Adenylyl cyclase activity of homogenates or membrane preparations from isolated rat pancreatic islets was slightly activated by 10 μm prostaglandin E1 (PGE1) or GTP. However, when both PGE1 and GTP were used in combination, adenylyl cyclase activity was increased twofold to a level that was 70% of that obtained with 10 mm sodium fluoride. In the presence of GTP (20 μm) PGE1 stimulation was evident at 0.2 μm and maximal at 10 μm. Prostaglandins E2, F2, and A1 (0.1 or 10 μm) had no effect on basal enzyme activity, but PGE2 or PGA1(10 μm) increased activity slightly in the presence of GTP (20 μm). Kinetic analysis indicated that PGE1 plus GTP increased both the apparent Michaelis constant for ATP and the maximum velocity of adenylyl cyclase. Neither compound had any effect on the activity of cyclic nucleotide phosphodiesterase in pancreatic islet homogenates.
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