Increased Glucose Sensitivity of Stimulus-Secretion Coupling in Islets FromPsammomys obesusAfter Diet Induction of Diabetes

Abstract

When fed a high-energy (HE) diet, diabetes-prone (DP) Psammomys obesus develop type 2 diabetes with altered glucose-stimulated insulin secretion (GSIS).␤-Cell stimulus-secretion coupling was investigated in islets isolated from DP P. obesus fed a low-energy (LE) diet (DP-LE) and after 5 days on a HE diet (DP-HE). DP-LE islets cultured overnight in 5 mmol/l glucose displayed glucose dose-dependent increases in NAD(P)H, mitochondrial membrane potential, ATP/ (ATP ؉ ADP) ratio, cytosolic calcium concentration ([Ca 2؉ ] c ), and insulin secretion. In comparison, DP-HE islets cultured overnight in 10 mmol/l glucose were 80% degranulated and displayed an increased sensitivity to glucose at the level of glucose metabolism, [Ca 2؉ ] c , and insulin secretion. These changes in DP-HE islets were only marginally reversed after culture in 5 mmol/l glucose and were not reproduced in DP-LE islets cultured overnight in 10 mmol/l glucose, except for the 75% degranulation. Diabetes-resistant P. obesus remain normoglycemic on HE diet. Their ␤-cell stimulus-secretion coupling was similar to that of DP-LE islets, irrespective of the type of diet. Thus, islets from diabetic P. obesus display an increased sensitivity to glucose at the level of glucose metabolism and a profound ␤-cell degranulation, both of which may affect their in vivo GSIS. Diabetes 51:2552-2560, 2002 S timulation of insulin secretion by glucose requires oxidative metabolism of the sugar in ␤-cells, with acceleration of ATP production and increase of the ATP/ADP ratio (1-4). The subsequent closure of ATP-sensitive K ϩ channels in the plasma membrane is followed by depolarization, opening of voltage-dependent Ca 2ϩ channels, and Ca 2ϩ influx. This leads to a rise with oscillations of the cytosolic calcium concentration ([Ca 2ϩ ] c ), which triggers exocytosis of insulin-containing granules (5-7). Glucose metabolism also amplifies the efficacy of [Ca 2ϩ ] c on exocytosis, but the nature of the coupling factor(s) remains elusive (8).Type 2 diabetes is characterized by the association of insulin resistance and inappropriate insulin secretion by pancreatic ␤-cells (9 -12). The gerbil Psammomys obesus is an attractive model of diet-induced type 2 diabetes because a diabetes-resistant (DR) subgroup has been separated from the diabetes-prone (DP) animals by assorted breeding (9,13). When fed a low-energy (LE) diet, DP P. obesus remain normoglycemic despite their marked insulin resistance. Within a few days on a high-energy (HE) diet, DP P. obesus develop hyperglycemia associated with hyperinsulinemia, rapidly followed by a progressive decline in ␤-cell function leading to absolute insulin deficiency (14,15). In contrast, DR P. obesus are less hyperinsulinemic and remain normoglycemic on HE diet (16). Previous studies have shown that after 5 days on HE diet, glucose-stimulated insulin secretion (GSIS) is markedly altered in islets from DP but not DR P. obesus, possibly as a result of decreased insulin content, lower oxidative capacity, and altered protein kinase C ac...

Psammomys Obesus, a Model for Environment-Gene Interactions in Type 2 Diabetes

“…This is true for glucosestimulated proinsulin biosynthesis (G.L., N.K., unpublished data) as well as the depletion of islet insulin content after prolonged exposure to high glucose in vitro (27). Therefore, the different incidence of diabetes between the two lines is not likely to be due to marked differences in ␤-cell function.…”

“…Furthermore, reduction of the secretory stimulus with diazoxide was accompanied by rapid refilling of the islet insulin reserve (25). Third, in vitro exposure of normoglycemic P. obesus islets to high glucose resulted in a marked depletion of islet insulin content within 24 h, irrespective of the genetic background of the animals (27). Thus, P. obesus islets fail to meet the sustained increase in secretory demand induced by glucose also when the islets are secluded from their in vivo insulin-resistant environment.…”

“…The leftward shift of the glucose-insulin concentration-response curve, characteristic of other models of hyperglycemia, was accompanied by a rise in cytosolic calcium concentration ([Ca 2ϩ ] c ), a critical step in insulin exocytosis (27). The glucose concentration-response curves for changes in mitochondrial membrane potential, NAD(P)H fluorescence, and [Ca 2ϩ ] c were also shifted to lower glucose concentrations in DP animals on HE diets relative to DP animals on LE diets (27).…”

“…Nutritional overload in DP animals resulted in further reduction of the threshold for insulin release and in the half-maximal stimulatory concentration (EC 50 ) of glucose (Fig. 4) (10,27).…”

“…To study the role of the endocrine pancreas in dietinduced diabetes and its dependence on the genetic background of P. obesus, we combined in vivo and in vitro studies in which kinetic parameters of insulin secretion and stimulus-secretion coupling signals were analyzed (10,26,27).…”