Exercise-stimulated glucose turnover in the rat is impaired by glucosamine infusion.

The infusion of glucosamine causes insulin resistance, presumably by entering the hexosamine biosynthetic pathway; it has been proposed that this pathway plays a role in hyperglycemia-induced insulin resistance. This study was undertaken to determine if glucosamine infusion could influence exercise-stimulated glucose uptake. Male SD rats were infused with glucosamine at 0.1 mg x kg(-1) x min(-1) (low-GlcN group), 6.5 mg x kg(-1) x min(-1) (high-GlcN group), or saline (control group) for 6.5 h and exercised on a treadmill for 30 min (17 m/min) at the end of the infusion period. Glucosamine infusion caused a modest increase in basal glycemia in both experimental groups, with no change in tracer-determined basal glucose turnover. During exercise, glucose turnover increased approximately 2.2-fold from 46 +/- 2 to 101 +/- 5 pmol x kg(-1) x min(-1) in the control group. Glucose turnover increased to a lesser extent in the glucosamine groups and was limited to 88% of control in the low-GlcN group (47 +/- 2 to 90 +/- 3 pmol x kg(-1) x min(-1); P < 0.01) and 72% of control in the high-GlcN group (43 +/- 1 to 73 +/- 3 pmol kg(-1) 1 min(-1); P < 0.01). Similarly, the metabolic clearance rate (MCR) in the control group increased 72% from 6.1 +/- 0.2 to 10.5 +/- 0.7 ml kg(-1) x min(-1) in response to exercise. However, the increase in MCR was only 83% of control in the low-GlcN group (5.2 +/- 0.5 to 8.7 +/- 0.5 ml x kg(-1) x min(-1); P < 0.01) and 59% of control in the high-GlcN group (4.5 +/- 0.2 to 6.2 +/- 0.3 ml x kg(-1) x min(-1); P < 0.01). Neither glucosamine infusion nor exercise significantly affected plasma insulin or free fatty acid (FFA) concentrations. In conclusion, the infusion of glucosamine, which is known to cause insulin resistance, also impaired exercise-induced glucose uptake. This inhibition was independent of hyperglycemia and FFA levels.

Improved insulin-sensitivity in mice heterozygous for PPAR-gamma deficiency.

The thiazolidinedione class of insulin-sensitizing, antidiabetic drugs interacts with peroxisome proliferator-activated receptor gamma (PPAR-gamma). To gain insight into the role of this nuclear receptor in insulin resistance and diabetes, we conducted metabolic studies in the PPAR-gamma gene knockout mouse model. Because homozygous PPAR-gamma-null mice die in development, we studied glucose metabolism in mice heterozygous for the mutation (PPAR-gamma(+/-) mice). We identified no statistically significant differences in body weight, basal glucose, insulin, or FFA levels between the wild-type (WT) and PPAR-gamma(+/-) groups. Nor was there a difference in glucose excursion between the groups of mice during oral glucose tolerance test, but insulin concentrations of the WT group were greater than those of the PPAR-gamma(+/-) group, and insulin-induced increase in glucose disposal rate was significantly increased in PPAR-gamma(+/-) mice. Likewise, the insulin-induced suppression of hepatic glucose production was significantly greater in the PPAR-gamma(+/-) mice than in the WT mice. Taken together, these results indicate that - counterintuitively - although pharmacological activation of PPAR-gamma improves insulin sensitivity, a similar effect is obtained by genetically reducing the expression levels of the receptor.

Troglitazone prevents hyperglycemia-induced but not glucosamine-induced insulin resistance.

Hyperglycemia can lead directly to a secondary state of insulin resistance or can worsen a preexisting insulin-resistant state. Troglitazone is an orally active hypoglycemic agent that has been shown to ameliorate insulin resistance and hyperinsulinemia in both diabetic animal models and NIDDM subjects. To determine whether this drug could prevent the development of hyperglycemia-induced insulin resistance and to investigate the mechanism by which this might occur, we studied troglitazone's effect on insulin action in rats made hyperglycemic or infused with glucosamine. Normal male SD rats were fed regular powdered diet with or without troglitazone as a food admixture (0.2%). After 2 weeks, rats were made hyperglycemic with glucose (52 mg x kg(-1) x min[-1]) and somatostatin (0.8 microg x kg(-1) x min[-1]) infusion or were infused with glucosamine (6.5 mg x kg(-1) x min[-1]) for 6.5 h. In vivo insulin action was measured by the hyperinsulinemic-euglycemic clamp technique at a submaximal (24 pmol x kg(-1) x min[-1]) or maximal (240 pmol x kg(-1) x min[-1]) insulin infusion rate. The infusion of glucose and somatostatin caused a pronounced rise in the plasma glucose concentration (19.8 +/- 0.6 mmol/l) compared with saline-infused animals (8.0 +/- 0.2 mmol/l; P < 0.001). Hyperglycemia resulted in insulin resistance, as evidenced by a marked reduction in the submaximal glucose disposal rate (GDR) (78 +/- 7 vs. 135 +/- 6 micromol x kg(-1) x min(-1); P < 0.01) and maximal GDR (141 +/- 9 vs. 237 +/- 6 micromol x kg(-1) x min(-1); P < 0.01) compared with the control group. Troglitazone treatment largely prevented the hyperglycemia-induced decline in submaximal (116 +/- 7 micromol x kg(-1) x min[-1]) and maximal GDR (209 +/- 9 micromol x kg(-1) x min(-1); P < 0.05). Glucosamine infusion also resulted in a marked reduction in the submaximal GDR (85 +/- 3 vs. 135 +/- 6 micromol x kg(-1) x min(-1); P < 0.01) and maximal GDR (137 +/- 14 vs. 237 +/- 6 micromol x kg(-1) x min(-1); P < 0.01) compared with the control group. In contrast to the results in the hyperglycemic animals, troglitazone treatment had no effect on glucosamine-induced insulin resistance. In summary, 1) in normal rats, experimental hyperglycemia, as well as glucosamine infusion, led to a marked state of peripheral and hepatic insulin resistance; 2) troglitazone treatment prevented the hyperglycemia-induced, but not the glucosamine-induced, insulin resistance; and 3) either troglitazone acts at one or more sites proximal to the entry of glucosamine into the hexosamine pathway, or the increased flux of glucose-derived products through the hexosamine pathway is not a major mechanism for the hyperglycemia-induced defect in insulin action in these animals.

Mechanisms of insulin resistance in experimental hyperinsulinemic dogs.

This study was undertaken to characterize the insulin resistance and the mechanism thereof caused by chronic hyperinsulinemia produced in dogs by surgically diverting the veins of the pancreas from the portal vein to the vena cava. Pancreatic venous diversion (PVD, n = 8) caused a sustained increase in arterial insulin and decrease in portal insulin concentration compared with the control group (n = 6). Hyperinsulinemic euglycemic clamps were conducted 4 wk after surgery. The increase in the glucose disposal rate (GDR) was significantly less in the PVD group (39.0+/-5.0 vs. 27.9+/-3.2 micromol/kg/min, P < 0.01) compared with the control group, but the suppression of hepatic glucose production by insulin was similar for both groups. Muscle insulin receptor tyrosine kinase activity (IR-TKA) increased from 6.2+/-0.4 to 20.3+/-2.7 in the control group, but from 5.8+/-0.5 to only 12.7+/-1.7 fmol P/fmol IR in the PVD group (P < 0.01). With respect to the periphery, the time to half-maximum response (t1/2a) for arterial insulin was the same for both groups, whereas the t1/2a for lymph insulin (30+/-3 vs. 40+/-4 min, P < 0.05) and GDR (29+/-3 vs. 66+/-10 min, P < 0.01) were greater for the PVD group. Chronic hyperinsulinemia led to marked peripheral insulin resistance characterized by decreased insulin-stimulated GDR, and impaired activation of GDR kinetics due, in part, to reduced IR-TKA. Transendothelial insulin transport was impeded and was responsible for one third of the kinetic defect in insulin-resistant animals, while slower intracellular mechanisms of GDR were responsible for the remaining two thirds.

TNF-alpha-induced insulin resistance in vivo and its prevention by troglitazone.

Tumor necrosis factor (TNF)-alpha may play a role in the insulin resistance of obesity and NIDDM. Troglitazone is a new orally active hypoglycemic agent that has been shown to ameliorate insulin resistance and hyperinsulinemia in both diabetic animal models and NIDDM subjects. To determine whether this drug could prevent the development of TNF-alpha-induced insulin resistance, glucose turnover was assessed in rats infused with cytokine and pretreated with troglitazone. Normal male Sprague-Dawley rats were fed normal powdered food with or without troglitazone as a food admixture (0.2%). After approximately 10 days, rats were infused with TNF-alpha for 4-5 days, producing a plasma concentration of 632 +/- 30 pg/ml. In vivo insulin action was measured by the euglycemic-hyperinsulinemic clamp technique at a submaximal (24 micromol x kg[-1] x min[-1]) and maximal insulin infusion rate (240 micromol x kg[-1] x min[-1]). TNF-alpha infusion resulted in a pronounced reduction in submaximal insulin-stimulated glucose disposal rate (GDR) (97 +/- 10 vs. 141 +/- 4 micromol x kg[-1] x min[-1], P < 0.05), maximal GDR (175 +/- 8 vs. 267 +/- 6 micromol x kg[-1] x min[-1], P < 0.01), and in insulin receptor-tyrosine kinase activity (IR-TKA) (248 +/- 39 vs. 406 +/- 32 fmol ATP/fmol IR, P < 0.05). It also led to a marked increase in basal insulin (90 +/- 24 vs. 48 +/- 6 micromol/l, P < 0.05) and free fatty acid (FFA) concentration (2.56 +/- 0.76 vs. 0.87 +/- 0.13 mmol/l, P < 0.01). Troglitazone treatment completely prevented the TNF-alpha-induced decline in submaximal GDR (133 +/- 16 vs. 141 +/- 4 micromol x kg[-1] x min[-1], NS) and maximal GDR (271 +/- 19 vs. 267 +/- 6 micromol x kg[-1] x min[-1], NS). The hyperlipidemia was partially corrected by troglitazone (1.53 +/- 0.28 vs. 0.87 +/- 0.13 mmol/l, P < 0.05), while IR-TKA and insulin concentration remained unaffected by the drug. Troglitazone restores insulin action possibly by lowering the FFA concentration of the blood and/or by stimulating glucose uptake at an intracellular point distal to insulin receptor autophosphorylation in muscle. If TNF-alpha plays a role in the development of the obesity/NIDDM syndrome, troglitazone may prove useful in its treatment.

Metabolic effects of troglitazone on fat-induced insulin resistance in the rat.

Troglitazone is a new orally active hypoglycemic agent that has been shown to ameliorate insulin resistance and hyperinsulinemia in both diabetic animal models and non-insulin-dependent diabetes mellitus (NIDDM) subjects. To determine whether this drug could prevent the development of diet-induced insulin resistance and related abnormalities, we studied its effect on insulin resistance induced by high-fat feeding in rats. Normal male Sprague-Dawley rats were fed a high-fat diet for 3 weeks with and without troglitazone as a food mixture (0.2%) or were fed normal chow. In vivo insulin action was measured using a euglycemic-hyperinsulinemic clamp at two different insulin infusion rates, 4 (submaximal stimulation) and 40 (maximal stimulation) mU/kg/min. Fat feeding markedly reduced the submaximal glucose disposal rate ([GDR], 26.4 +/- 1.3 v 37.5 +/- 1.4 mg/kg/min, P < .01) and maximal GDR (55.9 +/- 1.3 v 64.5 +/- 1.3 mg/kg/min, P < 0.5), reduced the suppressibility of submaximal hepatic glucose production ([HGP], 3.2 +/- 0.9 v 1.5 +/- 0.5 mg/kg/min, P < .05), and resulted in hyperlipidemia. Troglitazone treatment did not affect any of these parameters. Insulin resistance induced by fat feeding is the first experimental model in which troglitazone failed to correct or partially correct the insulin resistance.

Kinetics of insulin action in vivo. Identification of rate-limiting steps.

To examine the kinetic steps in insulin's in vivo action, we have assessed the temporal relationship between arterial insulin, interstitial insulin, glucose disposal rate (GDR), and insulin receptor kinase (IRK) activity in muscle and between portal insulin, hepatic glucose production (HGP), and IRK activity in liver. Interstitial insulin, as measured by lymph-insulin concentration (muscle only), and IRK activity were used as independent methods to determine the arrival of insulin at its tissue site of action. Euglycemic clamps were conducted in seven mongrel dogs and consisted of an activation phase with a venous insulin infusion (7.2 nmol.kg-1.min-1, 100 min) and a deactivation phase. Liver and muscle biopsies were taken to assess IRK activity. Arterial, portal, and lymph insulin rose to 636 +/- 12, 558 +/- 18, and 402 +/- 24 pmol/l, respectively. GDR increased from 13.9 +/- 0.6 to 41.7 +/- 2.8, and HGP declined from 14.4 +/- 0.6 to 1.1 +/- 0.6 mumol.kg-1.min-1. Muscle and liver IRK activity increased significantly from 5.9 +/- 0.9 to 14.6 +/- 0.6 and 5.5 +/- 0.7 to 23.7 +/- 1.9 fmol P/fmol insulin receptor (IR), respectively. The time to half-maximum response (t1/2a) for stimulation of GDR (19.8 +/- 4.8 min) and suppression of HGP (21.5 +/- 3.7 min) were similar. The t1/2a for stimulation of GDR, muscle IRK, and rise in lymph insulin were not significantly different from one another and were all markedly greater than that for the approach to steady state of arterial insulin (2.3 +/- 1.2 min, P < 0.01).(ABSTRACT TRUNCATED AT 250 WORDS)

Metabolic effects of troglitazone on fructose-induced insulin resistance in the rat.

Troglitazone is a new orally active hypoglycemic agent that has been shown to reduce insulin resistance and hyperinsulinemia in both diabetic animal models and non-insulin-dependent diabetes mellitus (NIDDM) subjects. To determine whether this drug could prevent the development of fructose-induced insulin resistance and related abnormalities, we studied the effects of troglitazone on the insulin resistance induced by fructose feeding in rats. Normal male Sprague-Dawley rats were fed a high-fructose diet for 3 weeks with and without troglitazone as a food admixture (0.2%) or were fed normal chow to serve as a control group. In vivo insulin resistnace was measured by the euglycemic hyperinsulinemic clamp technique at two different insulin infusion rates, 29 (submaximal stimulation) and 290 (maximal stimulation) pmol.kg-1.min-1. Fructose feeding markedly reduced submaximal glucose disposal rate (GDR) (113.8 +/- 8.3 vs. 176.0 +/- 5.6 mumol.kg-1.min-1, P < 0.05) and maximal GDR (255.9 +/- 5.6 vs. 313.6 +/- 10.5 mumol.kg-1.min-1, P < 0.05), reduced the suppressibility of submaximal hepatic glucose production (HGP; 45.5 +/- 5.0 vs. 11.7 +/- 5.0 mumol.kg-1.min-1, P < 0.05), and resulted in hypertriglyceridemia and hypertension. Troglitazone treatment completely restored the GDR (submaximal 158.2 +/- 5.6, maximal 305.3 +/- 6.1 mumol.kg-1.min-1) and submaximal HGP (9.4 +/- 2.8 mumol.kg-1.min-1) to control levels and also normalized the elevated plasma triglyceride concentration and systolic blood pressure levels in fructose-fed rats.(ABSTRACT TRUNCATED AT 250 WORDS)

Islet amyloid polypeptide (amylin) increases the renal excretion of calcium in the conscious dog.

Islet amyloid polypeptide (IAPP) is a member of the calcitonin/CGRP family and has been isolated from the beta-cell of pancreatic islets. Recent evidence suggests that this peptide may be involved in calcium metabolism in that its administration resulted in lowering of serum calcium levels. To determine the mechanism of IAPP-induced hypocalcemia, the peptide was infused at 50 pmol/min/kg for 90 minutes in conscious male mongrel dogs. Infusion of the peptide resulted in a modest decline in the total serum calcium concentration (10.4 +/- 0.2 to 9.4 +/- 0.2 mg/dl; P < 0.05) and a concomitant increase in urinary calcium excretion (3.6 +/- 0.6 to 6.9 +/- 2.0 mg/dl; P < 0.01). Based on an extracellular volume of 7 liter in a 28 kg dog, the total decrement in calcium due to IAPP was 41.3 +/- 2.4 mg, whereas the total increase in urinary calcium was 3.2 +/- 0.7 mg. There were no detectable changes in calcitonin. We conclude that IAPP lowers serum calcium and increases the renal excretion of calcium independently of calcitonin. However, the calciuria can only account for a small component of the hypocalcemic effect and therefore, an additional calcium lowering effect of IAPP exits.

Intracerebroventricular administration of somatostatin octapeptide counteracts the hormonal and metabolic responses to stress in normal and diabetic dogs.

Intracerebroventricular (ICV) injection of carbachol elicits hormonal and metabolic responses similar to moderate stress. In normal dogs, ICV carbachol stimulated marked counterregulatory hormone release, but altered plasma glucose only marginally because the marked increment in glucose production (Ra) was almost matched by the increment of utilization (Rd), even though plasma insulin was unchanged. In alloxan-diabetic dogs, Rd did not match Ra and plasma glucose increased substantially. Since somatostatin octapeptide (ODT8-SS) inhibits some sympathetic mechanisms of the stress response, we explored the extent to which ODT8-SS can alleviate the counterregulatory responses to stress induced by carbachol, and particularly whether it can restore glycemic control in diabetes. ODT8-SS (20 nmol) was ICV-injected (1) in normal dogs (n = 5), and (2) prior to ICV carbachol before (n = 7) and after (n = 6) the induction of alloxan-diabetes. ODT8-SS did not affect basal values, but when administered before ICV carbachol there were no significant increments in plasma epinephrine, cortisol, arginine vasopressin (AVP), insulin, glucose, or lactate. There were significant increases in norepinephrine, glucagon, Ra, Rd, and the glucose metabolic clearance rate (MCR), although they were much smaller than seen previously with ICV carbachol alone. After induction of alloxan-diabetes, Rd and MCR did not change with ICV ODT8-SS and carbachol as in normal dogs, but norepinephrine, epinephrine, glucagon, lactate, plasma glucose, and Ra increased, although with the exception of glucagon these increases were much smaller than seen previously with ICV carbachol alone. ODT8-SS administered before ICV carbachol in normal or diabetic animals resulted in increased free fatty acid (FFA) levels. The increases in glycerol were less than and those in FFA greater than seen previously with ICV carbachol alone. Since ODT8-SS does not alter basal counterregulatory hormone release but suppresses the release during stress, this is a useful probe to analyze some of the metabolic responses to stress. When the response to carbachol from our previous report is compared with the responses to carbachol + ODT8-SS, it is indicated that the stress-related increase in Ra was consistent with stimulation of the sympathetic nervous system, whereas increased Rd is related to an unknown stress-related neuroendocrine mechanism that requires a permissive effect of insulin, since it was not seen in the frankly diabetic animals. We hypothesize that the stress-induced increase in Rd occurs not only in muscle but also in adipocytes, and that the somatostatin-induced attenuation of Rd decreased FFA re-esterification and consequently markedly increased stress-induced FFA release.(ABSTRACT TRUNCATED AT 400 WORDS)

The roles of insulin and catecholamines in the glucoregulatory response during intense exercise and early recovery in insulin-dependent diabetic and control subjects.

Intense exercise is associated with a marked stimulation of glucose production (Ra), a somewhat smaller increment in its utilization (Rd) (and therefore hyperglycemia), large increases in plasma catecholamines, and moderate hyperglucagonemia. The hyperglycemia increases in recovery and is accompanied by hyperinsulinemia. Because these adaptations are unique to intense exercise, we tested the physiological significance of the hyperinsulinemia by exercising six fit, postabsorptive young male subjects with insulin-dependent diabetes mellitus (IDDM) after overnight glycemic normalization by iv insulin, keeping its infusion rate constant during and for 2 h after 100% maximum VO2 cycle ergometer exercise to exhaustion (12 min) (no postexercise hyperinsulinemia). Their responses were compared with those of matched control subjects studied on two separate occasions, once without intervention (physiological hyperinsulinemia, n = 6) and again with a 0.05 U/kg iv bolus at exhaustion (postexercise supraphysiological hyperinsulinemia, n = 5). In all three study protocols, Ra increased by 7-fold, and Rd by 4-fold at exhaustion, and Ra declined in early recovery at the same rates. Therefore, the early recovery hyperinsulinemia is not required to return Ra to preexercise levels, and excessive hyperinsulinemia does not accelerate this decline. We infer that the catecholamine increments and decrements are the prime regulators of Ra (correlations of Ra vs. norepinephrine or epinephrine, P < 0.001 in the three studies), with a smaller contribution from the concurrent hyperglucagonemia. Rd, in contrast, was significantly affected by insulin. In the IDDM subjects, Rd remained at the same rate as Ra through most of recovery, resulting in sustained hyperglycemia and decreased glucose MCR, vs. the control subjects. This hyperglycemia compensated for the abnormal MCR, such that Rd was comparable to that in the control subjects. With the insulin bolus, the Rd elevation was sustained longer compared to the study without bolus, resulting in mild hypoglycemia successfully counterregulated by an increase in Ra. Thus, the principal regulators of the marked exercise increase and rapid recovery decrease in Ra are probably the catecholamines. The postexercise hyperinsulinemia is required for the MCR response and to return plasma glucose concentrations to preexercise levels. Different therapeutic strategies are required in persons with IDDM undergoing strenuous vs. moderate exercise, because of their inability to generate the postexercise hyperinsulinemia.

Glucose turnover and its regulation during intense exercise and recovery in normal male subjects.

Intense exercise to exhaustion is expected to be associated with rapid and large changes in glucose production (Ra) and utilization (Rd). To quantify these, and to determine their mechanisms and those of the prolonged postexercise hyperglycemia, we measured circulating metabolic regulators and glucose kinetics, the latter by the method of enriched tracer [3-3H] glucose infusion during exercise. Eighteen fit, lean young male subjects exercised to exhaustion at 80% of maximal workload (approximately 100% VO2max) on a cycle ergometer. Plasma glucose was 4.90 +/- 0.08 mM/L at rest, increased during exercise, then abruptly to 6.91 +/- 0.40 mM/L at 4 min recovery then gradually declined. Plasma insulin was constant during exercise, then doubled to 162 +/- 28 pmol/l until 20 min recovery, before declining. Plasma glucagon increased by 71 +/- 11 pg/mL. Plasma norepinephrine increased 18-fold and epinephrine 14-fold, both declining by 20 min recovery. Ra increased 7-fold by exhaustion to 13.0 +/- 1.18 mg/kg/min, then decreased to 2.43 +/- 0.24 mg/kg/min by 9 min, then to about 2 mg/kg/min the rest of recovery. Rd rose 3-fold (6.61 +/- 0.70 mg/kg/min), and remained lower than Ra to 7 min recovery, but thereafter declined more slowly. Thus, the rapid and extremely large increase in Ra was not matched by the increment in Rd during exercise and early recovery. We suggest that unlike in exercise of lesser intensity, the major mediators of both the increase in Ra and the restraint of the increase in Rd are the catecholamines. The post exercise hyperglycemia and hyperinsulinemia are appropriate to muscle glycogen repletion.

Regulation of glucose turnover at the onset of exercise in the dog.

The early responses of endogenous glucose production (Ra), glucose utilization (Rd), and glucoregulatory hormones to moderate treadmill exercise (12% incline, 100 m/min, 60 min) were examined in dogs. Rd increased rapidly and progressively from the start of exercise. The change in Ra, as estimated with a variable-volume model of glucose kinetics, was biphasic, with an abrupt increase by 8.5 +/- 2.3 mumol.min-1.kg-1, followed by a delayed further increase that matched Rd 11-22 min after the onset of exercise. The plasma glucagon-to-insulin molar ratio fell slightly at the onset of exercise and then increased gradually. The glucagon-to-insulin ratio was correlated with Ra over the entire exercise period (r = 0.63, P less than 0.0001), but not during the early part of exercise, when Ra increased rapidly. The catecholamine- (epinephrine plus norepinephrine) to-insulin molar ratio was correlated with Ra during the early period (r = 0.52, P less than 0.01) and over the entire period of exercise (r = 0.66, P less than 0.0001). Our results confirm previous demonstrations that the glucagon-to-insulin molar ratio is an important regulator of Ra during exercise. We hypothesize that the catecholamine-to-insulin molar ratio is important during the early period of exercise and possibly during late exercise as an additional regulatory factor to the glucagon-to-insulin molar ratio.

Estimation of glucose production during exercise with a one-compartment variable-volume model.

A variable-volume one-compartment model of glucose kinetics and step increases in the rate of tracer infusion were examined for estimation of endogenous glucose production (Ra) during moderate exercise in dogs. A primed infusion of D-[3-3H]glucose was left constant or increased 1.5-, 2-, 3-, 4-, or 5-fold at the onset of a 60-min period of exercise. Application of a regression method, in which Ra and the effective distribution volume were estimated over time, revealed dynamic changes in Ra that were not evident during the constant tracer infusion with a fixed-volume model. Application of the fixed-volume model to studies performed with a two- or three-fold step increase in tracer resulted in the lowest sum-of-squares difference from the regression method. Our results demonstrate that application of a variable-volume model can be achieved during exercise by enrichment of the plasma specific activity through step increases in the rate of tracer infusion and application of a regression method. Alternately, estimates of Ra with a fixed-volume model can be improved by enrichment of the plasma specific activity through a single step increase in the rate of tracer infusion. Our results suggest that when endogenous Ra is changing rapidly, such as at the onset of exercise, these methods will provide a more accurate estimate of Ra than the standard fixed-volume model and constant tracer infusion.

Glucoregulatory and hormonal responses to repeated bouts of intense exercise in normal male subjects.

Glucose turnover and its regulation were studied during and after two identical bouts of intense exhaustive exercise separated by 1 h to define differences in response. Six lean young postabsorptive male subjects exercised at approximately 100% maximal O2 uptake (3.7 +/- 0.3 l/min) for 13.0 +/- 0.7 min for the first (EX1) and 13.2 +/- 0.8 min for the second (EX2) bout. Plasma glucose increased during EX1 and peaked at 7.0 +/- 0.6 mmol/l in early recovery but to 5.8 +/- 0.5 mmol/l (P less than 0.05) after EX2, and both the hyperglycemic and the hyperinsulinemic responses were less after EX2 (P less than 0.015, analysis of variance). The hyperglycemia was due to lesser increments in glucose utilization (Rd) (3-fold resting) than glucose production (Ra) (7-fold) toward exhaustion and for 7 min of recovery. The rise in Rd was more rapid (P less than 0.05) and metabolic clearance rate was greater during (P = 0.015) and from 9 to 60 min after EX2, and Ra also remained higher during recovery (P less than 0.05). Marked and similar increments in plasma norepinephrine (18-fold) and epinephrine (14-fold) occurred with both bouts. Plasma glucagon increments were small and not different. Therefore, 1) more circulating glucose was used with EX2, 2) greater metabolic clearance rate during and after EX2 suggests local muscle adaptations due to EX1, and 3) significant correlations (P less than 0.002) between plasma norepinephrine and Ra (r = 0.82) and Ra - Rd (r = 0.52) and between epinephrine and Ra (r = 0.71) and Ra - Rd (r = 0.48) suggest a major regulatory role for the catecholamine responses.

Mechanism of glucoregulatory responses to stress and their deficiency in diabetes.

During exercise, increased energy demands are met by increased glucose production that occurs simultaneously with the increased glucose uptake. We had previously observed that, during exercise, metabolic clearance rate of glucose (MCR) increases markedly in normal, but only marginally in poorly controlled diabetic dogs. We wished to determine (i) whether in a more general model of stress matched increases in rate of appearance of glucose and MCR also occur, or if MCR is suppressed, as during catecholamine infusion; and (ii) whether diabetes affects stress-induced changes in rate of glucose appearance and MCR. Therefore, we injected carbachol (27 nmol/50 microliters), an analog of acetylcholine, intracerebroventricularly in seven conscious dogs before and after induction of alloxan diabetes. In normal dogs, plasma epinephrine and cortisol increased 4- to 5-fold, whereas norepinephrine and glucagon doubled. Plasma insulin, however, remained unchanged. Tracer-determined hepatic glucose production increased rapidly, but transiently, by 2.5-fold. This increment can be fully explained by the observed increments in the counterregulatory hormones. Surprisingly, however, MCR also promptly increased, and therefore, plasma glucose changed only marginally. After induction of diabetes, the animals were given intracerebroventricular carbachol while plasma glucose was maintained at moderate hyperglycemia (9.0 +/- 0.4 mM). Increments in counterregulatory hormones were similar to those seen in normal dogs, except for exaggerated norepinephrine release. Peripheral insulin levels were higher in diabetic than in normal dogs; however, MCR was markedly reduced and the lipolytic response to stress increased, indicating insulin resistance. Interestingly, the hyperglycemic response to stress was 6-fold greater in diabetic than normal animals, relating mainly to the failure of MCR to rise. Plasma lactate increased equivalently in diabetic and normal animals despite suppression of MCR in the diabetics, indicating either greater muscle glycogenolysis and/or impairment in glucose oxidation. We conclude that in this stress model MCR increases as in exercise in normal but not in diabetic dogs. We speculate that glucose uptake in stress could be mediated through an insulin-dependent neural mechanism.

Serum immunoreactive insulin levels in intact and regenerating postmetamorphic Xenopus laevis.

In order to confirm the presence of immunoreactive insulin (IRI) in the serum of postmetamorphic Xenopus laevis, radioimmunoassay (RIA) methods were used. The concentration of hormone found in samples of blood serum taken from nonanaesthetized intact male and female animals by the guillotine method was 10.46 +/- 0.76 microU/ml. Significantly higher IRI concentrations were found in our intact animals anaesthetized in MS 222 at pH 3.5 (21.9 microU/ml) compared with intact controls anaesthetized in MS 222 adjusted to pH 7.0 (14.4 microU/ml). During the wound-healing stage subsequent to forelimb amputation in the experimental cases (0 hours to 3 days) anaesthetized in MS 222 pH 7.0, there were intervals of significantly elevated serum IRI followed by a period of decreased IRI concentration compared with the levels in anaesthetized (MS 222 pH 7.0) and nonanaesthetized intact controls. These fluctuations were due, presumably, to stress caused by amputational injury and/or anaesthetic. Serum IRI increased steadily from 3 to 14 days postamputation then remained stable for the balance of the regeneration period (28 days) compared with nonanesthetized intact controls. A positive correlation was found between immunoreactive insulin and glucose levels in the serum of our animals. However, no correlation exists between serum IRI levels and serum osmolality in the data.

The histamine H2 receptor agonist impromidine: synthesis and structure-activity considerations.

Impromidine (1) is a potent and selective histamine H2 receptor agonist and its structure comprises a strongly basic guanidine group containing two different imidazole-containing side chains. In this paper we report the synthesis of analogues in which both of the side chains and the guanidine group are modified and tested as agonists or antagonists at histamine H2 receptors on guinea pig atrium. A protonated amidine group linked by a chain of three carbon atoms to a tautomeric imidazole ring appears to be an essential feature for agonist activity and it is suggested that the second imidazole-containing side chain in impromidine mainly contributes toward affinity for histamine H2 receptors.

Cyanoguanidine-thiourea equivalence in the development of the histamine H2-receptor antagonist, cimetidine.

In the histamine H2-receptor antagonist metiamide (2a) isosteric replacement of thione sulfur (=S) by carbonyl oxygen (=O) or imino nitrogen (=NH) affords the urea 2c and guanidine 2d which are antagonists of decreased potency. The guanidine is very basic and at physiological pH is completely protonated. However, introduction of strongly electronegative substituents into the guanidine group reduces basicity and gives potent H2-receptor antagonists, viz. the cyanoguanidine 2b (cimetidine, "Tagamet") and nitroguanidine 2e. A correspondence between the activity of thioureas and cyanoguanidines is demonstrated for a series of structures 1-4. The close correspondence between cyanoguanidine and thiourea in many physicochemical properties and the pharmacological equivalence of these groups in H2-receptor antagonists leads to the description of cyanoguanidine and thiourea as bioisosteres. Acid hydrolysis of the cyanoguanidine 2b yields the carbamoylguanidine 2f at ambient temperatures and the guanidine 2d at elevated temperatures. Cimetidine is slightly more active than metiamide in vivo as an inhibitor of histamine-stimulated gastric acid secretion and has clinical use in the treatment of peptic ulcer and associated gastrointestinal disorders.