This study was designed to elucidate the role of adenosine 3′, 5′-monophosphate (cAMP) in the decreased glucose-induced insulin release observed in cultured pancreatic islets. Freshly coUagenase-isolated rat islets and islets that had been maintained in tissue culture medium for two to 140 hours (six days) were incubated for one hour with 3H-2-adenine, and insulin release and islet 3H-cAMP accumulation were measured in subsequent short-term (3 to 60 minutes) incubations.
There was a progressive decrease in the stimulatory effect of 20 mM glucose on insulin release in islets that had been cultured for 2, 4, and 20 hours, and, after 20 to 140 hours of culture, glucoseinduced insulin release was about one-third that in freshly isolated islets. Similarly, 20 mM glucose stimulated a threefold increase in 3H-cAMP content in fresh islets, but did not increase 3H-cAMP in islets cultured for four to 140 hours. Basal insulin release and islet 3H-cAMP levels were two- to threefold higher in islets cultured for 44 hours in 16.7 mM glucose rather than in 5.6 mM glucose. However, the sensitivity and capacity of glucose-induced insulin release were less in islets cultured in medium with either 5.6 or 16.7 mM glucose than in freshly isolated islets, and glucose did not further increase islet 3H-cAMP levels in the cultured islets. By contrast, 3-isobutyl-l-methylxanthine (IBMX, 1 mM) increased insulin release and 3H-cAMP content in both fresh and cultured islets. Glucagon (10 μM) increased insulin release and 3H-cAMP content in cultured islets better than in fresh islets; and glucagon (10 μM) plus IBMX (1 mM) increased insulin release and islet 3H-cAMP content similarly in fresh and cultured islets.
It is concluded that there is rapid and, apparently, selective loss in the short-term action of glucose to increase the cAMP content in islets maintained in vitro, and this may account for the impaired insulin-releasing action of glucose observed in cultured islets.