Insulin Secretion and Cyclic Adenosine 3',5'-Monophosphate Levels in Pancreatic Islets of Fed and Fasted Rats: Time Course and Dose Kinetics During Glucose Stimulation

Incubation of pancreatic islets of fed rats at glucose 10 and 15 mM induced a rapid rise of the islet cyclic adenosine 3',5'-monophosphate (cAMP) content. Maximum levels were attained at 15 min and lasted until 30 min, after which cAMP declined again. Insulin secretion increased most rapidly from 15 min onward, i.e. after the rapid rise of cAMP. Islet cAMP at either 15 or 30 min showed its major concentration-dependent increase between glucose 7.5 and 10 mM. Glucose 15 mM did not further enhance the cAMP response, although this concentration increased insulin secretion more than two-fold over values observed at glucose 10 mM. Thus, the glucose dose-response relations for cAMP levels and insulin secretion appear to be different, indicating that factors other than cAMP alone determine the secretory response to glucose. Fasting for 24 and 72 h progressively inhibited glucose-induced insulin secretion. At glucose 15 mM the secretory inhibition disappeared after 30–45 min, but at glucose 10 mM it persisted for at least 90 min. Increasing periods of fasting also progressively delayed and inhibited the cAMP response to glucose, most strongly at glucose 10 mM. Fasting for 24 h shifted the glucose dose-response curves for cAMP and insulin secretion to the right, but the maximum responses at glucose 37.5 mM were not significantly inhibited. The secretory inhibition appeared to be linearly related with the cAMP content in two different ways: (a) At fixed concentrations of glucose, the increasing inhibition of the cAMP response (at 15 min) as induced by 24 and 72 h of fasting correlated with the secretory inhibition over the initial 30 min. (b) At one fixed period of fasting (24 h), the variable percent inhibition of the cAMP response to graded concentrations of glucose (5–37.5 mM) correlated with the percent secretory inhibition at these concentrations. These correlations were no longer apparent after 30 min of incubation. The results support the view that inhibition of the adenylate cyclase-cAMP system is a major determinant factor in fasting-induced impairment of insulin secretion during the initial 30 min of glucose Stimulation.</jats:p

The efficacy of density gradients for islet purification: A comparison of seven density gradients

Seven different density gradient-forming materials were compared as to their efficacy for rat islet purification. Continuous density gradients were used in order to determine the bouant densities of the different pancreatic tissue components. Hand-picked islets served as a control. A significant separation of large numbers of slets from the exocrine tissue band was only seen in the albumin, dextran-40, and metrizamide gradients. Pure islet preparations could not be obtained with any of the gradients studied as none of the gradients completely separated lymph nodes, vessels, and ducts from the islets. Gradients containing sucrose resulted in low islet yields. The islet yields obtained with the other gradients were in the same range as those obtained by hand-picking. Metrizamide gave significantly higher yields than the widely used Ficoll. Judged both in terms of numbers of islets and their in vitro function, the best results were obtained with metrizamide and dextran-4