Progesterone inhibits insulin secretion by a membrane delimited, non-genomic action

In rat islets, progesterone caused a prompt concentration-dependent inhibition of glucose-stimulated insulin release with an IC50 of 10 microM at 8.4mM glucose. The inhibition was specific since both testosterone and 17beta-estradiol had no such effect. The degree of inhibition was similar in islets from male and female rats. The inhibition was not blocked in PTX-treated islets thus ruling out the Gi/Go proteins as mediators of the inhibition. Progesterone inhibited both glucose- and BayK-8644-stimulated insulin secretion in HIT-T15 cells and the IC50 vs. 10 mM glucose was also 10 microM. There was no effect on intracellular cyclic AMP concentration in the presence 0.2 and 10 mM glucose. Progesterone decreased [Ca2+]i under all conditions tested. The decrease in [Ca2+]i was due to blockade of the L-type voltage-dependent Ca2+ channels. Under Ca(2+)-free conditions, progesterone did not inhibit the stimulation of insulin release due to the combination of glucose, phorbol ester and forskolin. Thus blockade of Ca2+ entry appears to be the sole mechanism by which progesterone inhibits insulin release. As progesterone covalently linked to albumin had a similar inhibitory effect as progesterone itself, it is concluded that the steroid acts at the outer surface of the beta-cell plasma membrane. These effects would be classified as either AI or AIIb in the Mannheim classification of nongenomically initiated steroid actions.

Synthesis and biological activity of aminoguanidine and diaminoguanidine analogues of the antidiabetic/antiobesity agent 3-guanidinopropionic acid

3-Guanidinopropionic acid (1) has been demonstrated both to improve insulin sensitivity and to promote weight loss selectively from adipose tissue in animal models of non-insulin-dependent diabetes mellitus (NIDDM). However, 1 has also been shown to be a substrate for both the creatine transporter and creatine kinase, leading to marked accumulation in muscle tissue as the corresponding N-phosphate. The corresponding aminoguanidine analogue 2 was recently discovered to retain the antidiabetic activity of 1 while being markedly less susceptible to creatine-like metabolism, suggesting that it should have less potential to accumulate in muscle. Further structural modification of 2 was undertaken to investigate whether the antidiabetic potency could be augmented while maintaining resistance to creatine-like metabolism. Modifications such as alpha-alkylation, homologation, and bioisosteric replacement of the aminoguanidine all were detrimental to antidiabetic activity. However, the simple regioisomeric aminoguanidinoacetic acid 9 and diaminoguanidinoacetic acid analogue 7 were found to be equipotent to 2, leading eventually to the discovery of the significantly more potent diaminoguanidinoacetic acid regioisomers 52 and 53. Further attempts to modify the more active template represented by 52 led only to reductions in antidiabetic activity. Each of the new active analogues displayed the same resistance to creatine-like metabolism as 2. Further testing of 7, 9, and 53 in obese diabetic ob/ob mice confirmed that weight loss is induced selectively from adipose tissue, similar to the lead 1. Administration of 53 to insulin-resistant rhesus monkeys led to reductions in both fasting and post-prandial plasma glucose levels with concomitant reductions in plasma insulin levels, suggesting that the compound improved the action of endogenous insulin. Compounds 7 and 53 were selected for further preclinical development.

Synthesis and biological activity of analogues of the antidiabetic/antiobesity agent 3-guanidinopropionic acid: discovery of a novel aminoguanidinoacetic acid antidiabetic agent

3-Guanidinopropionic acid (1, PNU-10483) has been demonstrated to both improve insulin sensitivity and to promote weight loss selectively from adipose tissue in animal models of non-insulin-dependent diabetes mellitus (NIDDM). However, 1 has also been shown to be a substrate for both the creatine transporter and creatine kinase, leading to marked accumulation in muscle tissue as the corresponding N-phosphate 4. In an effort to identify novel entities that maintain antidiabetic potency without susceptibility to creatine-like metabolism, an analogue program was undertaken to explore the effects of various structural modifications, including homologation, simple substitution, single atom mutations, and bioisosteric replacements for the guanidine and carboxylic acid. Overall, the scope of activity encompassed by the set of new analogues proved to be exceedingly narrow. Notable exceptions demonstrating equivalent or improved antidiabetic activity included the alpha-amino derivative 29, aminopyridine 47, isothiourea 67, and aminoguanidine 69. On the basis of its superior therapeutic ratio, aminoguanidine 69 was selected for preclinical development and became the foundation for a second phase of analogue work. Furthermore, in vitro studies demonstrated that 69 is markedly less susceptible to phosphorylation by creatine kinase than the lead 1, suggesting that it should have less potential for accumulation in muscle tissue than 1.

Disruption of mitochondrial activities in rabbit and human hepatocytes by a quinoxalinone anxiolytic and its carboxylic acid metabolite

The quinoxalinone anxiolytic, panadiplon, was dropped from clinical development due to unexpected hepatic toxicity in human volunteers. Subsequent experimental studies in rabbits demonstrated a hepatic toxicity that resembled Reye's syndrome. In the present studies, we examined the effects of panadiplon and a metabolite, cyclopropane carboxylic acid (CPCA) on hepatic mitochondrial activities in vitro and ex vivo. Acute inhibition of beta-oidation of [14C]palmitate was observed in rabbit and human hepatocyte suspensions incubated with 100 microM panadiplon. Panadiplon (30 microM) also reduced mitochondrial uptake of rhodamine 123 (R123) in cultured rabbit and human, but not rat hepatocytes, following 18 h exposure. CPCA also impaired beta-oxidation and R123 uptake in rabbit and human hepatocytes. R123 uptake and beta-oxidation in cells from some donors was not impaired by either agent, and cell death was not observed in any experiment. Hepatocytes isolated from panadiplon-treated rabbits had reduced palmitate beta-oxidation rates and inhibited mitochondrial R123 uptake; R123 uptake remained inhibited until 48-72 h in culture. Rabbit mitochondrial respiration experiments revealed a slightly lower ratio of ATP formed/oxygen consumed in panadiplon-treated animals: direct exposure of normal rabbit liver mitochondria to panadiplon did not have this effect. Hepatocytes isolated from panadiplon-treated rabbits showed reduced respiratory control ratios and lower oxygen consumption compared to controls. Our results indicate that panadiplon induces a mitochondrial dysfunction in the liver, and suggest that this dysfunction may be attributed to the carboxylic acid metabolite.

Insulin secretory defect in Zucker fa/fa rats is improved by ameliorating insulin resistance

The role of insulin resistance in the impaired glucose-stimulated insulin release of Zucker fatty rats was investigated using the insulin-sensitizing thiazolidinedione drug pioglitazone. Fatty rats had fasting hyperinsulinemia yet a blunted secretory response to intravenous glucose compared with lean age-matched controls. Islets from fatty rats secreted less insulin (based on islet DNA) in response to high glucose than islets from lean rats but secreted normal amounts of insulin when tolbutamide or alpha-ketoisocaproic acid (alpha-KIC) was the stimulus. Administering pioglitazone for 9 days diminished basal hyperinsulinemia and increased the insulin response to high glucose by fatty rats but not by lean controls. Pioglitazone pretreatment augmented the secretory response by isolated islets to high glucose, alpha-KIC, and tolbutamide. Augmentation of islet insulin release was not associated with reduced plasma glucose concentration, suggesting that altered glycemia was not involved. Pancreas and islet insulin content was greater in fatty rats than in lean controls and was decreased by pioglitazone; hence, insulin stores and glucose-stimulated insulin release did not correlate. Pioglitazone treatment did not affect the rate of islet glucose usage or ATP/ADP in the presence of 2.75 or 16 mmol/l glucose. These data indicate that ameliorating insulin resistance reverses defective glucose-stimulated insulin release by Zucker fa/fa rats. After pioglitazone administration, insulin secretion may be augmented by increased generation of a metabolic coupling factor from glucose or at a later step in the secretory process that is common to both glucose and nonglucose secretagogues.

Pioglitazone increases insulin sensitivity, reduces blood glucose, insulin, and lipid levels, and lowers blood pressure, in obese, insulin-resistant rhesus monkeys

The antidiabetic effects of pioglitazone hydrochloride were evaluated in 6 spontaneously obese, insulin-resistant rhesus monkeys. The animals were studied during six successive 2-wk treatment phases separated by 2-wk rest periods: two placebo phases; 0.3, 1.0, and 3.0 mg.kg-1 x day-1 pioglitazone hydrochloride phases; and a final placebo phase. During the second week of each treatment phase, serum insulin (immunoreactive insulin [IRI]), plasma glucose, and serum triglyceride (TG) levels were measured after an overnight fast and after a standardized meal. Blood pressure was measured and glucose tolerance tests (modified minimal model protocol) were performed a few days after the meal tests. Pioglitazone hydrochloride significantly improved fasting and postprandial levels of IRI, plasma glucose, and TG in a dose-related manner (P < 0.05). Fasting values during treatment with 3.0 mg.kg-1 x day-1 were reduced by 64% for IRI, 19% for plasma glucose, and 44% for TG compared with the placebo phase before treatment. Efficacy of pioglitazone hydrochloride was more marked for those animals with fasting hyperglycemia. Insulin sensitivity was increased by pioglitazone hydrochloride (P = 0.05), whereas glucose effectiveness and glucose disappearance rate were not detectably affected. Systolic and mean arterial blood pressures were significantly decreased by pioglitazone hydrochloride (P < 0.05). No toxic side effects of pioglitazone hydrochloride treatment were noted.

Antihyperglycemic action of guanidinoalkanoic acids: 3-guanidinopropionic acid ameliorates hyperglycemia in diabetic KKAy and C57BL6Job/ob mice and increases glucose disappearance in rhesus monkeys

To evaluate the long-held concept that acidic guanidines lack glycemic effects, guanidinoalkanoic acids and the biguanide metformin (positive control) were administered to KKAy mice, a model of noninsulin-dependent diabetes. Two acidic guanidines, 3-guanidinopropionic acid (3-GPA) and guanidinoacetic acid, decreased the plasma glucose level; other compounds were ineffective. 3-GPA was more potent than even metformin. Insulin suppression tests in KKAy mice indicated that improved insulin sensitivity was the mode of action for 3-GPA. Glycemic effects in KKAy mice resulted from increased glucose disposal whereas gluconeogenesis, hepatic glycogen content and intestinal glucose absorption were unchanged. 3-GPA's glycemic effect was corroborated in two other models of noninsulin-dependent diabetes. In ob/ob mice, the compound reduced hyperglycemia, polyuria, glycosuria and hyperinsulinemia. In insulin-resistant rhesus monkeys, it increased the disappearance of i.v. glucose. The glycemic action of 3-GPA required the presence of some circulating insulin as well as hyperglycemia because the compound was ineffective in normoglycemic mice, insulinopenic Chinese hamsters and streptozotocin-diabetic rats. These data indicate that acidic guanidine derivatives can ameliorate hyperglycemia in animal models of noninsulin-dependent diabetes. Because acidic derivatives uniquely lack the propensity of guanidine compounds for inducing lactic acidosis, our finding suggests a new approach for developing improved antidiabetes compounds from this chemical class.

Effect of Ca++ channel blockers on energy level and stimulated insulin secretion in isolated rat islets of Langerhans

To investigate the effect of calcium channel blockade on intracellular energy levels and stimulated insulin secretion in isolated rat pancreatic islets, five different blockers of calcium channels were used. Insulin secretion stimulated with 16 mM glucose, 40 mM KCl or 20 mM alpha-ketoisocaproic acid was inhibited dose dependently by nifedipine, diltiazem, flunarizine and verapamil, albeit with different potencies. Nifedipine and flunarizine were more potent than diltiazem and verapamil. omega-Conotoxin GVIA (100 nM) had no significant effect on insulin release with all stimuli tested, although it caused approximately 20% inhibition of the late phase of secretion stimulated with high glucose. The doses of L-type channel antagonists and of flunarizine chosen for the measurements of cellular energy levels gave 60 to 80% inhibition of total stimulated insulin release. The [ATP]/[ADP] ratio, with 5 mM glucose in the perifusion medium, was greater when these channel blockers were present than in controls, whereas it was smaller with omega-conotoxin. The rises in the nucleotide ratio elicited by 16 mM glucose were not affected by any of the antagonists tested. Thus, influx of Ca++ and a consequent rise in its intracellular level are unlikely to be the primary causal event in stimulation of energy synthesis which occurs upon addition of high concentrations of metabolic secretagogues.

Relationships between energy level and insulin secretion in isolated rat islets of Langerhans. Manipulation of [ATP]/[ADP][Pi] by 2-deoxy-D-glucose

Perifusion of islets with nominally phosphate-free buffer containing increasing concentrations of 2-deoxy-D-glucose (2.5 to 10 mM) produced increments in high alpha-ketoisocaproic acid-induced secretion of insulin beyond those observed in the absence of the sugar analogue. 3-O-methyl-D-glucose, a poorly metabolized sugar, was without effect. Insulin release evoked by 40 mM KCl was not altered by 2-deoxyglucose. The concentration of intracellular inorganic phosphate was lower in islets perifused with 2-deoxyglucose and declined to a lower level after addition of 20 mM alpha-ketoisocaproic acid. The enhancement of alpha-ketoisocaproic acid-induced hormone secretion by 2-deoxyglucose was not seen in islets perifused with medium containing 1.5 mM phosphate; instead a small inhibition was observed. It is postulated that conditions which lower intracellular [Pi] facilitate, either directly or indirectly, hormone release although the mechanism of this effect remains to be elucidated.

Relationships between energy level and insulin secretion in isolated rat islets of Langerhans. A study at various pH values

To define better the role of [ATP]/[ADP] in insulin release from pancreatic islets, changes in the adenine nucleotide ratios elicited by alterations in external pH were correlated with the secretion profiles produced by administration of two metabolic secretagogues, 16 mM glucose and 10 mM alpha-ketoisocaproic acid. Experiments were carried out in buffers with and without bicarbonate, in the pH range 6.5-7.7. Insulin release was dependent on pHe irrespective of the secretagogue used. Secretion profiles for alpha-ketoisocaproic acid were the same both with and without bicarbonate; the release was decreased below pH 7.1 but maintained at 7.4-7.7. The same pattern was seen with glucose in media buffered with Hepes. With bicarbonate present, secretion caused by high glucose showed a bell-shaped dependence on [H+], with reductions at the acid and alkaline sides of pH 7.1-7.4. [ATP] and [ADP] were higher when Hepes was the buffer, at all pH values studied. The [ATP]/[ADP] declined with increasing pH under both basal and stimulated conditions; the values were always larger after stimulation although at pH 7.7 with bicarbonate present and glucose as the stimulant the difference was very small. It is concluded that: (i) the [ATP]/[ADP] in pancreatic islets is markedly dependent on pHe; (ii) there is no straight-forward correlation between either [ATP] or the absolute value for [ATP]/[ADP] and insulin secretion; and (iii) a rise in [ATP]/[ADP] is necessary for glucose-stimulated insulin release although it is not always the rate-determining event.

Oxygen and temperature dependence of stimulated insulin secretion in isolated rat islets of Langerhans

The effects of lowered O2 tension on insulin secretion and changes in cellular energy parameters were investigated in isolated rat pancreatic islets perifused with buffers equilibrated with 21, 9, 5, and 1% oxygen and containing 5 mM glucose. Decreasing the external [O2] reduced the amount of insulin released in response to 16 mM glucose, 20 mM alpha-ketoisocaproic acid, and 40 mM KCl. Secretion elicited by high glucose or KCl had declined significantly at 9% oxygen, whereas that caused by alpha-ketoisocaproic acid became inhibited below 5% O2. Lowering the oxygen tension also decreased the ability of islets to respond with a rise in [ATP]/[ADP] upon stimulation with metabolic secretagogues. This reduction in the evoked increase in the nucleotide ratios paralleled the inhibition of stimulated insulin secretion. Addition of 2 mM amytal markedly decreased the islet energy level and eliminated the secretory response to 16 mM glucose. The results suggest that enhancement of B-cell energy production and a consequent rise in [ATP] (or [ATP]/[ADP]) are a necessary event for the hormone release elicited by high glucose and alpha-ketoisocaproic acid. A decrease in temperature inhibited insulin secretion with all three secretagogues tested. The energies of activation were similar for high glucose and KCl-induced secretion, about 20 kcal/mol, but were higher for alpha-ketoisocaproic acid, about 35 kcal/mol. At 28 degrees C, the [ATP]/[ADP] was larger than that at 38 degrees C (8 versus 5) and was not increased further upon addition of 16 mM glucose. It is suggested that a decrease in the rate of energy production at lowered temperatures may contribute to the inhibition of insulin release caused by metabolic secretagogues.

Pancreatic beta-cell somatostatin receptors

The characteristics of somatostatin (SRIF) receptors in rat pancreatic beta-cells were investigated using rat islets and the beta-cell line HIT-T15 (HIT). The biochemical properties of the SRIF receptors were examined with 125I-labeled des-Ala-1,Gly-2-desamino-Cys-3-[Tyr-11]- dicarba3,14-somatostatin (CGP 23996). 125I-CGP 23996 bound to SRIF receptors in HIT cells with high affinity and in a saturable manner. The binding of 125I-CGP 23996 to SRIF receptors was blocked by SRIF analogues with a rank order of potency of somatostatin 28 (SRIF-28) greater than D-Trp-8-somatostatin greater than somatostatin 14 (SRIF-14). To investigate the physical properties of the HIT cell SRIF receptor, the receptor was covalently labeled with 125I-CGP 23996 using photo-cross-linking techniques. 125I-CGP 23996 specifically labeled a protein of 55 kDa in HIT cell membranes. The size of the SRIF receptor in HIT cells is similar to the size of the SRIF receptor labeled with 125I-CGP 23996 in membranes of freshly isolated islets, suggesting that the physical properties of SRIF receptors in HIT cells and rat islet cells are similar. The binding studies suggest that beta-cells predominantly express a SRIF-28-preferring receptor. In freshly isolated islets, glucose- and arginine-stimulated insulin release was effectively blocked by SRIF-28 but not by SRIF-14. SRIF-14 did inhibit arginine-stimulated glucagon secretion from freshly isolated islets. The dissociation of the inhibitory effects of SRIF-28 and SRIF-14 on insulin and glucagon release from freshly isolated islets suggests that the two peptides act through different receptors in islets to regulate hormone secretion.

Bioenergetic response of pancreatic islets to stimulation by fuel molecules

The relationship of fuel-stimulated insulin secretion and the beta-cell bioenergetic state was investigated in isolated rat islets. In islets perifused with 5 mmol/L glucose to maintain a high basal energy state, stimulation by 9 to 28 mmol/L glucose increased the [ATP]/[ADP] and [GTP]/[GDP]. The rise in the former occurred prior to, or coincident with, the onset of insulin secretion and was dependent on glucose concentration. The increase in the latter appeared to lag behind the alteration in the [ATP]/[ADP] and achieved statistical significance after 30 minutes of incubation. Addition of 20 mmol/L alpha-ketoisocaproic acid, a powerful secretagogue, also caused a rise in the [ATP]/[ADP]. By contrast, 20 mmol/L lactate, which affected insulin secretion only minimally, failed to alter nucleotide concentrations. These data support the hypothesis that an increase in the islet energy state is a metabolic signal linking fuel metabolism with initiation of insulin secretion.

alpha-Glycerophosphate shuttle in a clonal beta-cell line

It has been proposed that the alpha-glycerophosphate (alpha-GOP) shuttle plays a crucial role in regulation of glycolysis in beta-cells by linking reoxidation of cytosolic NADH to formation of ATP in the electron transport chain (J. Biol. Chem. 265: 8287, 1981). Direct evidence for this suggestion is still lacking, however. In this work the operation of the alpha-GOP shuttle was investigated in the insulin-secreting cell line HIT-T15. The constituent enzymes of the pathway were found to be present in HIT cells. Flavin-linked alpha-GOP dehydrogenase was associated with the mitochondrial fraction, whereas NAD+-dependent alpha-GOP dehydrogenase was localized in the cytosol. In the presence of amobarbital (used to preserve the function of the alpha-GOP shuttle under conditions where oxidation of NADH by the respiratory chain was blocked), glucose increased insulin secretion, O2 consumption, and the cell [ATP]/[ADP] when compared with amobarbital alone. These results indicate that the alpha-GOP shuttle contributes to ATP generation in HIT cells and that its activation may be necessary for the initiation of insulin secretion by glucose.

Fuel-stimulated insulin secretion by clonal hamster beta-cell line HIT T-15

Insulin secretion by monolayer cultures of HIT T-15 cells was measured in response to various fuel molecules (glucose, dihydroxyacetone, lactate, glutamine, alpha-ketoisocaproic acid, alpha-ketoisovaleric acid) and a nonmetabolized glucose analogue (3-O-methylglucose). HIT cells secreted insulin in response to fuel molecules, but 3-O-methylglucose was ineffective. Stimulation of insulin release by fuels was increased by isobutylmethylxanthine and blocked by antimycin A. Iodoacetate selectively inhibited glucose-stimulated insulin release but had little effect on alpha-ketoisocaproic acid-stimulated insulin secretion. These results indicate that HIT cells retain the capacity of normal beta-cells to act as fuel sensors. Thus, HIT cells may provide a well-defined and relatively abundant tissue source in studies of stimulus-secretion coupling in beta-cells stimulated by fuels.

Glucose transport by radiation-induced insulinoma and clonal pancreatic beta-cells

Sugar uptake was measured in dispersed cells prepared from radiation-induced insulinomas transplantable in NEDH rats and in three clonal beta-cell lines maintained in continuous culture (RIN m5F, RIN 1046, HIT). Uptake of D-glucose and 3-O-methyl-D-glucose by insulinoma cells was rapid so that the intracellular concentration of D-hexoses approximated the concentration in the incubation medium by 15-30 s. L-Glucose was taken up only slowly. 3-O-methyl-D-glucose uptake by RIN m5F, RIN 1046, and HIT cells was slow; with 1 mM 3-O-methylglucose in the medium, equilibrium was attained at 20 min, but with 10 mM 3-O-methylglucose, equilibrium was not attained even at 20 min. In HIT cells incubated with D-glucose for 30 min, the intracellular concentration of glucose was less than the medium glucose concentration, indicating glucose transport is a nonequilibrium reaction in this cell line. These data indicate that radiation-induced insulinoma cells retain the capacity of normal beta-cells to transport sugar at high rates. RIN m5F, RIN 1046, and HIT cells transport sugar slowly, however, and thus differ from normal beta-cells. In RIN m5F, RIN 1046, and HIT cells, unlike in normal beta-cells, glucose transport may be the site regulating glucose metabolism.

Acetylcholine stimulates glucose metabolism by pancreatic islets

Acetylcholine stimulates insulin secretion in the presence of physiological concentrations of glucose. Stimulation of insulin secretion by acetylcholine is accompanied by an increase in glucose usage by isolated rat islets. Acetylcholine increased glucose usage by 38%, 28%, and 12% at 3.5 mM, 5.5 mM, and 10 mM glucose, respectively, compared to glucose usage by isolated islets incubated with glucose alone. Data showing increased glucose usage in islets treated with acetylcholine converge with data from an earlier report (J. Biol. Chem. 254 3921-3929 [1979]) showing a crossover point for glycolytic metabolites at phosphofructokinase to indicate that activation of glycolysis by acetylcholine results from increased phosphofructokinase activity and coordinate activation of hexokinase in intact islets.

Metabolic concomitants in pure, pancreatic beta cells during glucose-stimulated insulin secretion

The role of the redox potential in insulin secretion by beta cells stimulated with high glucose was investigated using an in vitro pancreas perfusion system. To assess glycolytic flux the sum of fructose-1,6-P2 + triose-P was determined in pure beta cells microdissected from lyophilized sections of the isolated perfused pancreas quick frozen during the early insulin secretory response. L-Glycerol 3-phosphate and dihydroxyacetone phosphate were measured as indicators of the free cytosolic [NAD+]/[NADH] ratio and NADH and NADPH were also measured. Fructose-1,6-P2 + triose-P was increased in beta cells simultaneously with the onset of insulin secretion indicating an increase in glucose metabolism had occurred. The ratio of [dihydroxyacetone phosphate]/[L-glycerol 3-phosphate] increased simultaneously with the onset of insulin secretion. NADH content increased only after initiation of insulin secretion and NADPH levels remained unchanged during the early secretory response to high glucose. These data contradict the hypothesis that insulin secretion is triggered by a more reduced cytosolic redox state and instead indicate that insulin secretion is initiated by other metabolic coupling factor(s) generated in beta cells stimulated by high glucose.

Identification of glucokinase as an alloxan-sensitive glucose sensor of the pancreatic beta-cell

Alloxan inactivated glucokinase in intact, isolated pancreatic islets incubated in vitro. Inactivation of glucokinase was antagonized by 30 mM glucose present during incubation of islets with alloxan. Glucokinase partially purified from transplantable insulinomas or rat liver was inactivated by alloxan with a half-maximal effect at 2-4 microM alloxan. Inactivation of purified glucokinase was antagonized by glucose, mannose, and 2-deoxyglucose in order of decreasing potency but not by 3-O-methylglucose. Glucose anomers at 6 and 14 mM were discriminated as protecting agents, with the alpha-anomer more effective than the beta-anomer. Glucokinase was not protected from alloxan inactivation by N-acetylglucosamine, indicating that the reactive site for alloxan is not the active site; therefore, glucose may protect glucokinase by inducing a conformational change. Glucokinase is thought to be the glucose sensor of the pancreatic beta-cell. The finding that glucokinase is inactivated by alloxan and protected by glucose with discrimination of its anomers similar to inhibition of glucose-stimulated insulin secretion by alloxan supports this hypothesis and appears to explain the mechanism for inhibition of hexose-stimulated insulin secretion by this agent and the unique role of glucose and mannose as protecting agents.

Regulatory role of fructose-2,6-bisphosphate in pancreatic islet glucose metabolism remains unsettled

Fructose-2,6-P2 was measured in perifused, isolated rat pancreatic islets. Fructose-2,6-P2 was present in pancreatic islets at low levels approximately equal to fructose-2,6-P2 content of liver from fasted rats. In islets perifused with glucose at physiologic concentrations, fructose-2,6-P2 was increased from 0.8 microM in the presence of 5.5 mM glucose to 1.0 microM at 10 mM glucose and 1.3 microM at 16.7 mM glucose, but did not increase further at higher glucose concentration. Therefore, only modest increases in the phosphofructokinase-1 activator, fructose-2,6-P2, occur at glucose concentrations stimulating insulin secretion.

Radiometric oil well assay for glucokinase in microscopic structures

Glucokinase (ATP:D-glucose 6-phosphotransferase, EC 2.7.1.1) plays a pivotal role in hepatic glucose metabolism and serves as the glucose sensor in pancreatic islet beta-cells. Biochemical studies of this enzyme are complicated by the cellular heterogeneity of the liver and the pancreas and because the presence of hexokinases (ATP:D-hexose 6-phosphotransferases, EC 2.7.1.1) seriously interferes with currently available analytical procedures. A radiometric assay was designed to deal with these problems. It is based on the liberation of 3H2O from D-[2-3H(N)]glucose 6-phosphate, the product of the glucokinase reaction, using exogenous phosphoglucose isomerase (D-glucose-6-phosphate ketol-isomerase, EC 5.3.1.9). Interference by hexokinases was largely eliminated by using glucose 6-phosphate as inhibitor and the sensitivity of the assay was greatly increased by using small volumes with the oil well procedure. The assay was sufficiently sensitive to detect about 1 pg of glucokinase. It thus allowed the application of quantitative histochemical procedures to the study of intralobular hepatic glucokinase profiles and the pancreatic beta-cell glucose sensor. The quantitative histochemical procedures were sufficiently sensitive and reliable for measuring important kinetic constants of glucokinase (i.e., the Km and the Hill number) in microscopic samples of tissue.

Adrenergic regulation of insulin secretion from the chicken pancreas in vitro

Adrenergic regulation of insulin secretion in the chicken was studied using a perifused pancreas fragment preparation. Beta-adrenergic stimulation by 50 microM isoproterenol potentiated theophylline-stimulated insulin secretion. Glucose at 19.5 mM did not stimulate insulin secretion, a finding consistent with previous reports of chicken pancreas sensitivity in vitro. Pretreatment with 50 microM isoproterenol did not alter this glucose insensitivity. Alpha-adrenergic stimulation by 50 microM epinephrine in the presence of beta blockade by sotalol or by 50 microM phenylephrine did not alter insulin secretion. Inhibition of insulin secretion by somatostatin could be demonstrated, however. Epinephrine, 50 and 0.164 microM, potentiated theophylline-stimulated insulin release and at 50 microM stimulated insulin secretion as an off-response even in the absence of theophylline. It is concluded that adrenergic regulation of insulin secretion in the chicken is primarily mediated through beta-adrenergic receptors, resulting in stimulation of insulin secretion.

Computer modeling identifies glucokinase as glucose sensor of pancreatic beta-cells

Previous work from this laboratory indicates that glucokinase serves as the glucose sensor of pancreatic islets. Here we show by nonlinear computer optimization that the kinetic properties of glucokinase (together with hexokinase, known to be present in islets) account for the observed glycolytic rates in islets as a function of glucose level. Alternative enzymes that have been suggested to perform the same function as glucokinase, N-acetyl-D-glucosamine kinase and glucose-6-phosphatase, are shown to have incompatible properties, including a poor fit, different curve shapes, and unreasonable parameter values resulting from optimization. Their activities in islets are shown to be too low to account for observed glucose usage rates. This work endorses our previous proposal that glucokinase acts as the glucose sensor in pancreatic islet cells.

New perspectives on pancreatic islet glucokinase

Control of blood sugar involves the complex interaction of the pancreatic glucose-sensing beta-cells with the liver, which serves as the primary site of glucose disposal after a meal. Glucokinase occupies an important role in controlling glucose phosphorylation and metabolism both in the liver and in pancreatic islets. In the beta-cells, glucokinase functions as pacemaker of glycolysis at physiological glucose levels. It determines the unique characteristics of islet hexose usage, that is, the rate, affinity, cooperativity, and anomeric discrimination of glucose metabolism. Because glycolysis controls hexose-induced insulin release, glucokinase is considered the best-qualified candidate for the elusive glucose sensor of beta-cells. A deficiency of glucokinase would disturb glucose homeostasis. Decreased islet glucokinase would diminish islet glycolysis and would result in a higher set point of beta-cells for glucose-induced insulin release. Decreased liver glucokinase would cause less efficient hepatic glucose disposal. Human maturity-onset diabetes (type II diabetes) has these characteristics. It is thus conceivable that certain forms of type II diabetes are due to a glucokinase deficiency.

Mannose phosphorylation by glucokinase from liver and transplantable insulinoma. Cooperativity and discrimination of anomers

Glucokinase from rat liver or transplantable, radiation-induced insulinomas was partially purified by ion exchange chromatography using DEAE-Cibacron Blue F3GA agarose. Phosphorylation of alpha,beta-D-mannose by glucokinase occurred with cooperative rate dependence on mannose concentration (nH: 1.50). Half-maximal phosphorylation rate occurred at 14 mM alpha,beta-D-mannose. The alpha- and beta-anomers of mannose were phosphorylated with sigmoidal kinetics (nH: 1.57 and 1.42, respectively). The affinity of glucokinase for alpha-D-mannose is higher than for beta-D-mannose (S0.5: 12 mM versus 19 mM). The maximum phosphorylation rate is slightly higher, about 10%, with beta-D-mannose than with alpha-D-mannose. Islet glucokinase has previously been shown to be chromatographically and kinetically identical to glucokinase from insulinoma and liver; therefore, evidence that glucokinase from these two tissues phosphorylates mannose with cooperative rate dependence and differentiates mannose anomers supports the glucokinase-glucose sensor hypothesis.

Discrimination of glucose anomers by glucokinase from liver and transplantable insulinoma

Phosphorylation of alpha- and beta-D-glucose by glucokinase from rat liver or a radiation-induced, transplantable insulinoma was investigated. Glucokinase partially purified by ion exchange chromatography on DEAE-Cibacron blue F3GA agarose was incubated for brief periods (1 or 3 min) with glucose anomers. Glucokinase from both liver and insulinoma tissue had a higher affinity for alpha-D-glucose (S0.5 = 6-7 mM) than beta-D-glucose (S0.5 = 12-14 mM). The maximum velocity was 15-20% lower for alpha-D-glucose than beta-D-glucose. Cooperative rate dependence with respect to glucose concentration was observed with both anomers (nH = 1.4). These kinetic data imply that both anomers of glucose are phosphorylated by glucokinase, however, at the physiological range of glucose concentrations below 15 mM, the higher affinity of alpha-D-glucose results in higher rates than with beta-D-glucose. At clearly pathological glucose concentrations exceeding 20 mM, the observed velocities are slightly higher with beta- than alpha-D-glucose. Glucokinase is thought to be the glucose sensor of pancreatic beta cells. The present data indicating a preferential phosphorylation of alpha-D-glucose compared to beta-D-glucose by glucokinase, supports the glucokinase-glucose sensor hypothesis, because it parallels the well established greater potency of alpha-D-glucose as a stimulant of insulin release.

Adrenergic regulation of avian pancreatic polypeptide secretion in vitro

Adrenergic regulation of avian pancreatic polypeptide (APP) secretion from perifused microfragments of chicken pancreas was investigated with catecholamines and adrenoreceptor-specific analogues added to avian Krebs-Ringer perifusion medium containing 11.1 mM glucose. APP release was stimulated by the alpha-adrenoreceptor agonist phenylephrine (threshold 5 microM; maximum effective concn, 50 microM) and the beta-adrenoreceptor agonist isoproterenol (threshold 5 microM; maximum effective concn, 50 microM). Epinephrine (0.164 microM) and 50 microM norepinephrine also stimulated APP release. The results obtained suggest that APP secretion from chicken pancreas is regulated by dual, complementary alpha- and beta-adrenoreceptor mechanisms.

Identification and significance of glucokinase in transplantable insulinomas

Glucose 6-phosphotransferases were investigated in two transplantable rat insulinoma tumor lines. Homogenates of tumors contained glucose phosphorylating activities of both high (e.g. supernatant Km = 0.060 mM and pellet Km = 0.077 mM) and low (Km = 7.6 mM) affinities for glucose. Chromatography of supernatants (105,000 X g) of insulinomas on DEAE-Cibacron Blue F3GA agarose evidenced glucose 6-phosphotransferase activity which eluted similarly to glucokinase from rat liver. Kinetic studies of insulinoma glucokinase also indicated similarity with liver glucokinase, i.e. cooperative rate dependence on glucose concentration and comparable Km values for glucose, and it did not phosphorylate N-acetylglucosamine. These characteristics of glucose 6-phosphotransferase in insulinomas are similar to those of the enzyme found in islets of Langerhans. Since glucokinase is thought to serve as glucose sensor of insulin secretory pancreatic beta-cells, these transplantable insulinomas offer great promise for biochemical and biophysical studies of the nature of glucose-induced insulin release.

Chromatographic resolution and kinetic characterization of glucokinase from islets of Langerhans

Glucokinase (ATP:D-glucose 6-phosphotransferase, EC 2.7.1.2) from rat islets of Langerhans was partially purified by chromatography on DEAE-Cibacron blue F3GA agarose. The enzyme eluted in two separate peaks. Sigmoidal rate dependence was found with respect to glucose (Hill coefficient = 1.5) for both enzyme fractions. Km values for glucose were 5.7 mM for the major fraction and 4.5 mM for the minor fraction. Neither fraction phosphorylated GlcNAc. A GlcNAc kinase (ATP:2-acetamido-2-deoxy-D-glucose 6-phosphotransferase, EC 2.7.1.59)-enriched fraction, prepared by affinity chromatography on Sepharose-N-(6-aminohexanoyl)-GlcNAc, had a Km of 25 microM for GlcNAc. Islet tissue also contained hexokinase (ATP:D-hexose 6-phosphotransferase, EC 2.7.1.1) eluting in multiple peaks. The results are consistent with the concept that glucokinase serves as the glucose sensor of pancreatic beta cells.