tolerance. These results indicate that marked changes of peripheral glucose oxidation have little influence on glucose tolerance and suggest that another mechanism, i.e. glucose storage, plays a larger role in regulating plasma glucose levels during oral glucose tolerance tests.     

Comparison of glucose,fructose, sorbitol,and xylitol utilization in humans during insulin suppression

During fructose, sorbitol, and xylitol perfusions, carbohydrate utilization was studied by continuous indirect calorimetry and compared with glucose utilization during pharmacologic inhibition of endogenous insulin secretion. The experiment was performed in 28 normal volunteers divided into 5 groups (glucose, fructose, sorbitol, xylitol, and saline), each subject being its own control. Insulin suppression was obtained by means of a constant infusion of epinephrine (6 μg/min) and propranolol (0.08 mg/min). After 90 min, during plasma insulin steady state, each sugar or polyol was infused at a rate of 6 mg/kg/min for 120 min. In contrast with a rise in plasma glucose from 161 ± 6 mg/dl to 291 ± 14 mg/dl during glucose infusion, glucose levels remained unchanged during infusion of the glucose substitutes. Carbohydrate oxidation showed a rise of 24, 65, 76, and 44 mg/min during infusions of glucose, fructose, sorbitol, and xylitol, respectively. Lipid oxidation rates decreased by 7, 20, 33, and 23 mg/min during the same infusions. These results indicate that fructose, sorbitol, and xylitol are oxidized at a higher rate than glucose during suppression of endogenous insulin secretion, without any significant rise in glycemia.     

Specific inhibition of the renin-angiotensin system: A key to understanding blood pressure regulation

The availability of specific inhibitors of the renin-angiotensin system has made it possible to evaluate precisely the contribution of this system to the maintenance of normal blood pressure and of various hypertensive situations encountered in animal models and in man. Furthermore, by combining the blockade of the renin system with accurate measurements of sodium balance, it is possible to expose and quantify latent or manifest abnormalities in renal sodium handling that operate directly or by interacting with the renin system.Experimental and clinical observations made with these inhibitors support the hypothesis that the level of blood pressure, normal or abnormal, is largely determined by two components: (1) the renin system (i.e., the angiotensin II-vasoconstrictor factor) and (2) the sodium extracellular and intravascular volume factor, which in the longer term may be indirectly supported by the renin system via angiotensin-induced aldosterone secretion.Normally, without threat to the homeostasis, the sodium volume component appears to be the main factor determining blood pressure and sustaining renal perfusion via adequate or—if necessary—increased blood pressure. However, whenever renal perfusion is compromised due to such events as hemorrhage, sodium depletion, upright posture, exercise, cardiac failure, cirrhosis, or renal vascular impairment, renin is secreted by the kidney. It seems that the renin-angiotensin system is the main vasoconstrictor system used by the body to support the sodium volume component to maintain or restore adequuate renal perfusion. There is little evidence that the catecholamines and the nervous system play any major or direct role in sustaining blood pressure, although indirectly they may be critically involved in monitoring renal renin release.Two models of renal hypertension have served as the experimental basis for development of this concept. Established two-kidney Goldblatt hypertension (one renal artery clipped, contralateral kidney intact) was found to be renin-dependent, since angiotensin II blockade induced a marked fall in blood pressure. Clinical counterparts to this predominantly “vasoconstrictor” type of hypertension are renovascular hypertension with unilateral renal artery stenosis, malignant and essential hypertension with high renin levels, and possibly normotensive situations with reduced “effective” blood volume such as cirrhosis and congestive heart failure.Chronic one-kidney Goldblatt hypertension (one renal artery clipped, contralateral nephrectomy) under conditions of unrestricted sodium intake appeared on the other hand to be predominantly sodium-volume-dependent, so that angiotensin II blockade did not alter the pressure level. However, sodium (and volume) depletion did not lower the blood pressure either in this model but resulted instead in a compensatory rise of renin release, and thus in a transition from a sodium-volume to vasoconstrictor-maintained type of hypertension. Accordingly, under conditions of sodium depletion, angiotensin II blockade did markedly reduce the blood pressure. Clinical counterparts to this model in which simultaneous sodium depletion plus blockade of the renin system are necessary to reduce blood pressure appear to be most patients with normal renin essential hypertension, chronic renal failure with normal or low renin levels, renovascular hypertension with bilateral renal artery stenoses, and possibly coarctation of the aorta.In contrast to these two model situations, low-renin essential hypertension as well as the hypertension induced by an excess of various mineralocorticoids appears to represent a pure volume type of hypertension, in which a diuretic-induced volume reduction does not result in a compensatory rise in renin release and a shift to vasoconstrictor support but instead results in a parallel reduction of blood pressure.     

Role of the mitochondrial metabolism of pyruvate on the regulation of ketogenesis in rat hepatocytes

In hepatocytes isolated from fed rats the inhibition of lipogenesis (-80%) by 5-tetradecyloxy-2-furoate (an inhibitor of acetylCoA carboxylase) and alpha-cyano-3-hydroxycinnamate (an inhibitor of pyruvate entry into mitochondria) increases the oxidation of 0.35 mM oleate respectively by 70% and 90%. 5-tetradecyloxy-2-furoate increases ketone body production from oleate only by 30% and has no effect on ketogenesis from octanoate, whereas alpha-cyano-3-hydroxycinnamate mimics the effects of fasting on ketone body production: It increases ketogenesis from 0.35 mM oleate by 90%, from 0.78 mM oleate by 25% and from 1.57 mM butyrate by 37%. alpha-cyano-3-hydroxycinnamate also decreases the activity of tricarboxylic acid cycle and the production of malate and citrate. In hepatocytes from fasted rats, alpha-cyano-3-hydroxycinnamate does not modify ketogenesis from oleate, unless cells are incubated with a mixture of lactate and pyruvate. A lactate and pyruvate mixture decreases ketogenesis from oleate and octanoate and increases citrate and malate production without modifying the uptake of fatty acids. This effect is potentiated by 3-mercaptopicolinate, an inhibitor of phosphoenolpyruvate carboxykinase. The results cannot be interpreted only by the effects of malonylCoA on carnitine acyltransferase. They are discussed with respect to the possible involvement of mitochondrial oxaloacetate concentration in the regulation of ketogenesis.     

The stimulus-secretion coupling of glucose-induced insulin resease XXVI. Are the secretory and fuel functions of glucose dissociable by iodoacetate?

Iodoacetate inhibits glyceraldehyde-3-phosphate dehydrogenase activity in pancreatic islets and causes a time- and dose-related inhibition of glucose oxidation and lactate output by the islets. High concentrations of the drug (0.3 mM or more) fail to affect Ba2+-induced insulin secretion but inhibit glucose-stimulated proinsulin biosynthesis, 45Ca net uptake and insulin release. A mixture of fumarate, glutamate, and pyruvate, the oxidation of which is only partially reduced by iodoacetate, fails to protect the B-cell against the inhibitory effect of the drug. These findings are compatible with the view that glycolysis plays an essential role in the process of glucose-induced insulin release. At low concentrations of iodoacetate (up to 0.2 mM), the reduction in glucose metabolism coincides with a partial inhibition of proinsulin biosynthesis. However, the expected reduction in 45Ca net uptake and subsequent insulin release is masked by a concomitant facilitating action of iodoacetate, possibly due to interference with native ionophoretic processes. It is concluded that iodoacetate is not an adequate tool to dissociate, if they are dissociable, the fuel and secretory functions of glucose.     

Endocrine abnormalities in patients undergoing long-term hemodialysis. The role of prolactin

The possible role of hyperprolactinemia in the sexual and thyroid abnormalities of patients with end-stage renal failure was studied in 56 patients undergoing long-term hemodialysis. Seventy-three per cent of the women and 25 per cent of the men receiving no prolactin (PRL)-enhancing drugs had elevated PRL levels (25 to 200 ng/ml, normal <20 ng/ml), and all those patients who were receiving α-methyldopa treatment (13 patients) had even higher PRL levels (30 to 1,107 ng/ml). The response of PRL to the administration of thyrotropin-releasing hormone (TRH) was blunted and prolonged, suggesting that hyperprolactinemia was in part due to prolonged plasma half-life of the hormones; this was confirmed by the insufficient PRL lowering effect of a single dose of bromocriptine in a short-term test, whereas a longer trial of six weeks demonstrated the PRL-suppressive effect of the drug.Amenorrhea or oligomenorrhea was a constant feature in women, four of whom also showed galactorrhea; during bromocriptine treatment, recurrence of menses was observed in some of the hyperprolactinemic amenorrheic women. Men with impotence had higher PRL levels than men with normal potency. Lack of appropriate elevation of luteinizing hormone (LH) levels despite low estradiol or testosterone levels was observed in approximately one third of the hyperprolactinemic subjects of both sexes, but also in a similar proportion of normoprolactinemic ones. However, treatment with bromocriptine resulted in an increase in basal LH in 13 of 16 patients with hyperprolactinemia. These data demonstrate that PRL measured in patients with advanced renal failure is biologically active and that hyperprolactinemia is one of the major factors in the hypogonadism of these patients. In addition, integrated circulating LH after the injection of exogenous luteinizing hormone-releasing hormone (LHRH) was normal, despite prolonged plasma half-life, suggesting that pituitary response was in fact impaired. Unrelated to PRL levels, 47 per cent of the patients had slightly elevated thyroid-stimulating hormone (TSH) levels, with normal total and free thyroxine (T4). Although triiodothyronine (T3) tended to cluster in the low-normal area, reverse-T3 levels were not consistent with preferential deiodination of T4 to reverse-T3. Elevation of the basal TSH levels was, at least in part, due to prolonged plasma half-life of the hormone. These biochemical changes were not related to clinically detectable dysfunction of the thyroid.     

Experimental cystinuria: The cycloleucine model. II. Amino acid efflux from intestinal and renal tissues

Loading and unloading experiments using intestinal sacs and renal cortex slices were undertaken to ascertain the role of amino acid efflux in cycloleucine-induced amino-aciduria. The presence of cycloleucine, lysine, or valine on the luminal or antiluminal side of the intestine caused an increased leakage of [¹⁴C] cycloleucine, [¹⁴C] lysine, and [³⁵S] cystine from the tissue. Similar results were obtained when using kidney cortex slices, except for cystine efflux. The latter phenomenon was inhibited by cycloleucine and lysine. Data, also obtained with renal cortex slices, suggest that cystine and cysteine are recognized by different transport sites although one (the oxidized form) may be typically extracellular and the other (the reduced form), intracellular. A comparison of these data with previous works done in our laboratory¹⁴ shows that cycloleucine affects efflux less than influx and further suggests that in rats given cycloleucine, renal transport is impaired only at the brush border level for cystine and at both luminal and antiluminal membranes for dibasic amino acids.     

Stimulus-secretion coupling of glucose-induced insulin release. Timing of early metabolic,ionic, and secretory events

The timing of the early metabolic, ionic, and secretory responses to glucose in rat pancreatic islets was monitored by measuring, at 12 sec intervals, the concentrations of glucose, lactic, and pyruvic acids, ³²P, ⁸⁶Rb, ⁴⁵Ca, and insulin in the effluent of perifused prelabeled islets. The increase in glucose concentration from zero to 16.7 mM was complete within 133 sec. The output of organic acids increased after 24 sec of exposure to glucose and, in the case of lactic acid, fell slightly after the initial elevation. The phosphate flush was initiated only after 96 sec of exposure to glucose, whereas the decreases in ⁸⁶Rb and ⁴⁵Ca outflow were both detectable within 72 sec of stimulation. The secondary rise in ⁴⁵Ca efflux was first seen after 157 sec of stimulation and its time course was not vastly different from that of insulin release. These data indicate that, in the secretory sequence, metabolic changes precede both the remodelling of ionic fluxes and the stimulation of insulin release. The results are compatible with the view that the secondary rise in ⁴⁵Ca outflow is attributable, in part at least, to the glucose-induced decrease in K conductance (but not to the increase in phosphate outflow), with resulting membrane depolarization and gating of voltage-dependent Ca channels.     

Reduction of increased levels of blood glycerol and ketone bodies by propranolol in human hyperthyroidism: Role of the fall of the triiodothyronine level

Hyperthyroid patients in the postabsorptive state have elevated levels of blood glycerol and ketone bodies (KB): this is believed to be due to increased lipolysis and ketogenesis. These increased glycerol and KB levels return toward normal after oral propranolol administration. In order to investigate the mechanism of action of propranolol in hyperthyroid patients, we compared the effects of the oral administration of propranolol with those of timolol, propylthiouracil (PTU), and a placebo. The placebo had no effect. The free thyroxine index, immunoreactive insulin level and glucagon level were not modified by propranolol, timolol, or PTU. Propranolol decreased the pulse rate (P < 0.01) and the levels of serum triiodothyronine (T₃; P < 0.05), blood glycerol (P < 0.01), and KB (P < 0.01). Like propanolol, timolol decreased the pulse rate (P < 0.01) but had no effect on the T₃, glycerol, or KB levels. Propylthiouracil did not modify the pulse rate, but like propanolol, it decreased the T₃ (P < 0.05), glycerol (P < 0.01) and KB (P < 0.01) levels. These results suggest that the metabolic actions of propranolol are not caused by its hemodynamic effects nor its beta-blocking properties but are mediated by the decrease of the T₃ level.     

Distribution and metabolism of intravenously injected tritiated insulin in rats.

Semisynthetic [³H] insulin was used to follow the distribution and metabolism of intravenously injected insulin in rats. Chromatographic methods were used to separate intact labeled insulin from radioactive degradation products in the plasma, liver, kidneys, and skeletal muscle. The metabolic clearance rate of the injected insulin was 23.6 ± 1 ml/min/kg and the distribution space 85 ± 17 ml/kg for an injected dose of $\text{1.5 mU}\text{100}\text{g}$ body weight. The kidneys concentrated tritium relative to the plasma by up to ninefold; the liver also concentrated radioactivity, but to a lesser degree. Radioactive degradation products were found to be of either higher or lower molecular weight than insulin. The kidneys contained predominantly low-molecular-weight degradation products, accounting for 54% of the radioactivity in these organs even 5 min after injection. The liver, by contrast, contained predominantly high-molecular-weight degradation products. This material appeared in the liver before it was seen in the plasma, suggesting that the liver is responsible for its production. This suggestion was confirmed by analyzing plasma samples from rats injected intravenously with [³H] insulin following either functional hepatectomy or nephrectomy. The hepatectomized rats displayed less high-molecular-weight material in the plasma and 2–3 times more intact insulin when compared to controls. By contrast, nephrectomy resulted in no significant change in the percentage of either high- or low-molecular-weight degradation products in the plasma despite reduced insulin clearance. These data confirm the importance of the liver and kidneys in insulin metabolism. Since at least some of the high-molecular-weight-degradation products may be formed by reincorporation of [³H] phenylalanine (liberated by degradation of [³H] insulin) into newly synthesized protein, the importance of the liver in its production may be a reflection of the protein biosynthetic capacity of this organ.     

Glucose oxidation in relation to the size of the oral glucose loading dose

Using “naturally labeled ¹³C-glucose” the conversion to expired air CO₂ of 100, 66 and 33 g oral glucose loads was evaluated, during 7 hr, in 6 male healthy volunteers. The total amounts of exogenous glucose converted to expired air CO₂ were proportional to the loading doses and averaged, in g/7 hr, 14.6 ± 2.1 for 33 g, 20.8 ± 2.1 for 66 g and 31.2 ± 1.6 for 100 g. In contrast total glucose oxidation, evaluated by indirect calorimetry, was similar in the three groups and was not correlated with the dose of glucose given. The plasma insulin response was significantly correlated with the load of glucose (r = 0.86; p < 0.001) whereas such correlation was not observed with the blood glucose response. Comparison with data available in the literature on the influence of the size of the oral glucose load on endogenous glucose production and on splanchnic glucose output suggests that the greater oxidation of exogenous glucose seen when the oral load of glucose is increased essentially results from a greater enrichment of the systemic glucose pool with exogenous glucose.     

The effects of acetylstrophanthidin on the response of the AV junction to adrenergic stimulation studied in dogs.

The response of the AV junction to adrenergic stimulation was studied in 35 anesthetized open-chest dogs before and after the injection of acetylstrophanthidin (5mug) directly into the AV node artery. An AV junctional rhythm was obtained under control conditions by injecting norepinephrine (n = 9) or isoproterenol (n = 8) into the AV node artery and by stimulation of the left stellate ganglion (n = 11) after selectively injecting propranolol into the sinus node artery. Acetylstrophanthidin brought about various degrees of conduction block from simple PR interval prolongation to complete heart block, and decreased the chronotropic response of the AV junction to adrenergic stimulation. In seven animals the appearance of a spontaneous second degree AV block did not reduce the AV junctional response to adrenergic stimulation. Acetylstrophanthidin also reduced the ventricular acceleration produced by adrenergic stimulation during atrial fibrillation. These results suggest that the anti-adrenergic effect of cardiac glycosides may not only be involved in the mechanism of AV conduction disturbances during digitalis intoxication, but may also play a role in slowing the ventricular rate during atrial fibrillation.     

Stimulus-secretion coupling of amino acid-induced insulin release VII. The B-cell memory for L-glutamine

In the absence of another exogenous nutrient, L-glutamine does not stimulate insulin release from rat pancreatic islets or isolated perfused pancreases. L-glutamine, however, augments insulin release evoked by L-leucine. These two amino acids could interact by providing both the substrate (L-glutamate) and an activator (L-leucine) for the reaction catalyzed by glutamate dehydrogenase. Under suitable experimental conditions, as little as 0.5 mM L-glutamine is sufficient to enhance leucine-stimulated insulin release. When the pancreases or islets are first exposed to L-glutamine and then stimulated with L-leucine, the rate of secretion is much higher than that evoked by L-leucine in tissue not first exposed to L-glutamine. The memory of a prior exposure to L-glutamine persists for at least 25 min after removal of the latter amino acid from the extracellular fluid. This memory phenomenon is not dependent on the presence of Ca2+ in the extracellular fluid during the first exposure to L-glutamine, but is suppressed when such a prior exposure is performed in the absence of extracellular K+. The memory phenomenon could be due, in part at least, to inhibition by L-glutamine of K+ conductance in the B-cell plasma membrane. Moreover, the amount of L-glutamate which accumulates in islets exposed to L-glutamine is sufficient to maintain, for a much longer period than with other nutrient secretagogues, a sustained increase in catabolic fluxes after removal of the amino acid from the extracellular fluid.     

Reduction by propranolol of urinary hydroxyproline excretion in human hyperthyroidism: A beta-receptor blockade effect or a membrane stabilizing mechanism?

In order to investigate the mechanism whereby oral propranolol administration reduces the increased rate of urinary hydroxyproline excretion (UHxE) of patients with hyperthyroidism, a comparison was made of the effects of the oral administration of propranolol-timolol, propylthiouracil (PTU), and a placebo to patients with hyperthyroidism and to normal controls. Propranolol decreased the pulse rate (P < 0.01), serum triiodothyronine (T₃) level (P < 0.05), and UHxE (P < 0.01) without modifying the serum free thyroxine index (FT₄I) or parathormone (PTH) level. Timolol decreased the pulse rate (P < 0.01) to the same extent as propranolol, had no effect on T₃, FT₄I, or PTH, and failed to decrease UHxE. Administration of PTU decreased the T₃ level (P < 0.05) to a similar extent as propranolol without modifying the FT₄I or PTH level and had no effect on UHxE. Placebo administration had no effect. These results suggest that the reduction of UHxE by propranolol is not due to the beta-receptor-blocking properties of propranolol nor mediated by the propranolol-induced decrease in the level of T₃ but is probably due to the membrane-stabilizing properties of propranolol.     

Stimulus-secretion coupling of glucose-induced insulin release. Effect of intracellular acidification upon calcium efflux from islet cells

When pancreatic islets were exposed to a medium of normal pH (7.4) equilibrated with a gaz mixture containing 30% (instead of 5%) CO₂, the intracellular pH of the islet cells, as judged by the apparent space of distribution of ¹⁴C-DMO, was decreased. The intracellular acidification was associated with a delayed decrease of [U-¹⁴C] glucose oxidation, but no major change in glucose-stimulated proinsulin biosynthesis, ⁴⁵calcium net uptake, or insulin release. The increase in p_CO2 provoked an immediate and sustained decrease in the fractional outflow rate of ⁴⁵Ca from prelabeled and perfused islets. The latter decrease was most marked under conditions associated with stimulated ⁴⁰Ca⁴⁵Ca exchange (i.e., at glucose 16.7 mM and normal Ca²⁺ concentration), but was also present when a process of Na⁺Ca²⁺ countertransport accounted for the major part of ⁴⁵Ca efflux (e.g., in the absence of both glucose and extracellular Ca²⁺). These findings are compatible with the view that the generation of H⁺ derived from the metabolism of glucose in islet cells plays a role in the sugar-induced decrease in ⁴⁵Ca fractional outflow rate.     

Reduction of beta-thromboglobulin levels in diabetics controlled by artificial pancreas

Beta-Thromboglobulin (B-TG) levels were measured in 22 poorly controlled insulin-dependent diabetics and 27 nondiabetic controls from the same age groups. Mean B-TG level was higher in diabetics than in controls (62.9 ± 18.0 versus 26.9 ± 12.2 ng/ml p < 0.01), but was identical in diabetics with (62.7 ± 17.4 ng/ml) and without complications (63.0 ± 19.3 ng/ml). After 24 hr of strict blood glucose (BG) normalization using an artificial pancreas, B-TG mean level remained stable. It decreased significantly after 48 hr of glycemic normalization, (11 patients, 44.2 ± 12.1 ng/ml versus 66.2 ± 18.5 ng/ml, p < 0.01). These results support the hypothesis that B-TG levels in diabetics can be reduced by strict BG control. Therefore, the platelet hyperactivity as measured by B-TG levels must be carefully interpreted taking into account the metabolic control of the patients.