The central and renal hemodynamic effects of amino acid infusion in the study of renal reserve in the baboon.

Normal human renal function is characterized by a large renal reserve. Recruitment of this reserve is a compensatory and pathological response to renal injury. This study was designed to assess the renal reserve and central hemodynamics of young female baboons and, in doing so, the appropriateness of the use of these animals in a model of human renal disease. Eight female baboons completed the protocol. PAH and inulin clearances were measured before and after an amino acid infusion. Central hemodynamics were measured with arterial and pulmonary artery catheters. Effective renal plasma flow and glomerular filtration rate increased by 42% after amino acid infusion (P = .025). Expansion of renal function was not consistent among individual baboons; two of the eight animals did not demonstrate renal reserve. Central hemodynamics were unaffected by the protocol.

Endogenous somatostatin-28 modulates postprandial insulin secretion. Immunoneutralization studies in baboons.

Somatostatin-28 (S-28), secreted into the circulation from enterocytes after food, and S-14, released mainly from gastric and pancreatic D cells and enteric neurons, inhibit peripheral cellular functions. We hypothesized that S-28 is a humoral regulator of pancreatic B cell function during nutrient absorption. Consistent with this postulate, we observed in baboons a two to threefold increase in portal and peripheral levels of S-28 after meals, with minimal changes in S-14. We attempted to demonstrate a hormonal effect of these peptides by measuring their concentrations before and after infusing a somatostatin-specific monoclonal antibody (mAb) into baboons and comparing glucose, insulin, and glucagon-like peptide-1 levels before and for 4 h after intragastric nutrients during a control study and on 2 d after mAb administration (days 1 and 2). Basal growth hormone (GH) and glucagon levels and parameters of insulin and glucose kinetics were also measured. During immunoneutralization, we found that (a) postprandial insulin levels were elevated on days 1 and 2; (b) GH levels rose immediately and were sustained for 28 h, while glucagon fell; (c) basal insulin levels were unchanged on day 1 but were increased two to threefold on day 2, coincident with decreased insulin sensitivity; and (d) plasma glucose concentrations were similar to control values. We attribute the eventual rise in fasting levels of insulin to its enhanced secretion in compensation for the heightened insulin resistance from increased GH action. Based on the elevated postmeal insulin levels after mAb administration, we conclude that S-28 participates in the enteroinsular axis as a decretin to regulate postprandial insulin secretion.

Evidence that galanin is a parasympathetic, rather than a sympathetic, neurotransmitter in the baboon pancreas.

To determine whether galanin is a pancreatic sympathetic neurotransmitter regulating insulin secretion in the baboon, as it is in the dog, we evaluated galanin for inhibitory effects on insulin secretion in conscious baboons, determined if baboon pancreatic islets are innervated by galaninergic fibers using immunohistochemistry, and measured galanin content in the major sympathetic ganglion supplying the pancreas. Surprisingly, infusion of galanin (1 microgram/kg per min) had no effect on arginine-stimulated secretion of either insulin (71 +/- 14 vs. 88 +/- 17 microU/ml; P = NS) or glucagon (104 +/- 12 vs. 94 +/- 9 pg/ml; P = NS). By contrast, growth hormone secretion was markedly increased during galanin infusion. In the baboon celiac ganglion, no galanin immunoreactivity was detectable in sympathetic neuronal cell bodies by immunostaining and their content of galanin-like immunoreactivity, determined by radioimmunoassay, was only 3% of that in dog celiac ganglion (5.2 +/- 0.8 vs. 158 +/- 13 pmol/g; P < 0.001). By contrast, galanin immunoreactivity was observed in many nerve fibers in the baboon exocrine pancreas and occasionally in baboon pancreatic islets. Moreover, galanin content of the baboon pancreas was similar to that of dog (8.7 +/- 1.5 vs. 5.5 +/- 1.2 pmol/g; P = NS). The finding of galanin immunoreactivity in many neuronal cell bodies in baboon intrapancreatic ganglia suggests a parasympathetic source for these galaninergic fibers in the baboon. Together these data demonstrate that galanin is likely to be a parasympathetic neurotransmitter in the baboon pancreas, without major effects on insulin or glucagon secretion.

Elimination of the action of glucagon-like peptide 1 causes an impairment of glucose tolerance after nutrient ingestion by healthy baboons.

Glucagon-like peptide 1 (GLP-1) is an insulinotropic hormone released after nutrient ingestion which is known to augment insulin secretion, inhibit glucagon release, and promote insulin-independent glucose disposition. To determine the overall effect of GLP-1 on glucose disposition after a meal we studied a group of healthy, conscious baboons before and after intragastric glucose administration during infusions of saline, and two treatments to eliminate the action of GLP-1: (a) exendin-[9-39] (Ex-9), a peptide receptor antagonist of GLP-1; or (b) an anti-GLP-1 mAb. Fasting concentrations of glucose were higher during infusion of Ex-9 than during saline (4.44 +/- 0.05 vs. 4.16 +/- 0.05 mM, P < 0.01), coincident with an elevation in the levels of circulating glucagon (96 +/- 10 vs. 59 +/- 3 ng/liter, P < 0.02). The postprandial glycemic excursions during administration of Ex-9 and mAb were greater than during the control studies (Ex-9 13.7 +/- 2.0 vs. saline 10.0 +/- 0.8 mM, P = 0.07; and mAb 13.6 +/- 1.2 vs. saline 10.6 +/- 0.9 mM, P = 0.044). The increments in insulin levels throughout the absorption of the glucose meal were not different for the experimental and control conditions, but the insulin response in the first 30 min after the glucose meal was diminished significantly during treatment with Ex-9 (Ex-9 761 +/- 139 vs. saline 1,089 +/- 166 pM, P = 0.044) and was delayed in three of the four animals given the neutralizing antibody (mAb 946 +/- 262 vs. saline 1,146 +/- 340 pM). Thus, elimination of the action of GLP-1 impaired the disposition of an intragastric glucose meal and this was at least partly attributable to diminished early insulin release. In addition to these postprandial effects, the concurrent elevation in fasting glucose and glucagon during GLP-1 antagonism suggests that GLP-1 may have a tonic inhibitory effect on glucagon output. These findings demonstrate the important role of GLP-1 in the assimilation of glucose absorbed from the gut.

Estradiol inhibits the increase of hypothalamic neuropeptide Y messenger ribonucleic acid expression induced by weight loss in ovariectomized rats.

Anorexia and weight loss produced by estradiol (E2) may involve altered expression of neuropeptide Y (NPY) in the hypothalamus. We tested this hypothesis using ovariectomized (OVX) rats by replacing E2 with SILASTIC brand capsule implants and measuring NPY messenger RNA (mRNA) levels in the arcuate nucleus by in situ hybridization. To equalize the effects of weight loss on NPY mRNA expression, E2 deficient OVX rats were pair-fed (n = 10) for 2 days to an OVX group receiving E2 (n = 12). Compared with the weight gain (P < 0.02) of E2 deficient OVX rats (n = 10), OVX rats replaced with E2 and pair-fed OVX rats both had 12.5% lower food intake and weight (P < 0.05). E2 replacement elevated insulin 52% (P < 0.05) and lowered NPY hybridization 32% (P < 0.05) compared with pair-fed controls. During a 2-day fast, E2 replacement (N = 12) attenuated the elevation of NPY mRNA levels 50% (P < 0.01) compared with E2 deficiency (n = 15). Therefore, when E2 is administered to OVX rats, reduced NPY mRNA expression in the hypothalamus is unlikely to be a primary cause of weight loss, although it may contribute to the maintenance of reduced food intake and body weight.

Accelerated cholesteryl ester transfer in baboons with insulin-requiring diabetes mellitus.

Cholesteryl ester transfer (CET), plasma, lipoprotein lipid and phospholipid composition were studied in insulin-treated baboons with chronic streptozotocin-induced diabetes. In these diabetic animals, CET measured both as the mass (p < 0.001) and isotopic transfer (p < 0.05) of CE from HDL to the apo B-containing lipoproteins (VLDL+LDL) were significantly accelerated compared to controls and the response closely resembled that recently reported in diabetic humans. No significant differences were present in plasma triglyceride, cholesterol, or HDL-C or in lipoprotein core or surface lipid composition. Thus, despite the fact that they did not display the same spectrum of abnormalities in lipoprotein composition, these insulin-treated diabetic baboons demonstrated an abnormality in CET identical to that described in humans. These findings suggest that this non-human primate may provide a suitable diabetic animal model in which to better characterize the mechanisms that underlie this potentially atherogenic disturbance in lipoprotein transport.

Perfusion with anti-insulin gamma globulin indicates a B to A to D cellular perfusion sequence in the pancreas of the rhesus monkey, Macaca mulatta.

The cellular sequence of intraislet vascular perfusion has been shown to be important in the regulation of islet hormone secretion in the rat and dog islet. In order to test whether a B to A to D sequence of islet cellular perfusion is also present in a nonhuman primate, pancreata from the rhesus monkey, Macaca mulatta, were isolated and perfused in vitro in the presence and absence of anti-insulin gamma globulin. In the presence of the insulin antibody, efflux concentration of insulin decreased rapidly (-95 +/- 1.8%), whereas glucagon and somatostatin concentrations increased (111 +/- 28% and 239 +/- 38%, respectively). These results suggest the presence of a B-A-D cellular sequence of vascular perfusion within the monkey islet. The present results also strongly support the hypothesis that a B-A-D sequence of islet perfusion is important in the regulation of islet hormone secretion and further emphasize the central role of the B-cell in intraislet cellular interactions. The results also suggest that, despite differences in islet anatomy, a B-A-D order of islet cellular perfusion may be the preferred functional sequence among mammalian species.

Correlations of in vivo beta-cell function tests with beta-cell mass and pancreatic insulin content in streptozocin-administered baboons.

In vivo beta-cell function tests are used increasingly in humans during the preclinical phase of insulin-dependent diabetes mellitus (IDDM), but the severity of the beta-cell loss responsible for the abnormalities seen in these tests is unknown. We have measured several physiological beta-cell function tests--fasting plasma glucose, glucose disappearance constant, fasting insulin, acute insulin responses to arginine (AIRarginine) and glucose (AIRglucose), and glucose potentiation of AIRarginine (delta AIRarginine/delta G) and two direct objective measurements (pancreatic insulin content [PIC] and quantitative beta-cell mass)--in adolescent male baboons (Papio anubis/cyanocephalus). We have correlated in vivo measurements obtained within 3 days after the animals were killed with in vitro estimates of PIC and beta-cell mass in 15 animals, (2 nondiabetic requiring insulin treatment and 13 after varying doses of streptozocin to induce degrees of beta-cell damage ranging from normoglycemia to severe hyperglycemia). There was a strong linear correlation between beta-cell mass and PIC (r = 0.79, P less than 0.001). Physiological measures of beta-cell function were significantly correlated with both PIC and beta-cell mass. The correlations between physiological measures and beta-cell mass were linear and intercepted the beta-cell mass axis at 0.15-0.2 g, suggesting that in vivo measures of beta-cell function approach 0 when there is still approximately 40-50% of the beta-cell mass detectable histologically. With PIC, the linear correlations intercepted the axes close to 0. These findings provide considerable validity to the measurements of beta-cell function used in preclinical IDDM in humans.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of nicotinic acid-induced insulin resistance on pancreatic B cell function in normal and streptozocin-treated baboons.

To study the interaction between insulin secretion and insulin action in maintaining glucose homeostasis, we induced experimental insulin resistance in eight normal baboons, in six baboons treated with 40 mg/kg streptozocin (STZ-40), and in six baboons treated with 200 mg/kg streptozocin (STZ-200). Insulin resistance was induced by a 20-d continuous intravenous infusion of nicotinic acid (NA). Normal animals showed compensatory increases in several measures of insulin secretion (fasting insulin [FI], acute insulin response to arginine [AIRarg], acute insulin response to glucose [AIRgluc], and glucose potentiation slope [delta AIRarg/delta G]), with no net change in fasting plasma glucose (FPG) or glycosylated hemoglobin (HbAtc). STZ-40 animals showed compensatory increases in FI, AIRarg, and AIRgluc, but delta AIRarg/delta G failed to compensate. Although FPG remained normal in this group during NA infusion, HbA1c rose significantly. STZ-200 animals failed to show compensatory changes in both AIRgluc and delta AIRarg/delta G, with both HbA1c and FPG rising. These animals showed a paradoxical inhibition of insulin secretion in response to intravenous glucose during NA infusion, at a time when they were hyperglycemic. These data indicate that a significant degree of insulin resistance does not cause hyperglycemia in the presence of normal B cell function but, in animals with reduced B cell mass and superimposed insulin resistance, the degree of hyperglycemia is proportional to the degree of pancreatic B cell dysfunction.

In vitro pancreatic hormonal pulses are less regular and more frequent than in vivo.

Spontaneous in vivo cyclic secretion of insulin and glucagon displays a pulse interval of 10 +/- 0.3 (SE) min and a constant phase relationship in fasting rhesus monkeys. When pancreata from six normal rhesus monkeys were perfused in vitro, the insulin pulse interval averaged 6.3 +/- 0.23 (SE) min. Insulin, glucagon, and somatostatin displayed high-amplitude secretory pulses, and the average pulse interval did not differ among the three islet hormones. The islet pulses are less regular in vitro than in vivo, and the phase relationship among the three hormones is lost. The relative amplitude averaged 142 +/- 10, 110 +/- 18, and 81 +/- 11% of the mean hormone concentrations for insulin, somatostatin, and glucagon, respectively. Similar differences in secretory pattern were observed during perfusion of three baboon pancreata compared with the in vivo pattern in this second primate species. The data suggest that the frequency and phase relationship of the islet pulsatile secretory system is modulated by factors extrinsic to the pancreas in the intact nonhuman primate. The nature of these modulating factors remains to be established. The apparent phase independence of the three islet hormones suggests that each of the major endocrine cell types of the islet possess independent episodic secretory mechanisms.

Insulin binding to individual rat skeletal muscles.

Studies of insulin binding to skeletal muscle, performed using sarcolemmal membrane preparations or whole muscle incubations of mixed muscle or typical red (soleus, psoas) or white [extensor digitorum longus (EDL), gastrocnemius] muscle, have suggested that red muscle binds more insulin than white muscle. We have evaluated this hypothesis using cryostat sections of unfixed tissue to measure insulin binding in a broad range of skeletal muscles; many were of similar fiber-type profiles. Insulin binding per square millimeter of skeletal muscle slice was measured by autoradiography and computer-assisted densitometry. We found a 4.5-fold range in specific insulin tracer binding, with heart and predominantly slow-twitch oxidative muscles (SO) at the high end and the predominantly fast-twitch glycolytic (FG) muscles at the low end of the range. This pattern reflects insulin sensitivity. Evaluation of displacement curves for insulin binding yielded linear Scatchard plots. The dissociation constants varied over a ninefold range (0.26-2.06 nM). Binding capacity varied from 12.2 to 82.7 fmol/mm2. Neither binding parameter was correlated with fiber type or insulin sensitivity; e.g., among three muscles of similar fiber-type profile, the EDL had high numbers of low-affinity binding sites, whereas the quadriceps had low numbers of high-affinity sites. In summary, considerable heterogeneity in insulin binding was found among hindlimb muscles of the rat, which can be attributed to heterogeneity in binding affinities and the numbers of binding sites. It can be concluded that a given fiber type is not uniquely associated with a set of insulin binding parameters that result in high or low binding.

Decreased insulin- and glucagon-pulse amplitude accompanying beta-cell deficiency induced by streptozocin in baboons.

The effect of beta-cell deficiency on the spontaneous pulsatile secretory pattern of the islets of Langerhans was studied in the baboon. Measures of beta-cell function were correlated with the secretory pattern before and at intervals after streptozocin administration. The degree of insulin deficiency was variable and ranged from mild to moderate. Highly regular pulses were less prevalent in baboons compared with rhesus monkeys and humans, but the mean frequency was similar and was not affected by treatment. The principal effect of beta-cell destruction was to proportionately reduce the pulse amplitude of insulin (-39%, P less than .003) without detectable change in pulse frequency, interhormonal phase relationship, or the regularity of pulses. Glucagon-pulse amplitude also fell (-19%, P less than .09), but not significantly. However, glucagon-pulse amplitude was strongly correlated with insulin-pulse amplitude (r = -.59, P less than .002), whereas mean fasting plasma concentrations of insulin and glucagon were not significantly changed after treatment. Because streptozocin affects only the beta-cell, the data indicate a major influence of the insulin pulse on the alpha-cell secretory pulse. The data do not support the presence of a separate pacemaker for the alpha-cell but do not eliminate this possibility. The strong correlation of reduction in insulin-pulse amplitude with increasing fasting glucose and decreasing glucose disappearance lends support to growing evidence that the pattern of insulin secretion is an important determinant of normal glucose homeostasis.

Responses of the pancreatic A cell during hypoglycemia and hyperglycemia are dependent on the B cell.

It is controversial whether stimulation of glucagon secretion by hypoglycemia or suppression by hyperglycemia is a direct effect of glucose on the A cell or whether it is mediated indirectly through the B cell. To determine the role of the B cell in the mediation of glucagon secretion adolescent male baboons were studied before and after successive injections of streptozocin designed to cause B-cell destruction in a series of stages. Following one dose of streptozocin, B-cell function was impaired but fasting blood glucose remained normal. B-cell function declined further with additional doses of streptozocin. As B-cell function declined, there was a corresponding reduction in the glucagon response to hypoglycemia (Ghyp). There were significant correlations between the percentage reduction in Ghyp and the percentage reduction in B-cell function (acute insulin response to arginine, r = .47; acute insulin response to glucose, r = .69; Kg, r = .79). In a second study the ability of hyperglycemia to suppress the acute glucagon response to arginine (AGRarg) was studied. Before streptozocin AGRarg was 128 +/- 26 pg/mL at a glucose level of 80 +/- 4 mg/dL and was suppressed to 74 +/- 20 pg/mL when glucose was raised and clamped at 209 +/- 14 mg/dL. After the initial dose of streptozocin, with mild B cell damage and fasting normoglycemia, AGRarg was not suppressed by hyperglycemia. With severe B cell dysfunction and fasting hyperglycemia, there was paradoxical enhancement of AGRarg by additional hyperglycemia. In conclusion, the ability of the A cell to respond appropriately to hypoglycemia and to arginine during hyperglycemia is dependent on normal B-cell function.

Defects in beta-cell function and insulin sensitivity in normoglycemic streptozocin-treated baboons: a model of preclinical insulin-dependent diabetes.

During the preclinical period of human insulin-dependent diabetes, both impaired pancreatic beta-cell function and increased insulin resistance are found, although normoglycemia is preserved. To better understand the changes in beta-cell function and insulin sensitivity that occur in preclinical insulin-dependent diabetes, we performed a panel of in vivo beta-cell function tests and measured insulin sensitivity in adolescent male baboons both in normal health and after a small dose of streptozocin which did not induce hyperglycemia. Nine animals were studied before (stage 1) and 1 week after receiving a low dose of streptozocin (stage 2). There was no change in fasting plasma glucose or insulin. The mean glucose disposal rate (Kg) remained within the normal range, but dropped from 2.0 +/- 0.2% +/- SE) to 1.2 +/- 0.1%/min (P less than 0.01), the acute insulin response to arginine (AIR(arg)) fell from 67.7 +/- 19.4 microU/mL (485.8 +/- 139.2 pmol/L) to 32.8 +/- 7.2 microU/mL (235.3 +/- 51.7 pmol/L; P less than 0.05), and the acute insulin response to glucose (AIR(gluc)) fell from 881 +/- 243 microU/mL.10 min (6321 +/- 1744 pmol/L.10 min) to 334 +/- 82 microU/mL.10 min (2396 +/- 588 pmol/L.10 min; P less than 0.01). The most dramatic change, however, was in the ability of hyperglycemia to potentiate AIR(arg) (expressed as the slope of potentiation). This was reduced by 94% from 1.8 +/- 0.5 to 0.1 +/- 0.1 (P less than 0.01), with almost no overlap in values between stages 1 and 2. Insulin sensitivity was also lower 1 week after streptozocin treatment. When the animals were restudied 8 weeks after streptozocin treatment (stage 3) most measures of beta-cell function were not significantly different from those in stage 1. The fasting plasma glucose level was 85.4 +/- 4.3 mg/dL (4.7 +/- 0.2 mmol/L), Kg was 1.8 +/- 0.3%/min, fasting plasma insulin was 35.9 +/- 8.5 microU/mL (257.6 +/- 61.0 pmol/L), AIR(arg) was 67.0 +/- 15.4 microU/mL (480.7 +/- 110.5 pmol/L), and AIR(gluc) was 615.3 +/- 265.3 microU/mL.10 min (4413 +/- 1901 pmol/L.10 min), and tissue insulin sensitivity was 2.7 +/- 0.4 x 10(4) min/microU.mL. These values show extensive overlap with those of stage 1, from which they are not significantly different. The slope of glucose potentiation, however, remained low in all animals at stage 3.(ABSTRACT TRUNCATED AT 400 WORDS)

Effects of branched-chain amino acids on postprandial 3-OH butyrate and glucagon in the baboon.

Although chronic postprandial elevation of branched-chain amino acids (BCAAs) occurs in diabetic subjects and in subjects consuming high-protein diets, the metabolic effects of simultaneously increasing levels of these three amino acids are unclear. In this study, a mixture of the BCAAs was infused intravenously into baboons, beginning 30 minutes after the daily meal and continuing for 200 minutes on four consecutive days. Blood samples were collected on the last day of treatment. Infusion of the BCAAs into fed baboons promoted an increase in peak levels of glucagon, a decrease in postprandial levels of seven amino acids, and an increase in plasma levels of 3-OH butyrate. The ketone body response occurred despite an increase in the plasma ratio of insulin/glucagon in four of the five animals and was not associated with a change in the rate of lipolysis as indicated by plasma glycerol measurements. These findings raise the possibility that ketone bodies are one of the metabolic products of BCAA metabolism induced by high concentrations of leucine or ketoisocaproate. The observation that chronic elevation of BCAAs augments glucagon secretion may explain the parallel increases in plasma glucagon and plasma BCAAs observed in subjects fed high protein diets.

Beta-adrenergic modulation of insulin binding in skeletal muscle.

Both a high physiological concentration (13.1 nM) of epinephrine (E) and acute exercise (AEx) have previously been shown to increase 125I-insulin binding in skeletal muscle. To investigate the site and mechanism of the effect of epinephrine on binding and the possible link between epinephrine- and AEx-enhanced insulin binding, we measured insulin binding in three different preparations: 1) crude membranes derived from whole soleus muscle incubated in vitro with 13.1 nM E, 2) crude membranes with E present in the binding assay, and 3) purified plasma membranes with E present. Epinephrine enhanced binding in all three preparations by 169, 144, and 164%, respectively, at low concentrations of insulin but had little effect at high concentrations. Epinephrine, therefore appears to have its effect at the plasma membrane. Propranolol (10 microM), a beta-adrenergic antagonist, blocked E-enhanced insulin binding and when added to crude membranes made from soleus and extensor digitorum longus muscle of AEx rats reversed the increase in binding seen with exercise. This indicates that E-enhanced insulin binding is mediated by beta-adrenergic receptors and that AEx enhances insulin binding via beta-adrenergic receptors. Sodium orthovanadate (3 mM), a phosphotyrosyl-protein phosphatase inhibitor, also inhibited the increase in insulin binding due to E, implying that E may increase insulin binding by activating a phosphotyrosyl-protein phosphatase which decreases the phosphorylation of a plasma membrane protein, presumably the insulin receptor.

Metabolic changes during maturation of male monkeys: possible signals for onset of puberty.

There is a close relationship between the metabolic status of a maturing animal and the timing of puberty onset. However, the signals linking metabolic status to the maturation of the reproductive axis remain unknown. We looked for metabolic differences before and after puberty by comparing plasma profiles of insulin, glucose, amino acids, beta-hydroxybutyrate, and glycerol between juvenile and adult monkeys in fed and fasted states. Thirteen juvenile and 13 adult male crab-eating macaques (Macaca fascicularis) were fed a mixed meal, and blood samples were collected at intervals between 1.5 and 52 h after the meal. Plasma insulin concentrations decreased in a similar manner in both groups during the first 16 h of fasting. By 20 h after a meal, basal insulin levels were significantly lower (P less than 0.025) in juveniles compared with adults and remained so until the end of the fast. Circulating levels of glucose were similar in juveniles and adults immediately after a meal and then decreased significantly (P less than 0.025) in juveniles by 28 h of fasting and in adults by 52 h of fasting. Plasma concentrations of all large neutral amino acids (i.e., tyrosine, tryptophan, phenylalanine, valine, leucine, and isoleucine, LNAA) except tryptophan decreased more precipitously in juveniles than in adults during the first 20 h of fasting. However, the ratios of tyrosine to other LNAA and tryptophan to other LNAA were similar in juveniles and adults at all times. beta-Hydroxybutyrate concentrations were low in both groups until 24 h after a meal, at which time plasma levels increased more rapidly and attained higher values in juveniles compared with adults.(ABSTRACT TRUNCATED AT 250 WORDS)

A model for augmentation of hepatocyte response to pulsatile glucagon stimuli.

We have reported that in the physiological concentration range pulsatile glucagon delivery (6 pulses in 90 min) is a more effective stimulus of rat hepatocyte glucose production than is continuous infusion of the same amount of hormone (pulsatile EC50 = 186 +/- 41 pg/ml, continuous EC50 = 884 +/- 190 pg/ml). At supraphysiological glucagon concentrations, however, the maximal response to continuous glucagon infusion exceeds the response to pulses (241 +/- 14 vs. 140 +/- 11 mumol X G-1 X 90 min-1). In an effort to explain these observations we derived a model for the 90-min hepatocyte responses to pulsatile and continuous glucagon delivery based on the waveform of the hepatocyte response to a transient glucagon stimulus. The model demonstrated that the time constant for response decay was an important determinant of the relative efficacy of the two patterns of hormone delivery. For the observed decay constant value of 0.132 +/- 0.02 min-1 the model predicted the following dose-response parameters: pulsatile EC50 = 131 pg/ml, Rmax = 119 mumol X G-1 X 90 min-1, continuous EC50 = 656 pg/ml, Rmax = 272 mumol X G-1 X 90 min-1. The ability of a model based only on the kinetics of a single pulse to simulate the observed dose-response relationship suggests that pulsatile stimulation is intrinsically more effective than continuous hormonal stimulation.

Pulsatile release of growth hormone and prolactin from the primate pituitary in vitro.

Perifused anterior hemipituitaries from one male and 4 female monkeys released GH and PRL in a pulsatile pattern, with mean +/- SE interpulse intervals of 8.2 +/- 0.4 and 8.5 +/- 0.3 min, as determined by a cycle detection computer algorithm. Mean hormone concentrations in the perifusate fractions collected at 2-min intervals were 435 +/- 89 (GH) and 515 +/- 262 (PRL) ng/ml. Pulse amplitudes averaged 74 +/- 16 ng/ml for GH and 189 +/- 89 ng/ml for PRL. These findings suggest the presence of a high frequency pulsatile secretory mechanism within the primate pituitary.

Glucose counterregulation during recovery from neuroglucopenia: which mechanism is important?

Neuroglucopenia (NGP), which is a serious potential hazard for all insulin-treated diabetics, stimulates many neural and hormonal responses including increased glucagon secretion and activation of beta-adrenergic receptors of the autonomic nervous system. To determine which of these responses is important in recovery from NGP, we induced NGP in baboons by the intravenous (IV) injection of 2-deoxy-D-glucose with and without beta-adrenergic blockade (propranolol) and somatostatin. Thirty minutes after the induction of NGP the animals recovered, and the mean (+/- SEM) rise in arterial plasma glucose was 6.6 +/- 0.9 mmol/L, in glycerol 0.106 +/- 0.22 mmol/L, and in beta-hydroxybutyrate 0.091 +/- 0.22 mmol/L. Animal recovery and glucose rise were uninfluenced by the infusion of propranolol (mean 30 minute plasma glucose rise of 6.2 +/- 0.8 mmol/L) and somatostatin (6.8 +/- 0.8 mmol/L). However, the combined infusion of somatostatin and propranolol prevented animal recovery and glucose rise (1.0 +/- 0.1 mmol/L). The glycerol and beta-hydroxybutyrate rises were blocked by the propranolol infusion alone. Thus, recovery from NGP and the associated rise in plasma glucose, glycerol, and beta-hydroxybutyrate are prevented by the combination of the suppression of the glucagon and beta-adrenergic response to NGP. Furthermore, if the results of our study are extrapolated to insulin-dependent diabetic patients, most of whom have an impaired glucagon response to insulin-induced hypoglycemia/neuroglucopenia, they would be critically dependent on beta-adrenergic mechanisms for recovery from NGP.