Regulation of insulin secretion by phospholipase C

Biphasic insulin secretion in response to a sustained glucose stimulus occurs when rat or human islets are exposed to high levels of the hexose. A transient burst of hormone secretion is followed by a rising and sustained secretory response that, in the perfused rat pancreas, is 25- to 75-fold greater than prestimulatory insulin release rates. This insulin secretory response is paralleled by a significant five- to sixfold increase in the phospholipase C (PLC)-mediated hydrolysis of islet phosphoinositide (PI) pools by high glucose. In contrast, mouse islets, when stimulated under comparable conditions with high glucose, display a second-phase response that is flat and only slightly (two- to threefold) greater than prestimulatory release rates. The minimal second-phase insulin secretory response to high glucose is accompanied by the minimal activation of PLC in mouse islets as well. However, stimulation of mouse islets with the protein kinase C (PKC) activator tetradecanoyl phorbol acetate (TPA) or the muscarinic agonist carbachol, which significantly activates an isozyme of PLC distinct from that activated by high glucose, induces a rising and sustained second-phase insulin secretory response. When previously exposed to high glucose, both rat and human islets respond to subsequent restimulation with an amplified insulin secretory response. They display priming, sensitization, or time-dependent potentiation. In contrast, mouse islets primed under similar conditions with high glucose fail to display this amplified insulin secretory response on restimulation. Mouse islets can, however, be primed by brief exposure to either TPA or carbachol. Finally, whereas rat islets are desensitized by chronic exposure to high glucose, mouse islet insulin secretory responses are relatively immune to this adverse effect of the hexose. These and other findings are discussed in relationship to the role being played by agonist-induced increases in the PLC-mediated hydrolysis of islet phosphoinositide pools and the activation of PKC in these species-specific insulin secretory response patterns.

Time-dependent effects of cholinergic stimulation on beta cell responsiveness

The effects of cholinergic stimulation on beta cell insulin secretory and phosphoinositide (PI) responses were determined in freshly isolated rat islets. Increasing the glucose level perifusing the islet from 5.6 to 8mM was accompanied by a modest insulin secretory response. The further addition of 10 microM carbachol increased peak first- and second-phase responses by 2.6- and 6. 8-fold, respectively. In the presence of 5.6 mM glucose, this low level (10 microM) of carbachol increased insulin release two- to three-fold, a response that was maintained for at least 60 min. In contrast to these acute stimulatory actions in the presence of glucose, chronic 3.5-h exposure of islets to 10 microM carbachol abolished beta cell insulin secretory responses to stimulation, with the combination of 8 mM glucose plus 10 microM carbachol. However, the further addition of 200 microM tolbutamide to these islets increased insulin secretory rates significantly. To establish the role of islet cell PI hydrolysis in these secretory responses, additional studies were conducted with islets whose PI pools were labeled with [3H]inositol. Acute exposure to 10 microM carbachol alone significantly increased inositol phosphate accumulation and the efflux of [3H]inositol, even in the absence of glucose. Including 10 microM carbachol during the labeling period with [3H]inositol resulted in significant impairments in subsequently measured inositol phosphate accumulation and [3H]inositol efflux responses to 8 mM glucose plus carbachol stimulation. Prior long-term exposure to 10 microM carbachol also induced heterologous desensitization: 20 mM glucose-stimulated insulin release and inositol phosphate accumulation were impaired in a parallel fashion. Chronic carbachol exposure had no deleterious effect on the usage of 8 or 20 mM glucose or on the insulin content of the islet. The acute stimulatory effects of carbachol on inositol phosphate accumulation as well as its inhibitory effect on 20 mM glucose-stimulated insulin release after prolonged exposure to the muscarinic agonist were significantly reduced by atropine. These findings demonstrate that changes in PI hydrolysis parallel those observed with insulin secretion and suggest that alterations in phospholipase C activation may account, at least in part, for the insulin secretory responses observed.

Signal transduction in isolated islets from the ob/ob mouse: enhanced sensitivity of protein kinase C to stimulation

The insulin secretory responses of islets isolated from ob/ob mice or their lean litter mates to glucose or the phorbol ester tetradecanoyl phorbol acetate were determined. Glucose-induced phospholipase C activation was also monitored. Even though lean mouse islets contained more insulin than ob/ob mouse islets, the first and second phases of 15mM glucose-induced secretion were significantly greater from ob/ob mouse islets. The kinetics of this amplified response were similar to those seen from lean islets as was the ability of 15mM glucose to activate phospholipase C. A striking dichotomy in responsiveness to the protein kinase C activator tetradecanoyl phorbol acetate was observed between lean and ob/ob mouse islets: while islets from lean animals were unresponsive to tetradecanoyl phorbol acetate (500nM), a rising and sustained insulin secretory response was evoked from ob/ob mouse islets. The combination of 7.5mM glucose plus tetradecanoyl phorbol acetate resulted in dramatic and sustained insulin secretory responses from ob/ob mouse islets, responses that could be duplicated by stimulation with the combination of 3mM glucose, 500nM tetradecanoyl phorbol acetate and 30mM potassium. Significantly smaller responses to these agonist combinations were observed from lean mouse islets. These findings demonstrate that the sensitivity of ob/ob mouse islet protein kinase C to stimulation is markedly enhanced when compared to islets from lean mice and that the activation of protein kinase C or processes distal to and dependent on the enzyme may account, at least in part, for the amplified insulin secretory responses of these islets.

Species differences in the induction of time-dependent potentiation of insulin secretion

The secretory responsiveness of the pancreatic beta-cell can be markedly improved by prior short term exposure to a stimulatory glucose level. Termed time-dependent potentiation (TDP), priming, or sensitization, this phenomenon has been documented to occur in both human and rat islets and my involve, at least in part, information flow in the phospholipase C and protein kinase C (PKC) signal transduction pathway. In contrast to human and rat islets, however, mouse islets fail to exhibit TDP in response to priming with high glucose. In the present series of studies, we explored in more detail the conditions and stimulants necessary for the induction of TDP in mouse islets and compared these responses with those obtained in rat islets. In agreement with previous reports, high (15 mM) glucose alone primed the rat beta-cell, but not the mouse beta-cell, to subsequent restimulation with 15 mM glucose. However, muscarinic stimulation of mouse islets with carbachol (100 microM) in the presence of 15 mM glucose primed the beta-cell to a subsequent 15-mM glucose stimulus. In addition, prior exposure to 50 nM of the PKC activator tetradecanoyl phorbol acetate dramatically amplified the subsequent insulin secretory responses of mouse islets to 15 mM glucose. In contrast to its significant inhibitory effect on glucose-induced insulin release from rat islets, the PKC inhibitor staurosporine (50 nM) had not effect on 15 mM glucose-induced release from control or prior glucose-exposed mouse islets. However, staurosporine significantly reduced the priming effect of tetradecanoyl phorbol acetate or carbachol on 15 mM glucose-induced insulin secretion from mouse islets. These findings emphasize the dramatic species differences that exist in the capacity of prior high glucose stimulation to induce TDP in rat and, presumably, human islets, on the one hand, and mouse islets, on the other. They also serve to emphasize the role of phosphoinositide hydrolysis and PKC activation in the induction of TDP.

Glucagon-like peptide-1 stimulates insulin secretion but not phosphoinositide hydrolysis from islets desensitized by prior exposure to high glucose or the muscarinic agonist carbachol

In the present series of experiments, the ability of the postulated incretin factor, glucagon-like peptide-1 (GLP-1), to stimulate insulin release from desensitized islets was determined. Compared with responses observed from control islets incubated for 3.5 hours with 5.6 mmol/L glucose alone, prior exposure to 10 mmol/L glucose, 20 mmol/L glucose, or 10 micromol/L carbachol reduced peak second-phase insulin release rates to a subsequent 20-mmol/L glucose stimulus by 63%, 81%, or 70%, respectively. Efflux of 3H-inositol from prior high-glucose- or carbachol-exposed islets was abolished and accumulation of inositol phosphates (IPs) in response to 20 mmol/L glucose was reduced. Further addition of 10 nmol/L GLP-1 together with 20 mmol/L glucose significantly increased insulin output from desensitized islets. Carbachol (10 micromol/L) preexposure also abolished the subsequent insulin secretory and 3H-inositol efflux responses to 8 mmol/L glucose plus 10 micromol/L carbachol. Inclusion of 10 nmol/L GLP-1 together with 8 mmol/L glucose plus 10 micromol/L carbachol improved but did not normalize secretion from these islets. These improvements in secretory responsiveness from high-glucose- or carbachol- desensitized islets occurred despite the lack of any apparent restorative effect of GLP-1 on agonist-induced increases in phosphoinositide (PI) hydrolysis. Finally, unlike the situation observed with carbachol or high-glucose preexposure, chronic exposure of islets to GLP-1 (100 nmol/L) did not desensitized islets to a subsequent 20 mmol/L glucose stimulus. We conclude from these studies that the incretin factor GLP-1 may play an important role in maintaining insulin output from islets in which phospholipase C (PLC)-mediated hydrolysis of islet PI pools in impaired. GLP-1 may prevent a further decline in beta-cell function and the associated deterioration in glucose tolerance that accompanies chronic exposure of islets to one of several agonists, including high glucose.

Regulation of insulin release by phospholipase C activation in mouse islets: differential effects of glucose and neurohumoral stimulation

Rat islets respond to glucose stimulation with a marked first and second phase increase in insulin secretion. In contrast, mouse islets have a similar first phase response but little second phase secretion. In these studies, we determined if activation of phospholipase C (PLC) accounts for these differences in second phase insulin secretion in these two species. Stimulation of freshly isolated mouse and rat islets with 15 mM glucose resulted in comparable first phase insulin secretion; however, the second phase response from mouse islets was only doubled from 28 +/- 6 to 60 +/- 7 pg/islet.min compared with an increase from 24 +/- 4 to 1064 +/- 93 pg/islet.min from rat islets. The addition of the muscarinic agonist carbachol (100 microM) in the presence of 15 mM glucose, however, markedly increased second phase insulin release from mouse islets to 801 +/- 80 pg/islet.min. Similar increases in second phase insulin release from mouse islets were obtained with the addition of 500 nM of the protein kinase C activator tetradecanoyl phorbol acetate in the presence of 15 mM glucose. However, the incretin factor glucagon-like peptide-1, which elevates islet cAMP levels, had little effect on second phase insulin release in the mouse. An analysis of PLC-mediated phosphoinositide (PI) hydrolysis revealed that 15 mM glucose increased inositol phosphate (IP) accumulation 0.5-fold above baseline in mouse islets compared with 3.7-fold in rat islets. In contrast, carbachol stimulated IP accumulation 3.5-fold in both mouse and rat islets. Analysis of PLC isozymes with isozyme specific monoclonal antibodies, demonstrated that mouse islets express 14 +/- 4% of PLC-delta 1 and 18 +/- 6% of PLC-beta 1 compared with rat islets but similar amounts of the PLC-gamma 1 (117 +/- 16%). These findings suggest that the decreased second phase insulin secretory response in mouse compared with rat islets results, at least in part, from an inability of high glucose to stimulate comparable increments in PI hydrolysis. This lack of glucose responsiveness may be due to the pronounced underexpression of specific PLC isozymes in the mouse.

Effects of short-term culturing on islet phosphoinositide and insulin secretory responses to glucose and carbachol

The ability of glucose and carbachol, alone or in combination, to stimulate islet cell phosphoinositide (PI) hydrolysis and insulin secretory responses in freshly isolated or in 20-24 h cultured rat islets was assessed. In freshly isolated, 3H-inositol-prelabeled islets, 20 mM glucose alone or 1 mM carbachol alone stimulated significant increments in 3H-inositol efflux and inositol phosphate (IP) accumulation. When stimulated with both agonists, a dramatic and synergistic effect on IP accumulation was noted. Carbachol (1 mM) alone had no sustained stimulatory effect on insulin secretion. Glucose (20 mM) alone induced a biphasic insulin secretory response. When compared to prestimulatory secretory rates of 18 +/- 4 pg/islet/min, peak first and second phase responses now averaged 422 +/- 61 and 1016 +/- 156 pg/islet/min, respectively. In contrast to freshly studied islets, culturing islets for 20-24 h in CMRL-1066 medium attenuated all measured responses. The increases in 3H-inositol efflux rates in response to glucose, carbachol, or their combination were significantly less than those observed with fresh islets. The IP responses were also attenuated. Second phase insulin secretory responses to 20 mM glucose alone 68 +/- 9 pg/islet/min) or the combination of 20 mM glucose plus 1 mM carbachol (358 +/- 85 pg/islet/min) were also significantly decreased when compared with fresh islets. We conclude from these studies that the process of culturing islets for one day in CMRL-1066 significantly decreases islet cell PI hydrolysis and insulin secretory responsiveness. These observations may help to explain the discordant conclusions reached concerning the involvement of PI hydrolysis and protein kinase C activation in the regulation of insulin release from freshly isolated versus cultured islets.

Synergistic interaction of glucose and neurohumoral agonists to stimulate islet phosphoinositide hydrolysis

The interaction between neurohumoral agonists and glucose to stimulate phosphoinositide (PI)-specific phospholipase C (PLC) and insulin release was examined. In freshly isolated rat islets, maximal glucose (40 mM), cholecystokinin (CCK; 300 nM), or carbachol (CCh; 1 mM) stimulated PI hydrolysis 6.5-, 9.8-, and 5.7-fold, respectively, above basal. The combination of glucose and CCK or of glucose and CCh, but not of CCK and CCh, synergistically increased PI hydrolysis 23.2- and 21.6-fold, respectively, indicating that these secretagogues activate PLC by distinct pathways and that there is an interaction between them. This synergy was maximal at physiological concentrations of stimulatory glucose (8-10 mM) and was paralleled by a marked synergistic stimulation of insulin secretion. The enhanced PI response was partially Ca2+ dependent and may involve the activation of distinct isozymes of PLC, which we identify in islets. These studies demonstrate for the first time a unique and highly sensitive synergistic interaction between glucose and neurohumoral agonists to stimulate PI hydrolysis, and they suggest that enhanced PI hydrolysis is important in the potentiation of glucose- and neurohumoral-stimulated insulin secretion.

Islet phosphoinositide hydrolysis and insulin secretory responses from prediabetic fa/fa ZDF rats

The sequence of events that culminate in the development of diabetes in the fa/fa Zucker diabetic fatty (ZDF) rat is unclear. In the present series of experiments islets from 5 week old prediabetic fa/fa male rats were isolated and their phosphoinositide (PI) hydrolysis and insulin secretory responses compared to those obtained from lean nondiabetic age- and weight-matched control rats. Peak first and second phase insulin secretory responses to 20mM glucose averaged 77 +/- 10 (mean +/- SE, n = 7) and 491 +/- 47 pg/islet/min from lean, nondiabetic control islets. The comparable responses from fa/fa prediabetic rat islets were significantly greater, 264 +/- 51 and 810 +/- 78 pg/islet/min. In a parallel fashion 3H-inositol efflux and inositol phosphate responses from prediabetic rat islets were also greater than comparable control responses. These findings demonstrate that significant increases in the phospholipase C-mediated hydrolysis of islet PI pools and insulin release in response to hyperglycemic stimulation can be detected prior to the emergence of diabetes in the fa/fa ZDF rat. These early changes in beta cell responsiveness to glucose may contribute to the hyperinsulinemia and subsequent insulin resistance characteristic of this animal model of non-insulin dependent diabetes mellitus.

Metabolic activation of Ca(2+)-independent phosphoinositide hydrolysis in beta-cells and its role in the control of insulin secretion

Recent studies have led to the proposal that the oxidative metabolism of glucose leads to the generation of messengers, in addition to ATP, that are important in the ability of changes in extracellular glucose concentration to stimulate insulin secretion from pancreatic beta-cells. In particular, there is now evidence that glucose induces both a Ca(2+)-dependent and Ca(2+)-independent increase in phosphoinositide (PI) hydrolysis. To explore the relationship between oxidative metabolism and PI hydrolysis, we examined the effect of low concentrations (2.5 mM) of alpha-ketoisocaproate (KIC) and monomethylsuccinate (MMSucc) either alone or in combination on insulin secretion and PI hydrolysis in isolated rat pancreatic islets incubated with either no glucose, 5 mM glucose, or 20 mM glucose. A combination of KIC and MMSucc leads to a marked increase in largely (80%) Ca(2+)-independent PI hydrolysis in either the absence or presence of 5 mM glucose. When glucose is absent, this combination of substrates induces a very small and transient first phase of insulin secretion but no significant second phase of secretion. In the presence of 5 mM glucose, either KIC or MMSucc alone induces a first phase of insulin secretion with a peak secretory rate 10-fold greater than the basal rate but only a small second phase of secretion approximately 5-fold above control. However, in the presence of 5 mM glucose, the combination of KIC plus MMSucc induces a large biphasic increase in insulin secretion: peak first-phase secretion is increased 30-fold, and second-phase 40-fold. These response are comparable to those induced by 20 mM glucose and are completely inhibited by 0.5 microM nitrendipine. In contrast, KIC plus MMSucc do not enhance the insulin secretory response induced by 20 mM glucose. Previous data showed that when 20 mM glucose acts, the resulting increase in PI hydrolysis is only partially Ca2+ dependent. A reanalysis of these data shows that raising the glucose concentration from 5 to 7 mM causes a 2-fold increase in Ca(2+)-independent PI hydrolysis, and a further increase to 20 mM leads to a further 2-fold increase in Ca(2+)-dependent PI hydrolysis. These data show that these two pathways are regulated by different ranges of glucose concentration. They raise the interesting possibility that these distinct pathways have different signaling functions. In particular, raising the glucose concentration from 5 to 7 mM is known to alter the responsiveness of beta-cells to a variety of neurohumoral agonists and to tolbutamide.(ABSTRACT TRUNCATED AT 400 WORDS)

Calcium and a mitochondrial signal interact to stimulate phosphoinositide hydrolysis and insulin secretion in rat islets

Fuel metabolism generates multiple signals that interact to stimulate insulin secretion. These studies explored the mechanism by which fuels activate phosphoinositide (PI) hydrolysis and the role of this signal transduction pathway in fuel-stimulated insulin secretion. High potassium (30 mM), which depolarizes the membrane and increases Ca2+ influx, caused only a transient monophasic release of insulin. In contrast, glucose (20 mM) or monomethylsuccinate (MMSucc; 10 mM) markedly stimulated a sustained insulin secretory response, indicating that fuel metabolism generates a signal(s) in addition to Ca2+ influx that is required for a sustained secretory response. On the other hand, diazoxide, an ATP-sensitive K+ channel activator that prevents membrane depolarization and Ca2+ influx in response to fuel metabolism, reduced the secretory responses to glucose and MMSucc to baseline levels, demonstrating that Ca2+ influx was essential to fuel-stimulated insulin secretion. The further addition of high K+ bypassed the diazoxide block and restored insulin secretory rates. The insulin secretory response to glucose or MMSucc in the presence of diazoxide and K+ was inhibited by the Ca2+ channel antagonist nitrendipine and the protein kinase-C inhibitor staurosporine. Changes in PI hydrolysis paralleled those in insulin secretion. High potassium alone induced only a modest 2.5-fold increase in inositol phosphate accumulation. This response was significantly less than that to glucose or MMSucc, which increased inositol phosphate accumulation by 6.8- or 5.2-fold, respectively. Like its effect on secretion, diazoxide markedly reduced glucose- or MMSucc-stimulated PI hydrolysis, and this inhibition was reversed with high K+. In contrast, diazoxide had no effect on receptor-activated PI hydrolysis stimulated by 100 nM cholecystokinin (CCK), and the effects of CCK were not dependent on added fuel, indicating that fuel and CCK activate PI hydrolysis by distinct pathways. These findings demonstrate that mitochondrial metabolism of glucose or MMSucc generates a signal(s) that interacts with Ca2+ influx to stimulate PI hydrolysis and sustained insulin secretion. This pathway of fuel-activated PI hydrolysis is distinct from that of CCK receptor-activated PI hydrolysis. These studies suggest that fuel-activated PI hydrolysis plays an important role in fuel-stimulated insulin secretion.

Comparative effects of monomethylsuccinate and glucose on insulin secretion from perifused rat islets

In the absence of any other exogenously added fuel, monomethylsuccinate, the methyl ester of succinic acid, at 10-20 mM stimulates insulin release in a biphasic pattern. In quantitative terms, first-phase release evoked by 20 mM MMSucc was comparable to that observed with 20 mM glucose but second-phase release was only 20% of the glucose-induced response. Secretion to both MMSucc and glucose was virtually abolished by the calcium channel antagonist nitrendipine (0.5 microM). In islets that had phosphoinositide pools labeled with [3H]inositol for 2 h, subsequent stimulation with 20 mM MMSucc results in dramatic and sustained increases in [3H]inositol efflux rates. Inositol phosphate levels are also increased. In contrast to secretion, the increase in phosphoinositide hydrolysis caused by MMSucc was largely resistant to nitrendipine, whereas significant reductions in glucose-induced phosphoinositide hydrolysis were observed in the presence of the calcium channel antagonist. MMSucc (2.75-10 mM) substitutes for glucose in that MMSucc supported the insulinotropic effects of the sulfonylurea tolbutamide (200 microM) and the gut hormone cholecystokinin (200 nM). A prior 15-min exposure to 20 mM MMSucc also sensitized islets to the stimulatory effects of 7.5 mM glucose. Finally, a 2-h exposure to 20 mM MMSucc desensitized the islet, in terms of both phosphoinositide hydrolysis and insulin secretion, to a subsequent exposure to 10 mM glucose. Thus, appropriate concentrations of MMSucc can cause qualitatively many of the effects induced by glucose.(ABSTRACT TRUNCATED AT 250 WORDS)

Influence of glucagon-like peptide-1 on beta cell responsiveness

The postulated incretin factor glucagon-like peptide-1 (GLP-1) causes a glucose-dependent increase in insulin secretion from perifused rat islets. In the presence of 6 mM glucose the response to 10 nM GLP-1 is characterized by a large initial spike of secretion, followed by a brief, slowly rising phase. However, after 30-40 min of stimulation, this phase subsides to prestimulatory secretory rates. Raising the glucose level to 8 mM, however, amplifies and sustains the stimulatory effect of 10 nM GLP-1. The response to GLP-1 (10 nM) in the presence of 8 mM glucose is abolished by the metabolic inhibitor mannoheptulose (15 mM), and reduced by the calcium channel antagonist nitrendipine (5 microM), or the protein kinase C inhibitor of staurosporine (20 nM). A significant synergistic effect of GLP-1 (10 nM) and 10 microM carbachol, a cholinergic agonist, on insulin secretion was observed in the presence of 6 mM glucose. In the presence of either 6 or 8 mM glucose, GLP-1 (10 nM) has no significant effect on glucose usage or on inositol phosphate generation in [3H]inositol prelabeled islets. The results support the concept that GLP-1 may function as an important physiologic incretin factor, particularly when accompanied by agonists that activate phosphoinositide hydrolysis.

Biochemical mechanisms involved in monomethyl succinate-induced insulin secretion

Esters of succinic acid stimulate insulin secretion from pancreatic beta-cells. Using collagenase-isolated rat islets, the transduction mechanisms involved were investigated. In freshly isolated perifused islets, monomethyl succinate (MMSucc), in the presence of basal (2.75 mM) glucose, stimulated insulin release in a biphasic pattern. This secretory response was dependent on extracellular calcium movement into the beta-cell, since the calcium channel blocker nitrendipine (5 microM) abolished it. The glucokinase inhibitor mannoheptulose (20 mM) had no effect on its secretory action, while the protein kinase-C inhibitor staurosporine (20 nM) reduced secretion to MMSucc. In addition, while ineffective alone, the diacylglycerol kinase inhibitor monooleoylglycerol (25 microM) potentiated MMSucc-induced insulin release. A similarly amplified response occurred in the presence of forskolin (0.25 microM), a compound that elevates islet cAMP levels. The sodium salt of succinic acid (20 mM) had no effect on insulin release in the presence or absence of forskolin. Prior treatment with MMSucc in the presence of 2.75 mM glucose sensitized islets to the usually weak insulin secretory effect of 7.5 mM glucose. Other groups of islets were incubated for 2 h with myo-[2-3H]inositol to label their phosphoinositide pools. These islets were subsequently stimulated, and the kinetics of [3H]inositol efflux and insulin secretion were measured. MMSucc induced a rapid and sustained dose-dependent increase in [3H]inositol efflux rates. In batch-incubated islets, MMSucc increased inositol phosphate levels. Finally, MMSucc (20 mM), in the presence of 8 mM glucose, did not influence the detritiation of [5-3H]glucose, but reduced the oxidation of [U-14C] glucose. These results support the following conclusions. First, MMSucc is a potent activator of islet phosphoinositide hydrolysis. Second, the activation of protein kinase-C appears to contribute to the acute insulin secretory effect of MMSucc. Third, MMSucc-induced increases in phosphoinositide hydrolysis contribute at least in part to its ability to acutely stimulate insulin release and prime the beta-cell to subsequent stimulation. Finally, mitochondrial events associated with the oxidative metabolism of MMSucc may underlie its insulinotropic action.

Glucosamine-induced desensitization of beta-cell responses: possible involvement of impaired information flow in the phosphoinositide cycle

The influence of glucosamine on beta-cell response characteristics of collagenase-isolated rat islets was determined. Groups of islets were incubated for 2 h with myo-[2-3H]inositol to label their phosphoinositide (PI) pools. Also included in some experiments was glucosamine (0.1-10 mM). Subsequently, these islets were perifused, and their responses to 10 mM glucose, 10 mM alpha-ketoisocaproate (KIC), and 1 microM of the phorbol ester phorbol 12-myristate 13-acetate were assessed. Increases in PI hydrolysis were monitored during the perfusion by measuring fractional efflux rates of [3H]inositol. The accumulation of inositol phosphates after the perifusion was also determined. In other experiments, the use of 10 mM glucose was measured after a 2-h exposure to 5 or 10 mM glucosamine. Finally, the ability of glucosamine itself to augment release and activate PI hydrolysis was assessed. The following observations were made. 1) A prior 2-h exposure to 5-10 mM glucosamine resulted in parallel dose-dependent impairments in 10 mM glucose-induced insulin release and PI hydrolysis. 2) Glucosamine (5-10 mM) also impaired the subsequent response to alpha-ketoisocaproate (KIC). Parallel deficits in KIC-induced PI hydrolysis were noted under conditions where insulin secretion was impaired. 3) Under several conditions where glucosamine impaired glucose-induced secretion, it had no adverse effect on phorbol 12-myristate 13-acetate-induced release. 4) The desensitizing effect of 10 mM glucosamine on 10 mM glucose-induced release and PI hydrolysis developed within 30 min of exposure to it. 5) Glucosamine (5-10 mM) preexposure had no adverse effect on the use of 10 mM glucose by desensitized islets. 6) Short term (5-min) exposure to glucosamine (10 mM) alone stimulated PI hydrolysis, while a 30-min exposure to the same level of the hexosamine depressed it. 7) In the presence of 0.25 microM forskolin, 10 mM glucosamine also had a transient stimulatory effect on insulin release. These findings support the concept that the acute and chronic effects of glucosamine on the beta-cell result at least in part from its ability to influence PI hydrolysis in islets.

Influence of staurosporine, nitrendipine and monooleoylglycerol on interleukin-1-induced insulin secretion and phosphoinositide hydrolysis

The monokine interleukin-1 alpha (IL-1) induces a glucose-dependent increase in insulin secretion, an effect tentatively attributed to its ability to increase beta cell phosphoinositide (PI) hydrolysis. In the present experiments, the effects of the protein kinase C inhibitor staurosporine (20 nM), the calcium channel antagonist nitrendipine (5 microM), and the diacylglycerol kinase inhibitor monooleoylglycerol (MOG, 25 microM) on 40 nM IL-1-induced increments in insulin release from perifused islets and inositol phosphate levels in [3H]inositol prelabeled islets were assessed. In perifused islets, insulin secretion in response to IL-1 in the presence of 7 mM glucose averaged 313 +/- 43 pg/islet/min 35-40 min after the onset of stimulation. Release from control islets perifused in the presence of 7 mM glucose alone averaged 56 +/- 6 pg/islet/min at this time point. The addition of staurosporine together with IL-1 reduced insulin secretion at this time point to 88 +/- 21 pg/islet/min. This level of IL-1 caused significant increases in inositol phosphate accumulation in the presence of 7 mM glucose but not 2.75 mM glucose. Staurosporine was without a significant effect on inositol phosphate accumulation in response to the monokine. In contrast, nitrendipine (5 microM) inhibited insulin release and inositol phosphate accumulation in a parallel fashion. Finally, MOG significantly amplified release to the monokine without significantly affecting its impact on inositol phosphate accumulation. Nitrendipine or staurosporine blocked this amplifying effect of MOG on secretion. These results emphasize the role of PI hydrolysis in IL-1-induced insulin secretion and suggest further that calcium influx is essential for IL-1 to fully activate both PI hydrolysis and insulin secretion.

Influence of staurosporine on glucose-mediated and glucose-conditioned insulin secretion

The effect of staurosporine, a putative inhibitor of protein kinase C (PKC), on insulin secretion induced by glucose and 4-methyl-2-oxopentanoate (KIC) was examined. In addition, the effects of staurosporine on the actions of other agonists, for which glucose acts as a conditional modifier, were also examined. At 20 nM, staurosporine caused a marked inhibition of second-phase insulin secretion, whether it was stimulated by 10 mM- or 20 mM-glucose, by 15 mM-KIC, or by carbachol or tolbutamide in islets co-perifused with 7.0 mM-glucose. In each case, the second-phase secretory response was inhibited by 70-85%. In contrast, in all cases there was no effect of staurosporine on the magnitude of the first phase of insulin secretion, nor on the time course of first-phase secretion, except when glucose alone was the secretagogue. With either 10 mM- or 20 mM-glucose, the peak of the first phase of insulin secretion was delayed. Staurosporine does not alter glucose metabolism, or the ability of glucose to activate phosphoinositide hydrolysis or to cause the translocation of alpha-PKC to the membrane. These findings support the concept that PKC activation plays an important role in fuel-induced or fuel-conditioned insulin secretion.

Effects of the phorbol ester phorbol 12-myristate 13-acetate (PMA) on islet-cell responsiveness

Collagenase-isolated rat islets were labelled for 2 h in myo-[2-3H]inositol solution supplemented with 2.75 mM-glucose. The phorbol ester phorbol 12-myristate 13-acetate (PMA; 0.1 or 1 microM) was also present in some experiments. After labelling, islets were washed and then perifused in 2.75 mM-glucose to establish basal [3H]inositol-efflux and insulin-secretory rates. Subsequently, the responses of these islets to stimulation with various agonists were assessed. Inositol phosphate accumulation was measured at the termination of the perifusion. In separate experiments, the cellular location of protein kinase C (PKC) after PMA pretreatment was measured by quantitative immunoblotting of membrane and cytosolic fractions. The following observations were made. (1) Labelling in 0.1-1 microM-PMA had no deleterious effect on total [3H]inositol incorporation during the 2 h labelling period. However, islets labelled for 2 h in 1 microM-PMA were unable to respond, in terms of increases in insulin release, to a 1 microM-PMA stimulus during the subsequent perifusion. (2) As compared with the responses of control islets labelled in 2.75 mM-glucose alone, islets labelled in the additional presence of 1 microM-PMA displayed a significant impairment in phosphoinositide (PI) hydrolysis, but an enhancement of both first-and second-phase insulin secretion, in response to subsequent 20 mM-glucose stimulation. (3) Decreasing extracellular Ca2+ level to 0.1 mM and including the Ca(2+)-channel antagonist nitrendipine (0.5 microM) along with 1 microM-PMA during the [3H]inositol-labelling period did not alter the response of the islets to the subsequent addition of 20 mM-glucose. Glucose-induced PI hydrolysis was still inhibited and 20 mM-glucose-induced insulin release was still enhanced. (4) A markedly amplified and sustained insulin-secretory response to 200 microM-tolbutamide in the presence of 2.75 mM-glucose was also obtained from 1 microM-PMA-pretreated islets. This contrasts sharply with the small and transient response to tolbutamide noted in control islets. (5) When present only during the perifusion phase of the experiments, nitrendipine (0.5 microM) abolished the amplified insulin-secretory responses to both 20 mM-glucose and 200 microM-tolbutamide noted in PMA-pretreated islets. (6) Prior labelling in 1 microM-PMA dramatically amplified the insulinotropic effect of 25 mM-K+ or 5 microM-A23187 stimulation. The amplified insulin-secretory response to K+, but not to A23187, was abolished by inclusion of nitrendipine during the perifusion.(ABSTRACT TRUNCATED AT 400 WORDS)

Physiology and pathophysiology of insulin secretion

Mechanisms by which various classes of extracellular signals regulate insulin secretion are discussed regarding their cellular and molecular actions. Under physiological circumstances, the small postprandial changes in plasma glucose concentrations (approximately 4.4-6.6 mM) primarily serve as a conditioned modifier of insulin secretion and dramatically alter the responsiveness of islets to a combination of neurohormonal agonists. These agonists have two functions. Cholecystokinin (CCK) and acetylcholine activate the hydrolysis of polyphosphoinositides, and gastric inhibitory polypeptide (GIP) and glucagonlike peptide 1 activate adenylate cyclase. These two functional classes of neurohumoral agonists act synergistically to enhance insulin secretion when plasma glucose is greater than 6.0 mM but not when it is less than or equal to 4 mM. On the other hand, an increase in plasma glucose concentration to 8-10 mM induces an increase in insulin secretory rate in the absence of any of the neurohormonal agonists. Remarkably, high glucose leads to an increase in the same intracellular signals, as does a combination of acetylcholine and GIP. On the basis of these data, a model of how insulin secretion is regulated under physiological circumstances is proposed. This model emphasizes that the regulation of insulin secretion occurs in three stages: cephalic, early enteric, and later enteric. In this view, the crucial event occurring during the first two phases is the agonist-induced, translocation of protein kinase C (PKC) to the plasma membrane under conditions in which an increase in Ca2+ influx does not occur. PKC is now in a cellular location and a Ca2(+)-sensitive conformation such that an increase in Ca2+ influx rate occurring during the third phase leads to its immediate activation and an enhanced rate of insulin secretion. Furthermore, under physiological circumstances, an optimal insulin secretory response is dependent on a correct temporal pattern of signals arising from neural and enteric sources. If this pattern is deranged, an abnormal pattern of insulin secretion is observed. An important new insight is provided by the observation that agonists (e.g., CCK or acetylcholine) that act to stimulate the hydrolysis of phosphatidylinositides, when acting for a short period (10-20 min), induce an enhanced responsiveness of islets to glucose, i.e., proemial sensitization. However, when acting unopposed for several hours, these agonists will induce a time-dependent suppression of responsiveness to glucose and other agonists. The latter observation implies that optimal insulin secretion is dependent on periodic rather than a continuous exposure to the correct pattern of extracellular signals.(ABSTRACT TRUNCATED AT 400 WORDS)

Forskolin-induced desensitization of pancreatic beta-cell insulin secretory responsiveness: possible involvement of impaired information flow in the inositol-lipid cycle

Isolated rat islets of Langerhans were incubated for 2 h in a myo-[2-3H]inositol-containing solution to label their phosphoinositides. Also included during this labeling period was forskolin (0.1-5 microM), a compound established to elevate islet cAMP levels. These islets were subsequently perifused, and their insulin secretory responses to 20 mM glucose or 1 microM of the phorbol ester 12-O-tetradecanoyl phorbol-13-acetate (TPA) were assessed. Determined in parallel with secretion were [3H] inositol efflux patterns and, at the termination of the perifusion, labeled inositol phosphate accumulation. The following major observations were made. 1) Forskolin had no deleterious effect on the total amount of [3H]inositol incorporated by the islets during the labeling period. 2) However, labeling in forskolin resulted in subsequent dose-dependent decreases in 20 mM glucose-induced insulin secretion, [3H]inositol efflux and inositol phosphate accumulation. 3) Inclusion of the diacylglycerol (DAG) kinase inhibitor monooleoylglycerol (50 microM) restored to a significant degree glucose-induced release from forskolin-desensitized islets. 4) Pretreatment with 5 microM forskolin had no deleterious effect on TPA-induced insulin release. 5) Prior exposure to forskolin also impaired phosphoinositide hydrolysis in response to cholecystokinin stimulation. 6) Similar to forskolin, labeling in isobutylmethylxanthine (1 mM) reduced in a parallel fashion islet [3H]inositol efflux and insulin secretion in response to 20 mM glucose stimulation. These findings demonstrate that prior chronic elevation of islet cAMP levels suppresses the activation of phospholipase-C in response to subsequent stimulation. Defective insulin secretory responsiveness of these islets appears to be the result of impaired generation of phosphoinositide-derived second messenger molecules, particularly DAG. By substituting for DAG, however, TPA circumvents this biochemical lesion and evokes a normal insulin secretory response from forskolin-pretreated islets.

Influence of monooleoylglycerol on islet cell phosphoinositide hydrolysis and insulin secretion

The diacylglycerol kinase inhibitor monooleoylglycerol (MOG) produced a dose-dependent increase in both phosphoinositide (PI) hydrolysis and insulin secretion in the presence of a substimulatory glucose level (2.75 mM). This effect could not be reproduced by the combination of oleic acid plus glycerol, potential metabolic products derived from MOG catabolism. At a level (25 microM) which has no significant effect on beta cell insulin secretion or PI hydrolysis in the presence of 2.75 mM glucose, MOG significantly potentiated the insulin stimulatory effect of the sulfonylurea tolbutamide (200 microM) in the presence of 7 mM glucose. This heightened insulin secretory response and PI hydrolysis were effectively attenuated by the calcium channel blocker nitrendipine (0.5 microM). These findings indicate that MOG has complex effects on beta cell performance. It promises, however, to be a useful probe in assessing how events associated with increases in PI hydrolysis influence insulin secretion.

Chronic in vivo hyperglycemia impairs phosphoinositide hydrolysis and insulin release in isolated perifused rat islets

We examined the effect of chronic hyperglycemia on phosphoinositide hydrolysis and insulin secretion in isolated perifused rat islets. Rats were infused for 44 h with 40% dextrose in order to raise and maintain the plasma glucose concentration at 350 mg/dl. Control animals were infused with equiosmolar amounts of mannitol. In vivo insulin secretion and rats of glucose disposal were monitored throughout the study. At the end of the infusion, islets were collagenase isolated, and phosphoinositide (PI) hydrolysis (assessed by measuring the increment in [3H]inositol efflux as well as labeled inositol phosphates) and insulin output in response to a 20-mM glucose challenge were quantitated. Plasma insulin concentration and in vivo glucose disposal rates decreased significantly, by 47% and 35% respectively, after 6-8 h of hyperglycemia. In islets perifused immediately after isolation, prior in vivo hyperglycemia markedly altered the pattern of insulin output in response to 20-mM glucose challenge. Compared to mannitol infusion, 20 mM glucose stimulation resulted in an exaggerated first phase insulin secretory response (1121 +/- 88 vs. 467 +/- 75 pg/islets.min) and a blunted second phase insulin secretory response (392 +/- 90 vs. 1249 +/- 205 pg/islet.min). In islets prelabeled with myo-[2-3H]inositol for 2 h, PI hydrolysis, particularly [3H]inositol efflux in response to glucose stimulation was also reduced (0.28 +/- 0.03%/min) compared to that in mannitol-infused animals (0.53 +/- 0.08%/min). Two hours of preincubation in a low glucose medium (2.75 mM) were able to completely reverse the islet defect in both PI hydrolysis and insulin secretion. Our results demonstrate that chronic in vivo hyperglycemia impairs PI hydrolysis in perifused rat islets and suggest that this defect accounts in part for the abnormal pattern of glucose-induced insulin secretion.

Cholinergic agonists prime the beta-cell to glucose stimulation

The ability of the cholinergic agonists carbachol or acetylcholine to stimulate insulin release, activate phosphoinositide hydrolysis, and prime the beta-cell to the insulin stimulatory effect of 7.5 mM glucose was assessed. In the presence of 7 mM glucose, but not 2.75 mM glucose, 1 mM carbachol evoked a sustained insulin secretory response. At both glucose levels, carbachol stimulated phosphoinositide hydrolysis, an event monitored in myo-[2-3H]inositol-prelabeled islets by increases in [3H]inositol efflux and labeled inositol phosphate accumulation. Prior exposure to carbachol (0.1-1 mM) resulted in a dose-dependent increase in the subsequent insulin secretory response to 7.5 mM glucose. This sensitization developed within 2 min and lasted for at least 45 min after carbachol removal from the perifusion medium. Carbachol pretreatment also sensitized the islet to either 200 microM tolbutamide or 10 mM arginine. Prior exposure to 1 mM acetylcholine induced a similar proemial sensitization to a subsequent challenge with glucose. These results demonstrate that even though cholinergic stimulation increases phosphoinositide hydrolysis, this event is insufficient to initiate sustained insulin secretion from islets exposed to a low (2.75 mM) glucose concentration. However, this increase in phosphoinositide hydrolysis sensitizes islets to a subsequent challenge with one of several different stimuli, including glucose. Hence, this sensitization of islets to physiologically relevant glucose concentrations may represent the major contribution of vagal stimulation to the regulation of insulin secretion.

Interactions between lithium, inositol and mono-oleoylglycerol in the regulation of insulin secretion from isolated perifused rat islets

In response to stimulation by 20 mM-glucose, 15 mM-4-methyl-2-oxopentanoate or 10 mM-glyceraldehyde, isolated perifused rat islets respond with brisk biphasic insulin-secretory responses. The inclusion of 10 mM-LiCl significantly decreased second-phase insulin release in response to all agonists. Inositol, at a concentration (10 mM) which has no effect on secretion in the presence of 2.75 mM-glucose, restored significantly glucose-, 4-methyl-2-oxopentanoate- or glyceraldehyde-induced second-phase release from Li+-treated islets. The addition of the diacylglycerol kinase inhibitor mono-oleoylglycerol, at a concentration (25 microM) which has no stimulatory effect on insulin secretion in the presence of 2.75 mM-glucose, significantly amplified both the first- and second-phase insulin responses to 20 mM-glucose. This amplifying effect of mono-oleoylglycerol was readily reversible and dependent on Ca2+ influx into the beta-cell. Li+ decreased the amplified insulin response to 20 mM-glucose plus mono-oleoylglycerol. Inositol restored release under this condition. These findings suggest that Li+ inhibits release by sequestering inositol into biosynthetically ineffective inositol phosphates. By limiting phosphoinositide resynthesis, the continued hydrolysis of phosphoinositides is diminished. Our results with mono-oleoylglycerol suggest further that diacylglycerol content may play a critically important role in the regulation of both the first and second phases of insulin secretion.

Interactions between cholinergic agonists and enteric factors in the regulation of insulin secretion from isolated perifused rat islets

The ability of the cholinergic agonist carbachol to sensitize islets to the action of combined glucose, cholecystokinin and gastric inhibitory polypeptide was determined in isolated rat islets. In response to this combination, peak first phase insulin secretion from control islets averages 85 +/- 5 pg.islet-1.min-1 (mean +/- SEM) and the insulin secretory rates measured 35-40 min after the onset of stimulation averages 127 +/- 34 pg.islet-1.min-1. A prior 20 min exposure to 1 mmol/l carbachol potentiates the modest insulin stimulatory response to this combination of stimulants: peak first phase release is 354 +/- 61 pg.islet-1.min-1, and release measured 35-40 min after the onset of stimulation is 179 +/- 34 pg.islet-1.min-1. This sensitizing effect of carbachol lasts for at least 40 min and can be duplicated by the natural in vivo agonist acetylcholine. These results demonstrate that cholinergic stimulation of isolated islets primes them to the subsequent stimulatory effect of a moderate increase in the circulating glucose level and to several postulated incretin factors. If operative in vivo, this communications network between cephalic and enteric factors represents a remarkable control system to ensure the release of insulin in amounts commensurate to meet the anticipated and actual insulin requirements for insulin-mediated fuel disposition.