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

Enhanced activation of phospholipase C and insulin secretion from islets incubated in fatty acid-free bovine serum albumin

Incubation in 100 micromol/L fatty acid-free bovine serum albumin (FAF-BSA) significantly amplifies insulin secretion from isolated, perifused rat islets. When compared with the responses of control islets incubated in 100 micromol/L radioimmunoassay-grade BSA, insulin secretion rates were increased 2- to 3-fold when these islets were stimulated with 10 mmol/L glucose alone or with the combination of 10 mmol/L glucose, 15 mmol/L KCl, and 100 micromol/L diazoxide. These amplified secretory responses were paralleled by significant increases in the phospholipase C (PLC) activation monitored by fractional increases in (3)H-inositol efflux from these same islets. Amplified PLC responses were also observed with the cholinergic agonist carbachol (50 micromol/L). No differences in the secretory responses to the protein kinase C activator phorbol 12-myristate 13-acetate (200 nmol/L) could be detected between control and FAF-BSA-pretreated rat islets. Mouse islets were also immune to the amplifying impact of this treatment protocol. These findings demonstrate that short-term incubation in FAF-BSA significantly augments the activation of PLC in rat islets by a number of agonists. This proximal event provides the impetus for the distal activation of protein kinase C. If applicable to human islets, this manipulation may provide a mechanism to enhance the secretory responses from islets destined for transplantation, thus improving their in vivo secretory capacity.

Biphasic insulin secretion from freshly isolated or cultured, perifused rodent islets: comparative studies with rats and mice

In the present report, we compared the insulin secretory responses of freshly isolated, perifused rat and mouse islets to glucose. Prestimulatory glucose levels were changed to assess their influence on the subsequent secretory responses. Additional studies included experiments with the incretin factor glucagon-like peptide-1 (GLP-1), the cholinergic agonist carbachol, and the alpha2 agonist epinephrine. Our findings demonstrate that under conditions where glucose (8.5-11.1 mmol/L) evokes a dramatic biphasic insulin secretory response from perifused rat islets, mouse islets exhibit little response. Increasing the prestimulatory glucose level to 8.5 mmol/L dramatically distorts subsequently measured glucose-induced insulin secretion from rat islets but allows the evocation of a modest but clear biphasic response from mouse islets in response to 30 mmol/L, but not 11.1 or 16.7 mmol/L, glucose. In the presence of a minimally effective glucose level (10 mmol/L), mouse islets remain exquisitely sensitive to the combined stimulatory effects of GLP-1 (2.5 nmol/L) plus carbachol (0.5 micromol/L) and to the inhibitory influence of epinephrine (10 nmol/L). Short-term culture of rat islets in CMRL 1066 containing 5.6 mmol/L glucose resulted in a significant decrease in the secretory response to 11.1 mmol/L glucose, whereas the same manipulation improved mouse islet responses. It is concluded that the process of collagenase isolating islets does not alter mouse islet sensitivity in any adverse way and that increasing the prestimulatory glucose level can indeed alter the pattern of insulin secretion in either a positive or negative manner depending upon the species being investigated. Prior short-term culture of rodent islets differentially affects secretion from these 2 species.

Divergent effects of epinephrine and prostaglandin E2 on glucose-induced insulin secretion from perifused rat islets

The impact of the catecholamine epinephrine and the postulated inhibitory second messenger prostaglandin E(2) (PGE(2)) on the kinetics and magnitude of glucose-induced insulin secretion were compared and contrasted. In agreement with a number of studies, epinephrine was a most effective antagonist of glucose-induced insulin secretion. Dose-response studies using 8 to 10 mmol/L glucose as stimulant established that levels as low as 1 to 10 nmol/L of the catecholamine were effective at inhibiting release. Glucose (20 mmol/L) caused an approximately 25-fold increase in insulin secretion, an effect that was completely abolished by 1 micromol/L epinephrine. Under conditions where it completely abolished 20 mmol/L glucose-induced insulin release, epinephrine (1 micromol/L) reduced, but did not abolish, the stimulatory effect of glucose on phospholipase C activation. Chronic 3-hour exposure to 10 mmol/L glucose alone desensitized the islet to subsequent stimulation by glucose. Despite its ability to completely suppress secretion to 10 mmol/L glucose, epinephrine failed to protect the islet from hyperglycemia-induced desensitization. In sharp contrast to epinephrine, PGE(2) at levels ranging from 1 to 10 micromol/L had no discernible adverse effect on 10 mmol/L glucose-induced secretion. These findings suggest that multiple mechanisms contribute to the inhibitory impact of epinephrine on release and, in conjunction with other studies, cast serious doubt on the concept that PGE(2) plays any significant inhibitory role in the regulation of glucose-induced secretion.

Desensitization of the pancreatic beta-cell: effects of sustained physiological hyperglycemia and potassium

The impact of modest but prolonged (3 h) exposure to high physiological glucose concentrations and hyperkalemia on the insulin secretory and phospholipase C (PLC) responses of rat pancreatic islets was determined. In acute studies, glucose (5-20 mM) caused a dose-dependent increase in secretion with maximal release rates 25-fold above basal secretion. When measured after 3 h of exposure to 5-10 mM glucose, subsequent stimulation of islets with 10-20 mM glucose during a dynamic perifusion resulted in dose-dependent decrements in secretion and PLC activation. Acute hyperkalemia (15-30 mM) stimulated calcium-dependent increases in both insulin secretion and PLC activation; however, prolonged hyperkalemia resulted in a biochemical and secretory lesion similar to that induced by sustained modest hyperglycemia. Glucose- (8 mM) desensitized islets retained significant sensitivity to stimulation by either carbachol or glucagon-like peptide-1. These findings emphasize the vulnerability of the beta-cell to even moderate sustained hyperglycemia and provide a biochemical rationale for achieving tight glucose control in diabetic patients. They also suggest that PLC activation plays a critically important role in the physiological regulation of glucose-induced secretion and in the desensitization of release that follows chronic hyperglycemia or hyperkalemia.

Acute and chronic effects of glucose and carbachol on insulin secretion and phospholipase C activation: studies with diazoxide and atropine

The acute and chronic effects of 20 mM glucose and 10 microM carbachol on beta-cell responses were investigated. Acute exposure of rat islets to 20 mM glucose increased glucose usage rates and resulted in a large insulin-secretory response during a dynamic perifusion. The secretory, but not the metabolic, effect of 20 mM glucose was abolished by simultaneous exposure to 100 microM diazoxide. Glucose (20 mM) significantly increased inositol phosphate (IP) accumulation, an index of phospholipase C (PLC) activation, from [(3)H]inositol-prelabeled islets. Diazoxide, but not atropine, abolished this effect as well. Unlike 20 mM glucose, 10 microM carbachol (in the presence of 5 mM glucose) increased IP accumulation but had no effect on insulin secretion or glucose (5 mM) metabolism. The IP effect was abolished by 50 microM atropine but not by diazoxide. Chronic 3-h exposure of islets to 20 mM glucose or 10 microM carbachol profoundly reduced both the insulin-secretory and PLC responses to a subsequent 20 mM glucose stimulus. The adverse effects of chronic glucose exposure were abolished by diazoxide but not by atropine. In contrast, the adverse effects of carbachol were abolished by atropine but not by diazoxide. Prior 3 h of exposure to 20 mM glucose or carbachol had no inhibitory effect on glucose metabolism. Significant secretory responses could be evoked from 20 mM glucose- or carbachol-pretreated islets by the inclusion of forskolin. These findings support the concept that an early event in the evolution of beta-cell desensitization is the impaired activation of islet PLC.

Targeted expression of the anti-apoptotic gene CrmA to NOD pancreatic islets protects from autoimmune diabetes

The activation of apoptosis is a critical mechanism by which pancreatic beta cells are destroyed in type 1 diabetes (T1DM). Strategies aimed at interfering with the apoptotic pathways could therefore be of potential therapeutic value. To this end, we generated NOD transgenic mice with targeted expression of the anti-apoptotic gene Cytokine response modifier A (CrmA) to pancreatic beta cells using the rat insulin promoter and the reverse tetracycline transactivator to express CrmA in a temporally controlled manner. Two lines of transgenic mice were studied whose expression of CrmA occurred only after feeding doxycycline food. Islet expression of CrmA partially protected pancreatic beta cells from the cytokine-mediated cytotoxicity in vitro and reduced modestly the spontaneous development of diabetes in NOD mice in vivo. In addition, beta cells from NOD CrmA mice were significantly protected from the destruction by diabetogenic T cells after adoptive transfer. More strikingly, NODCrmA mice were significantly resistant to the diabetogenic activity of a potent insulin-specific CD8 T-cell clone. Since these adoptive transfer models mainly represent the effector phase rather than the initiation phase of autoimmune diabetes, our data suggest that the latter is more sensitive to CrmA protection. We conclude that anti-apoptotic genes such as CrmA might be potential candidates to enhance islet graft survival in T1DM.

Are 5-hydroxytryptamine-preloaded beta-cells an appropriate physiologic model system for establishing that insulin stimulates insulin secretion?

The release and oxidation of 5-hydroxytryptamine from 5-hydroxytryptamine-preloaded beta-cells has been used as a surrogate marker for insulin secretion. Findings made using this methodology have been used to support the concept that insulin stimulates its own release. In the present studies, the effects of 5-hydroxytryptamine on stimulated insulin secretion from isolated perifused rat islets was determined. When added together with stimulatory glucose, 5-hydroxytryptamine (0.5 mm) significantly reduced both phases of 8 mm glucose-induced secretion and reduced the first phase of 15 mm glucose-induced release by 60% without any effect on sustained insulin release rates. Preloading of beta-cells with 0.5 mm 5-hydroxytryptamine for 3 h resulted in a more severe impairment of 15 mm glucose-induced secretion. First and second phase release rates were reduced by 70 and 55%, respectively. In addition, this pretreatment protocol also abolished 200 microm tolbutamide-induced insulin secretion from perifused islets. These findings confirm that 5-hydroxytryptamine is a powerful inhibitor of stimulated insulin secretion. The responses of 5-hydroxytryptamine-preloaded beta-cells may not accurately reflect the biochemical events occurring during the physiologic regulation of insulin secretion. The suggestion that insulin stimulates its own secretion based exclusively on amperometric measurements should be reconsidered.

Fuel and hormone regulation of phospholipase C beta 1 and delta 1 overexpressed in RINm5F pancreatic beta cells

The mechanism by which glucose and other fuels stimulate phosphoinositide-specific phospholipase C (PLC) in pancreatic islet beta cells is not known. Previous studies have suggested that glucose may couple to PLC beta 1 and PLC delta 1. To determine directly if fuels activate these PLC isozymes, clones stably overexpressing PLC beta 1 or PLC delta 1 were generated in the fuel-sensitive beta cell line RINm5F, and secretagogue regulation of these PLC isoforms was determined. Overexpression of PLC beta 1 or PLC delta 1 significantly increased PLC activity in isolated cell fractions, consistent with overexpression of active PLC isoforms in these clones. In paired experiments, stimulation of inositol phosphate (IP) accumulation by the fuel glyceraldehyde was enhanced in clones overexpressing PLC beta 1, in parallel with the G-protein alpha subunit activator, AlF(4)(-), suggesting a coupling between glyceraldehyde and this PLC isoform. In contrast, overexpression of PLC delta 1 had no effect on glyceraldehyde- or AlF(4)(-)-stimulated IP accumulation. Similarly, IP accumulation stimulated by ionomycin was enhanced in PLC beta 1, but not PLC delta 1 clones, indicating that increases in intracellular free calcium [Ca(2+)](i) can regulate PLC beta 1 but not PLC delta 1 overexpressed in this cell line. Interestingly, [Arg(8)] vasopressin-stimulated, but not carbachol-stimulated, IP accumulation was significantly increased in clones overexpressing either PLC beta 1 or PLC delta 1. These studies illustrate unique pathways coupling diverse secretagogues to specific PLC isoforms in islet beta cells, and demonstrate that glyceraldehyde can activate PLC beta 1 but not PLC delta 1; whereas, vasopressin, but not carbachol, can stimulate either isoform.

Chronic exposure to TPA depletes PKC alpha and augments Ca-dependent insulin secretion from cultured rat islets

The insulin secretory responses of rat islets to glucose (15 mM), 12-O-tetradecanoylphorbol 13-acetate (TPA; 500 nM), and potassium (30 mM) were determined from perifused islets cultured for 22-24 h in CMRL-1066 medium (control cultured) or islets cultured in the additional presence of 500 nM TPA. Islet content of protein kinase C alpha (PKC alpha) and serine and threonine phosphoprotein patterns were also monitored after the culture period. Compared with freshly isolated islets, culturing alone had no adverse effect on the capacity of TPA or 30 mM potassium to stimulate secretion or on the islet content of PKC alpha. In agreement with previous studies, culturing in TPA reduced the islet content of immunoreactive PKC alpha by > 95% and abolished the capacity of the phorbol ester to stimulate secretion during a subsequent dynamic perifusion. Culturing in TPA slightly improved the insulin secretory response to 15 mM glucose compared with control-cultured islets; however, sustained rates of 15 mM glucose-induced secretion from these islets were significantly less than the responses of freshly isolated islets. Islets cultured in TPA responded to 30 mM potassium with a markedly amplified insulin secretory response that was abolished by nitrendipine. Enhanced phosphorylation of several islet proteins was also observed in TPA-cultured islets compared with control-cultured islets. These findings demonstrate that culturing alone impairs glucose-induced secretion, a response that is improved but still subnormal compared with freshly isolated islet responses, if TPA is included in the culture medium. Sustained phosphorylation of several islet proteins in TPA-cultured islets may account, at least in part, for augmented calcium-dependent secretion.

Low extracellular calcium enhances beta cell sensitivity to the stimulatory influence of 1,25-dihydroxyvitamin D3 on insulin release by islets from vitamin D3-deficient rats

The beneficial effect of 1,25-dihydroxyvitamin D3 [1,25 (OH)2 D3] on insulin secretion from beta cells in hypocalcemic vitamin D3-deficient rats is now well established. Moreover, few data concerning the mechanism of 1,25 (OH) 2D3 efficiency as a function of the severity of hypocalcemia. In the present experiment, we submitted islets from vitamin D3-deficient rats to in vitro exposure to a range of decreasing extracellular Ca2+ concentrations ([Ca2+]ex), from 0.5 mM to 0.6 mM, during a 6-h 10-8 M 1,25 (OH) 2D3 induction. Thereafter, we compared the effect of this pretreatment on the islets' insulin response to a given stimulus. Various stimuli were used, and we measured in parallel the variations of 86Rb+ and 45Ca2+ efflux and insulin release into the perifusion medium. In the presence of 1,25 (OH) 2D3, we observed an inverse correlation between the [Ca2+]ex pre-exposure and the amplitude of the insulin response to certain stimuli studied, suggesting that beta cells that were pre-exposed to low [Ca2+]ex became more sensitive to the beneficial effect of 1,25 (OH) 2D3 on insulin release. This effect was observed when beta cells were activated by acetylcholine but only during its second phase of stimulation, and more particularly with the barium plus theophylline stimulus. In contrast, insulin release was not affected by [Ca2+]ex pre-exposure during 1,25 (OH) 2D3 induction in response to acetylcholine during its first phase of stimulation, thus excluding any mechanism mediated via nutrient pathways, membrane depolarization, or inositol triphosphate (IP3)-dependent events. Moreover, the islets that were pre-exposed to a 10-fold [Ca2+]ex exhibited only a 50% lower 45Ca2+ content after 45Ca2+ loading, suggesting a different or relatively more efficient storage capacity in the presence of low extracellular calcium. Studies of 45Ca2+ efflux showed that the mobilization of Ca2+ stores induced by a barium plus theophylline stimulus, in the absence of calcium in the perifusion medium, was more efficient in islets pre-exposed to low [Ca2+]ex, whereas the acetylcholine-IP 3-induced mobilization of Ca2+ from reticular stores was not affected. These results generated the hypothesis that 1,25 (OH)2D3 may prepare the beta cells during their pre-exposure to low [Ca2+]ex to become more efficient as concerns insulin release via a more efficient mobilization of 45Ca2+ stores (mitochondrial?) and by an activation of release potentiating systems via protein kinase C protein kinase A pathways.