Adrenergic receptor control mechanism for growth hormone secretion.

The influence of catecholamines on growth hormone secretion has been difficult to establish previously, possibly because of the suppressive effect of the induced hyperglycemia on growth hormone concentrations. In this study, an adrenergic receptor control mechanism for human growth hormone (HGH) secretion was uncovered by studying the effects of alpha and beta receptor blockade on insulin-induced growth hormone elevations in volunteer subjects. Alpha adrenergic blockade with phentolamine during insulin hypoglycemia, 0.1 U/kg, inhibited growth hormon elevations to 30-50% of values in the same subjects during insulin hypoglycemia without adrenergic blockade. More complete inhibition by phentolamine could not be demonstrated at a lower dose of insulin (0.05 U/kg). Beta adrenergic blockade with propranolol during insulin hypoglycemia significantly enhanced HGH concentrations in paired experiments. The inhibiting effect of alpha adrenergic receptor blockade on HGH concentrations could not be attributed to differences in blood glucose or free fatty acid values; however, more prolonged hypoglycemia and lower plasma free fatty acid values may have been a factor in the greater HGH concentrations observed during beta blockade. In the absence of insulin induced hypoglycemia, neither alpha nor beta adrenergic receptor blockade had a detectable effect on HGH concentrations. Theophylline, an inhibitor of cyclic 3'5'-AMP phosphodiesterase activity, also failed to alter plasma HGH concentrations. These studies demonstrate a stimulatory effect of alpha receptors and a possible inhibitory effect of beta receptors on growth hormone secretion.

Loss of hepatic autoregulation after carbohydrate overfeeding in normal man.

To determine the effect of increased glycogen stores on hepatic carbohydrate metabolism, 15 nondiabetic volunteers were studied before and after 4 d of progressive overfeeding. Glucose production and gluconeogenesis were assessed with [2-3H] glucose and [6-14C] glucose (Study I, n = 6) or [3-3H] glucose and [U-14C]-alanine (Study II, n = 9) and substrate oxidation was determined by indirect calorimetry. Overfeeding was associated with significant (P < 0.01) increases in plasma glucose (4.97 +/- 0.10 to 5.09 +/- 0.11 mmol/liter), insulin (18.8 +/- 1.5 to 46.6 +/- 10.0 pmol/liter) and carbohydrate oxidation (4.7 +/- 1.4 to 18.0 +/- 1.5 mumol.kg-1.min-1) and a decrease in lipid oxidation (1.2 +/- 0.2 to 0.3 +/- 0.1 mumol.kg-1.min-1). Hepatic glucose output (HGO) increased in Study I (10.2 +/- 0.5 to 13.1 +/- 0.9 mumol.kg-1.min-1, P < 0.01) and Study II (11.17 +/- 0.67 to 13.33 +/- 0.83 mumol.kg-1.min-1, P < 0.01), and gluconeogenesis decreased (57.6 +/- 6.4 to 33.4 +/- 4.9 mumol/min, P < 0.01), indicating an increase in glycogenolysis. The increase in glycogenolysis was only partly compensated by an increase in glucose cycle activity (2.2 +/- 0.2 to 3.4 +/- 0.4 mumol.kg-1.min-1, P < 0.01) and the fall in gluconeogenesis, thus resulting in increased HGO. The suppression of gluconeogenesis despite increased lactate and alanine (glycerol was decreased) was associated with decreased free fatty acid (FFA) oxidation and negligible FFA enhanced gluconeogenesis. These studies suggest that increased liver glycogen stores alone can overwhelm normal intrahepatic mechanisms regulating carbohydrate metabolism resulting in increased HGO in nondiabetic man.

Effect of lipids on growth hormone secretion in humans.

To determine the effect of elevations of plasma lipids on growth hormone secretion in humans, paired insulin hypoglycemia tests and paired arginine infusion tests were performed on eight and six normal female volunteers respectively. On 1 of the 2 test days for each growth hormone stimulus, subjects were given 60 g corn oil (Lipomul) 3 hr before testing followed by intravenous heparin (5000 U) at the time of insulin or arginine administration. Lipomul plus heparin administration inhibited both insulin- and arginine-induced plasma HGH elevations with almost complete suppression of the response to arginine. The plasma HGH (human growth hormone) inhibition was associated with elevation in plasma triglycerides and inhibition of plasma FFA (free fatty acid) depression after insulin or arginine. Neither the hypoglycemic response to insulin nor the blood glucose and plasma immunoreactive-insulin responses to arginine were altered by Lipomul plus heparin administration. In four additional subjects in whom Lipomul was given without heparin, the elevated plasma triglyceride values were not associated with suppression of arginine-induced plasma HGH elevations. In the same four subjects, heparin administration without Lipomul neither suppressed arginine-induced plasma HGH elevations nor prevented the depression in plasma FFA after arginine as much as when Lipomul plus heparin had been given. These latter observations suggest that the elevation in plasma FFA was responsible for suppression of growth hormone secretion by Lipomul plus heparin. These studies indicate a possible role of plasma FFA in regulation of growth hormone secretion.

Proinsulin, insulin, and C-peptide concentrations in human portal and peripheral blood.

Concentrations of insulin, proinsulin, and C-peptide were measured in portal and peripheral venous blood in six nondiabetic, nonobese subjects. Portal vein samples were obtained by umbilical vein catheterization. Three subjects were studied with intravenous infusion of 25 g glucose, and three with 30 g arginine. Insulin and proinsulin were determined in the insulin immunoassay after separation by gel filtration, and C-peptide was measured by direct immunoassay. With both glucose and arginine stimulation, portal vein levels of all three peptides peaked at 90-120 s after the onset of the stimulus. Relative increases in insulin concentration were greater than those of proinsulin or C-peptide. In peripheral venous blood, maximal levels of the three peptides were observed later (2-5 min), and the increase in insulin relative toproinsulin and C-peptide was not as great. At the time of peak secretion, portal vein insulin and C-peptide approached equimolar concentrations, and proinsulin, as measured against an insulin standard, comprised approximately 2.5% of the total immunoreactive insulin. After stimulation by glucose or arginine, portal insulin, proinsulin and C-peptide levels were not correlated with the concentrations measured in simultaneously drawn peripheral samples. At all sampling times, however, significant correlation was found between insulin and C-peptide in both peripheral and portal blood. The results indicate that under the conditions studied, insulin and C-peptide are secreted in equimolar concentrations in man, and that proinsulin is secreted in the same proportion to insulin as found in the pancreas. Consideration of the relative secretory and metabolic rates of the three beta cell peptides explains their peripheral concentrations. The data further support the use of plasma C-peptide as an indicator of beta cell secretory function.

Characterization of circulating insulin and proinsulin-binding antibodies in autoimmune hypoglycemia.

Five patients with fasting and(or) postprandial hypoglycemia were found to have insulin antibodies in the absence of previously documented immunization. Studies on the equilibrium-binding of insulin to the autoantibodies revealed two classes of binding sites with association constants and binding capacities analogous to those of insulin antibodies from insulin-treated diabetic patients. Similarly, no consistent differences in these parameters were found in both groups of patients with insulins of bovine, porcine, and human origin. Proinsulin (C-segment directed) antibodies capable of binding bovine or porcine proinsulin were present in 10 of 10 and 9 of 10 insulin-treated diabetics serving as controls, respectively, and, when present, provide incontrovertible evidence of exogenous insulin administration. No such antibodies could be detected in the hypoglycemic patients with autoimmune insulin antibodies. The kinetics of dissociation of the insulin-antibody complexes were consistent with the existence of two classes of antibody sites. The corresponding dissociation rate constants were large enough to predict that significant amounts of free hormone may be generated by this mechanism and provide a plausible pathogenesis for the hypoglycemia in these patients.

Suppression of gluconeogenesis after a 3-day fast does not deplete liver glycogen in patients with NIDDM.

To determine the effect of inhibition of gluconeogenesis on liver glycogen stores in patients with non-insulin-dependent diabetes mellitus (NIDDM) after a 3-day fast, 10% ethanol (EtOH) was administered intravenously to nine obese patients with NIDDM and six obese nondiabetic subjects. Rates of glucose appearance (3-[3H]glucose) and [U-14C]alanine incorporation into glucose (alanine gluconeogenesis [Ala-GNG]) were determined before and during EtOH administration, and residual glycogen stores were assessed by the incremental glucose response to glucagon (glucoseAUC). Hepatic glucose output (HGO) was closely correlated with plasma glucose levels (r = 0.71, P < 0.001) after the 3-day fast and was significantly greater in the diabetic compared with the nondiabetic subjects (13.8 +/- 1.4 vs. 7.6 +/- 0.6 mumol.kg-1 FFM.min-1, P < 0.01). During the 120-min EtOH infusion, Ala-GNG fell by more than 50% in both groups and did not increase after intravenous glucagon administration. HGO fell modestly in both the diabetic and nondiabetic subjects during the first 30 min of EtOH infusion and stabilized thereafter. In contrast to Ala-GNG, HGO increased significantly after intravenous glucagon administration in both the diabetic and nondiabetic subjects, but the increase was significantly greater in the patients with NIDDM (P < 0.01). The glucose area under the curve in response to glucagon (glucoseAUC) was lower in the presence of EtOH than in its absence (14.9 +/- 7 vs. 68 +/- 15.6 mM/min, P < 0.01) in the obese nondiabetic subjects, which suggests a decrease in liver glycogen stores.(ABSTRACT TRUNCATED AT 250 WORDS)

Downregulation of insulin receptors in obese man.

In an attempt to determine whether the decreased number of insulin's receptors in obesity is a result of downregulation of the receptors, diazoxide (5 mg/kg/d) was given to 10 obese subjects. Insulin's suppression by diazoxide in these 10 subjects resulted in a mild glucose intolerance and an increase in insulin's receptors in seven of the 10 subjects. The subjects could be divided into three groups by analyzing the Scatchard plots of their insulin receptor studies before and after diazoxide. Four subjects exhibited an increase in both high affinity and low affinity receptors, three showed an increase only in high affinity receptors, and three failed to demonstrate any change in receptors in response to diazoxide. These studies support the concept that the decreased number of insulin's receptors observed in obesity is a result of the downregulation of the receptors and is not the primary, underlying cause of insulin resistance in obesity, although a contributory role cannot be ruled out.

Pump-induced insulin aggregation. A problem with the Biostator.

A fall in plasma IRI despite constant C-peptide levels during prolonged insulin euglycemic clamp studies using the Biostator (Ames Division, Miles Laboratories, Elkhart, Indiana) prompted a meticulous evaluation of the Biostator's insulin delivery system. At slow infusion rates, a striking loss of immunoreactive and biologically active insulin was observed after 6 h of the Biostator run. Studies with labeled insulin indicated that the loss of insulin was not due to adsorption of insulin to the tubing since recovery of labeled insulin was close to 100%. A variety of techniques (gel filtration, polyacrylamide gel electrophoresis, centrifugation, and Coomassie Brilliant Blue protein assay) indicated that the loss of insulin activity was due to insulin coming out of solution. The insoluble nature of the immunologically and biologically inactive insulin was confirmed by centrifugation, i.e., 88% 125I-insulin precipitated into the pellet. The dependency of this loss of insulin activity on flow rate was clearly demonstrable with activity (IRI) less than 20% of expected at flow rates of 2.1 ml/h, and 30% at 4.2 ml/h. Full recovery was observed only with flow rates of 16.8 ml/h or greater. At each flow rate, IRI rose only after delivery of the effluent between the pump and exit port, demonstrating that insulin alteration occurs within the pump assembly presumably from heat-induced aggregation. Investigators employing the Biostator should carefully examine their systems for this time- and flow rate-dependent alteration of insulin. The loss of IRI at low flow rates (low-dose insulin clamp or insulin delivery during basal periods) will profoundly influence data generated from the Biostator.

Morning insulin requirements. Critique of dawn and meal phenomena.

Morning insulin resistance has frequently been invoked to explain early-morning increases in both basal and breakfast-associated insulin requirements in diabetic patients. This increase in insulin requirements and plasma glucose from 0600 to 0900, when compared with midnight to 0600, has been termed the dawn phenomenon. We believe that the increased need for insulin in the morning has been misinterpreted. Data are reviewed that suggest the major perturbation overnight is a sleep-associated fall in hepatic glucose output, with a return to basal production rates on arousal in the morning. Moreover, the apparent increased insulin requirement for breakfast compared with lunch or supper (meal phenomenon) appears to be related more to lack of residual insulin effect from a preceding meal than to any putative morning insulin resistance. Thus, we found little evidence to support morning insulin resistance as a cause of either the dawn phenomenon (more appropriately designated the sleep phenomenon) or the meal phenomenon. A proper understanding of these phenomena is essential to the management of diabetic patients receiving insulin.

Sleep-associated fall in glucose disposal and hepatic glucose output in normal humans. Putative signaling mechanism linking peripheral and hepatic events.

Values reported for basal hepatic glucose production and glucose utilization do not reflect metabolic changes occurring during sleep. To determine the effect of sleep with its associated lowered metabolic rate and thermogenesis on glucose kinetics and gluconeogenic substrate availability, 11 normal volunteers underwent an overnight study in which [3-3H]glucose was infused. Despite decreased insulin secretion, a fall in hepatic glucose output was observed with sleep that was synchronous with a reduction in glucose utilization and lipolysis (decreased plasma glycerol and free fatty acids). When activity was increased, these parameters rose toward previously reported basal levels. Prevention of sleep in 6 additional subjects attenuated the fall in glucose utilization and production as well as the fall in glycerol and free fatty acids despite similar insulin and counterregulatory hormone profiles. We suggest that sleep-associated metabolic changes produce a peripheral signal(s) that modulates hepatic glucose production in humans.

Failure of a midnocturnal insulin infusion to suppress the increased insulin need for breakfast in insulin-dependent diabetic patients.

Insulin requirements for meals were measured in eight insulin-dependent diabetic patients, using a closed-loop insulin infusion system. Patients required more insulin for breakfast than for an isocaloric lunch (35.7 +/- 5.5 mU/kcal/3 h versus 26.9 +/- 5.1 mU/kcal/3 h, P less than 0.02) or an isocaloric supper (35.7 +/- 5.5 mU/kcal/3 h versus 26.6 +/- 6.6 mU/kcal/3 h, P = 0.05). To determine whether this insulin resistance at breakfast might be due to low basal insulin levels overnight, the insulin needs for breakfast were compared after an overnight fast (day 1) and after a midnocturnal (0200 h-0500 h) insulin infusion (day 2). Breakfast insulin requirements were similar on both days (35.7 +/- 5.5 mU/kcal/3 h versus 37.7 +/- 5.1 mU/kcal/3 h, P = NS). Whereas nonobese diabetic patients required approximately 60% more insulin for breakfast than for other meals, obese diabetic patients in this study did not demonstrate insulin resistance at breakfast. These findings provide a basis for the common clinical practice of allocating more insulin for breakfast than for other meals. The absence of an increased insulin need at breakfast in our obese patients cautions against a similar algorithm for obese diabetic patients. We postulate that growth hormone may be a cause for morning insulin resistance.

Evidence for dual control mechanism regulating hepatic glucose output in nondiabetic men.

We previously reported a fall in hepatic glucose output (HGO) during sleep accompanied by reductions in glucose utilization (Rd) and free fatty acids (FFAs). This study was undertaken to determine the potential role of changes in Rd and FFA on HGO in nondiabetic men. To determine if the fall in HGO during sleep could be reversed by FFA elevation, seven nondiabetic men underwent [3-3H]glucose infusions from 2200 to 0800, with heparin (90 mU.kg-1.min-1) added at 0200. Glucose appearance (Ra) fell from 11.7 +/- 1.1 at 2430 to 8.9 +/- 0.8 mumol.kg-1.min-1 (P less than 0.05) at 0200. The fall in Ra was associated with decreases in FFA (0.57 +/- 0.10 to 0.48 +/- 0.07 mM) and glycerol (0.08 +/- 0.01 to 0.06 +/- 0.01 mM). Infusion of heparin significantly increased FFA and glycerol (1.09 +/- 0.21 and 0.11 +/- 0.01 mM, respectively, P less than 0.01) and resulted in a significant fall in plasma alanine, suggesting that gluconeogenesis had been increased. However, rates of glucose turnover were indistinguishable from overnight studies without heparin. In additional studies (n = 6), intralipid and heparin-induced FFA elevation (from 0.61 +/- 0.07 to 0.95 +/- 0.05 mM, P less than 0.01) stimulated gluconeogenesis ([U-14C]alanine to glucose) twofold (188 +/- 22% increase compared to 114 +/- 6% in saline control studies, P less than 0.01). However, despite increasing gluconeogenesis, overall HGO did not change (10.6 +/- 0.5 vs. 10.7 +/- 0.6 mumol.kg-1.min-1) during lipid infusion.(ABSTRACT TRUNCATED AT 250 WORDS)

Increased transcapillary escape rate of albumin in nondiabetic men in response to hyperinsulinemia.

Diabetic patients manifest increased vascular permeability. To determine whether insulin per se might increase vascular permeability, five nondiabetic men were studied by the hyperinsulinemic-euglycemic clamp technique. Each subject received a 0.72-nmol/kg body wt i.v. insulin bolus, followed by a 72-pmol.kg-1.min-1 insulin infusion for 4 h. Euglycemia was maintained by the Biostator glucose controller. At 7 h of study, 10 microCi i.v. 125I-labeled albumin was injected as bolus dose. Frequent blood samples were drawn during the next 70 min for determination of the transcapillary escape rate (TER) of albumin. Subjects returned 1-2 wk later for a control study, during which 0.45% saline was infused at a rate identical to the dextrose and insulin infusion rates during the hyperinsulinemic clamp. The mean +/- SE serum insulin levels during the hyperinsulinemic clamp and saline infusion were 9786 +/- 126 and 46 +/- 4 pM, respectively, whereas serum glucose during the two sessions was similar (5.0 +/- 0.2 vs. 4.8 +/- 0.1 mM, NS). Identical fluid volumes were infused during the two sessions (1767 +/- 197 ml/7 h), and urine outputs did not differ significantly (1615 +/- 309 vs. 1035 +/- 248 ml/7 h). The TER of albumin was greater in all five men after hyperinsulinemia than after saline infusion (18.3 +/- 2.7 vs. -2.8 +/- 2.3%/h, P = 0.01).(ABSTRACT TRUNCATED AT 250 WORDS)

Sleep apnea in active acromegaly.

Previous case reports have shown an association between acromegaly and the sleep apnea syndrome (SAS). Some of the patients described had central SAS, raising the possibility that an elevation of the growth hormone (GH) level may cause a defect in respiratory drive. We determined the prevalence of SAS in 21 patients with a history of acromegaly. We separated them into two groups based on serum GH concentrations. Ten patients had active acromegaly (mean GH concentration, 62.2 ng/mL; range, 12.6 to 148 ng/mL), while 11 patients had inactive acromegaly (mean GH, 3.2 ng/mL; range, 0.7 to 6.4 ng/mL). Four of the ten patients with active acromegaly had SAS; none of the 11 patients with inactive acromegaly had SAS. Three patients with SAS had the purely obstructive type, and one had the mixed central and obstructive type. The hypercapnic ventilatory response was normal in all patients tested and was not influenced by the GH level. We conclude that SAS is associated with active acromegaly and that the GH level does not affect the hypercapnic ventilatory response. The absence of SAS in successfully treated patients suggests that it may resolve after a normal GH level is restored.

The treatment of acromegaly by transsphenoidal surgery.

Pituitary tumors were removed transsphenoidally in 32 patients with acromegaly. Ten patients had high prolactin (PL) levels as well as elevated growth hormone (GH) levels. In 24 patients, GH levels were lowered to 5 ng/mL or less and remained low. The PL level was reduced to normal in eight patients. Of three patients with postoperative GH levels of 6 to 10 ng/mL, one patient remains in this range but two have relapsed. Five patients showed only a partial lowering in GH level. Pituitary irradiation lowered the level further. Hypopituitarism developed in four patients. Permanent diabetes insipidus occurred in one patient and meningitis developed in another. They subsequently recovered. There were no deaths. Abnormal GH responses to hyperglycemia, hypoglycemia, and levodopa returned to normal following surgery. Transsphenoidal tumor removal appears to be an effective treatment for acromegaly.

Absorption characteristic of breakfast determines insulin sensitivity and carbohydrate tolerance for lunch.

To test the hypothesis that prolonging absorption of breakfast might improve the glucose tolerance of the subsequent meal served at lunch, normal male volunteers were administered the same carbohydrate in either a rapidly absorbed (sucrose, S) or slowly absorbed (sucrose with guar, S + G) form for breakfast (0800) and lunch (1145). Area under the curve (AUC) for glucose did not differ for S at breakfast vs. S + G at breakfast, although AUCinsulin for S at breakfast was greater than that for S + G at breakfast (3389 +/- 608 vs. 1523 +/- 246 microU.min.ml-1, P less than .002). Plasma glucose and insulin profiles for the two breakfast meals differed markedly. Once S was ingested, plasma glucose and insulin returned to baseline after 120 and 160 min, respectively. However, once S + G was ingested, plasma glucose and insulin were still significantly above baseline values after 180 min. When S was eaten for breakfast, AUCglucose for lunch was similar to that for breakfast, regardless of whether lunch consisted of S or S + G. However, if S + G was eaten for breakfast, AUCglucose for S + G or S at lunch was 44% (P less than .005) and 75% of that for breakfast, respectively. Only one of five subjects who ingested S + G for breakfast failed to exhibit a fall in AUCglucose when S was eaten for lunch. The beneficial effect of prolonged absorption of breakfast on the glucose tolerance of lunch was not observed if the timing of lunch was delayed by 2 h (i.e., served at 1345).(ABSTRACT TRUNCATED AT 250 WORDS)

Central nervous system insulin receptors in normal and diabetic rats.

The presence of specific insulin receptors in homogenates of various parts of the rat brain was demonstrated. Hypothalamic preparations exhibited the greatest insulin binding. Scatchard plots of the binding data were typically curvilinear. Insulin binding to the brain homogenates was influenced by pH, incubation time, temperature, and ionic milieu. The specificity of insulin binding to the brain homogenates was demonstrated by partial displacement of labeled insulin by insulin analogs according to their biological activities and by inhibition of insulin binding by serum containing an antibody to insulin receptors. Insulin binding to brain homogenates from streptozotocin-induced diabetic rats was similar to that from control animals and failed to indicate up-regulation of these receptors.

Effect of lithium on pancreatic islet insulin release.

Lithium exerts an inhibitory effect on glucose and amino acid-induced insulin release. The inhibitory effect of Li+ persists even in subsequent Li+-free conditions, indicating an only slowly reversible effect. Lithium fails to inhibit glucagon-induced insulin release. The exact mechanism of the lithium effect is as yet undetermined, but interference with calcium flux and/or microtubular function is an attractive hypothesis. The inhibitory effect of lithium on insulin release cannot be reversed by alteration in ionic (Ca++, Mg++, K+) concentrations in the incubation media. Studies involving the effect of low sodium on insulin release in which lithium had been used as an osmotic replacement for sodium must be carefully reassessed because of these findings.

Effect of down-regulation of insulin receptors on receptor-antibody binding.

The extraction of insulin receptor antibody (ARAB) by IM-9 lymphoblastoid cells was measured using nonisotopic ARAB, ARAB extraction was expressed as a clearance, with ARAB determinations made indirectly by a method dependent on its inhibition of insulin binding. Control cells and down-regulated IM-9 cells (cells exposed to 10(-7) M insulin for 16 h) at a cell density of 8 X 10(7) cells/ml were incubated in the presence of a 1:75 dilution of serum from a patient with insulin resistance due to insulin receptor antibodies. After 60 min at room temperature, control cells cleared 42 +/- 3% of ARAB compared to only 18 +/- 2% by the down-regulated cells. ARAB clearance was directly proportional to maximal tracer insulin binding, as determined in parallel experiments. ARAB clearance was similarly reduced in a dose-related fashion by 16-h exposure to Concanavalin A. These results indicate that ARAB extraction by IM-9 cells is reduced by chronic exposure to insulin. The observation that ARAB binding to insulin down-regulated cells is reduced despite the ability of ARAB to attach normally to cells whose receptors have been saturated with insulin supports the notion that down-regulation is due to a loss of membrane receptor rather than to simple receptor occupancy. (Endocrinology 108: 478, 1981)

Down regulation of insulin receptors in primary cultures of adult rat hepatocytes in monolayer.

Down regulation of insulin receptor number by insulin at physiological concentrating (10(-9) M) has been demonstrated in primary cultures of adult rat liver parenchymal cells in monolayer. However, because insulin is rapidly degraded in this culture system, special efforts are necessary to maintain the concentration of insulin in the medium in order to evoke the down regulation phenomenon.