Differential effects of age versus glycemic stimulation on the maturation of insulin stimulus-secretion coupling during culture of fetal rat islets.

Abstract

We have cultured islets from 21.5-day-old fetal rats for 1-7 days in RMPI 1640/10% fetal calf serum containing 2.8 or 11.1 mM glucose to evaluate the differential effects of age vis-à-vis glycemic stimulation on the maturation of selected components of stimulus-secretion coupling. After 1 day of culture in either media, acute stimulation with 3.0 mg/ml glucose during basal perifusion with 0.5 mg/ml glucose elicited only a small first phase of stimulated insulin secretion and no second phase. The acute exposure to 3.0 mg/ml glucose produced no change in the prevailing high rates of oxygen consumption (QO2) and caused only minor increments in phosphate efflux (i.e., peak values for phosphate flush of 126 +/- 16% of baseline for islets that had been cultured in 11.1 mM glucose and 162 +/- 30% for islets cultured in 2.8 mM glucose). After 7 days of culture in 11.1 mM glucose, acute stimulation with 3.0 mg/ml glucose increased Qo2 (as in adult islets) and effected acute increases in the AT32P and GT32P content of prelabeled islets. The 3.0 mg/ml glucose also triggered phosphate flush to 599 +/- 45% of baseline and elicited first as well as early second phases of stimulated insulin secretion that replicated the performance of adult islets. By contrast, after 7 days of culture in 2.8 mM glucose, similar acute exposures of fetal islets to 3.0 mg/ml glucose effected only a small first phase and a negligible second phase of stimulated insulin secretion despite the occurrence of the same increments in Qo2 as after culture in 11.1 mM glucose, highly significant increases in AT32P and GT32P, and phosphate flushes that peaked at 306 +/- 16% of basal values. Thus, the ontogeny of individual components of stimulus-secretion coupling may occur in asynchronous fashion and varying require glycemic stimulation in addition to aging per se. The capacities to augment efflux of orthophosphate, enhance respiration, and heighten nucleotide turnover in response to acute stimulation with glucose seem to mature in large measure in time-dependent fashion, whereas some chronic exposure to ambient glucose in excess of basal levels may be required to establish and/or maintain the other coupled components that underlie biphasic stimulated insulin release. However, we did not achieve full exocytotic maturation even after 7 days of culture with 11.1 mM glucose.(ABSTRACT TRUNCATED AT 400 WORDS)