Failure of glucose to elicit a normal secretory response in fetal pancreatic beta cells results from glucose insensitivity of the ATP-regulated K+ channels.

Abstract

Fetal pancreatic beta cells demonstrate a deficient insulin release in response to glucose, but the underlying mechanism at the cellular level is unknown. By using beta cells from 21-day fetal rats we made an attempt to clarify the mechanism(s) behind this reduced glucose response. In addition to measuring insulin release, glucose metabolism, and cellular ATP content, ATP-regulated K+ channels (G channels) and voltage-activated Ca2+ currents were investigated with the patch-clamp technique. It was thus demonstrated that the ATP-regulated K+ channels in fetal beta cells were not sensitive to glucose but otherwise had similar characteristics as those of adult beta cells. Also, the characteristics of the voltageactivated Ca2+ currents were similar in adult and fetal beta cells. However, as judged from measurements of both glucose oxidation and glucose utilization, glucose metabolism was impaired in fetal beta cells. In addition, there was no increase in the ATP content, even when the cells were stimulated for 30 min. It is therefore concluded that the attenuated glucoseinduced insulin release in fetal pancreatic beta cells is due to an immature glucose metabolism resulting in impaired regulation of the ATP-sensitive K+ channels. These rmdings may be relevant to the understanding of the deficient stimulus-secretion coupling associated with non-insulin-dependent diabetes.In adult beta cells glucose promotes the closure of a K+ channel that is regulated by intracellular ATP (G channel) (1-4), resulting in depolarization, opening of voltage-activated Ca2l channels, and insulin release (1). The significance of the G channel is further emphasized by the finding that hypoglycemic sulfonylureas, drugs used in the treatment of noninsulin-dependent diabetes (type II diabetes), initiate insulin release by a direct closure of this channel (1, 5-7). Thus, one can postulate that failure of glucose to promote insulin release in type II diabetes might result from either the G channel or its regulation being defective. In vitro studies of the endocrine pancreas of both man and animals have demonstrated a poor coupling between ambient glucose concentration and fetal insulin discharge (8)(9)(10)(11)(12)(13)(14)(15)(16)(17)(18)(19)(20). So far it has not been possible to define the step(s) in the stimulus-secretion coupling responsible for the reduced glucose response. In the present study we intended to clarify whether the relative inability of glucose to promote insulin release from fetal pancreatic beta cells can indeed be explained in terms of an altered function of the G channel protein and/or a defect in G channel regulation. Information obtained from studies of the stimulus-secretion coupling in fetal beta cells is relevant also for the understanding of the mechanism(s) responsible for the defect in insulin release in type II diabetes. MATERIALS AND METHODSPreparation and Dispersion of Fetal Pancreatic Islets as Well as Measurements of Insulin Release. Fetal rat islets were isolated from 21-day pregnant rats as in He...

Functional characteristics of neonatal rat β cells with distinct markers

“…This glucose unresponsiveness was attributed to an impartial differentiation, in particular relating to specialized features of glucose-sensing b cells such as NADH shuttles (Tan et al 2002), K C ATP channels (Rorsman et al 1989) or general immaturity with expression of disallowed genes (Aye et al 2010, Jermendy et al 2011. Indeed, also our neonatal b cell isolates showed a consistent flattening of glucose concentration-response curves at all tested levels: metabolic rate (NAD(P)H), insulin synthesis, and insulin secretion.…”

“…Fetal and neonatal b cells have also been reported to have a lower glucose-stimulated insulin secretion (GSIS) than adult b cells (Asplund et al 1969, Grill et al 1981, Hellerstrom & Swenne 1991. Both their higher propensity to proliferate and their lower functional glucose responsiveness are generally considered as two sides of a same coin: both properties are assumed to reflect an incompletely differentiated b cell phenotype that has, on the one hand, not yet assumed full expression of all genes that are quintessential for the specialized glucose--sensing function of a mature b cell (Rorsman et al 1989, Tan et al 2002, Jermendy et al 2011 and, on the other hand, likely maintains expression of protein markers typically encountered in developing tissues. There are, to date, relatively few independently confirmed gene or protein markers that can discern mature from immature b cells.…”

Role of NADH Shuttles in Glucose-Induced Insulin Secretion From Fetal β-Cells

“…This is shown by normal glucose utilization and normal activity of the rate-limiting enzyme, glucokinase (5). A reduction of glucose oxidation has been observed more distally (8). Several factors may be responsible for this reduction: 1) immaturity in the TCA cycle; 2) the lack of substrate for the TCA cycle, e.g., the accumulation of lactate due to high lactate dehydrogenase activity, which results in the unavailability of pyruvate for glucose oxidation; or 3) reduced NADH shuttle activity.…”

The anterior chamber of the eye is a transplantation site that supports and enables visualisation of beta cell development in mice

“…We now demonstrate that pancreatic buds engrafted in the ACE develop similarly to the native pancreas, with cells expressing PDX1, somatostatin and glucagon. Moreover, developing islets in the ACE respond to both glucose and forskolin, similar to fetal islets [7]. Therefore, isolated E10.5 pancreatic buds transplanted into the ACE recapitulate the development of the embryonic pancreas in terms of morphology, gene expression and function.…”

“…In parallel, we performed the same assay on pancreatic buds cultured in vitro for 10 days. As forskolin has been shown to stimulate glucose responsiveness in immature beta cells [7], we co-incubated buds with forskolin (10 μmol/l) and glucose (11 mmol/l) and found enhanced insulin secretion only from ACE-transplanted buds (tenfold) and not from in vitro cultured buds (Fig. 2, ESM Fig.…”