Forskolin activation of adenylate cyclase in mouse parotid membranes

In mouse parotid membranes forskolin activated adenylate cyclase four-fold; maximal activation of the enzyme occurred with 10 microM forskolin. Activation was not dependent on the guanyl nucleotide GTP nor on the inhibitory guanine nucleotide 5'-0-(2-Thiodiphosphate), GDP beta S. In contrast, stimulation of adenylate cyclase by isoproterenol required GTP and was antagonized by GDP beta S in a dose-dependent manner. These results indicate that the guanyl-binding protein of mouse parotid adenylate cyclase is not a requisite for forskolin activation and lends support for direct interaction of forskolin at the catalytic subunit.