Physiologic implications of phosphoinositides and phospholipase C in the regulation of insulin secretion.

The secretion of insulin from the pancreatic beta-cell must be commensurate to satisfy the insulin requirements of the organism. This cell has a great flexibility to meet these requirements which are increased not only by the ingestion of nutrients (increase of plasma glucose) but also by the sensitivity of target tissues to insulin as well. The insulin secretion is a complex biochemical event regulated by a host of potential second messenger molecules acting alone or in concert.

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.

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.

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.

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.

Dexamethasone suppresses phospholipase C activation and insulin secretion from isolated rat islets.

Dexamethasone inhibits insulin secretion from isolated islets. In the present experiments, possible underlying biochemical mechanisms responsible for defective secretion were explored. Dexamethasone (1 micromol/L) had no immediate deleterious effect on 15 mmol/L glucose-induced insulin release from perifused rat islets.

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.

Aggregation and lack of secretion of most newly synthesized proinsulin in non-beta-cell lines.

Myoblasts transfected with HB10D insulin secrete more hormone than those transfected with wild-type insulin, as published previously, indicating that production of wild-type insulin is not efficient in these cells. The ability of non-beta-cells to produce insulin was examined in several cell lines. In clones of neuroendocrine GH(4)C(1) cells stably transfected with proinsulin, two thirds of (35)S-proinsulin was degraded within 3 h of synthesis, whereas (35)S-prolactin was stable.

Effects of prior 5-hydroxytryptamine exposure on rat islet insulin secretory and phospholipase C responses.

Glucose-induced insulin secretion is inhibited by 5-hydroxytryptamine (5HT). In the present studies the specificity of 5HT inhibition of release and the potential biochemical mechanisms involved were investigated. Dose-dependent inhibition of 15 mM glucose-induced secretion was induced by a prior 3 h incubation with 5HT.

Effects of muscarinic receptor type 3 knockout on mouse islet secretory responses.

The impact of muscarinic type 3 receptor knockout (M3KO) on the cholinergic regulation of insulin secretion and phospholipase C (PLC) activation was determined. Islets isolated from control, wild-type mice or heterozygotes responded with comparable insulin secretory responses to 15 mM glucose. This response was markedly amplified by the inclusion of 10 microM carbachol.

Contrasting effects of nateglinide and rosiglitazone on insulin secretion and phospholipase C activation.

When stimulated with 6 mmol/L glucose, a minimal, transient insulin secretory response was observed from perifused rat islets. The inclusion of 5 micromol/L nateglinide significantly amplified release. Elevating glucose to 8 or 10 mmol/L resulted in an increasing insulin secretory response that was again markedly potentiated by the further inclusion of nateglinide.

Estrogen can prevent or reverse obesity and diabetes in mice expressing human islet amyloid polypeptide.

Type 2 diabetes is characterized by loss of beta-cell mass and concomitant deposition of amyloid derived from islet amyloid polypeptide (IAPP). Previously we have shown that expression of human IAPP (huIAPP) in islets of transgenic mice results in either a rapid onset of hyperglycemia in mice homozygous for the huIAPP transgene on a lean background (FVB/N) or a gradual hyperglycemia in mice hemizygous for the huIAPP transgene on an obese background (A(vy)/A). In both strains, only the males routinely develop diabetes.

Effects of glucose, exogenous insulin, and carbachol on C-peptide and insulin secretion from isolated perifused rat islets.

Isolated perifused rat islets were stimulated with glucose, exogenous insulin, or carbachol. C-peptide and, where possible, insulin secretory rates were measured. Glucose (8-10 mm) induced dose-dependent and kinetically similar patterns of C-peptide and insulin secretion.