The adenylate cyclase activity of isolated hepatocytes actions of glucagon and sodium fluoride.

Isolated hepatocytes converted exogenous [alpha-32P]ATP to cyclic [32P]AMP at high rates. This system was used for kinetic studies of the effects of glucagon, fluoride, free magnesium and free ATP4- on adenylate cyclase. In the absence or presence of glucagon, free Mg2+ activated adenylate cyclase by decreasing the Km for MgATP2- without changing V. Free ATP4- was not a potent inhibitor of adenylate cyclase and the only effect of glucagon was to increase V. Fluoride also increased the V of adenylate cyclase, but, in contrast to the results obtained with glucagon, the effect increased as the concentration of free Mg2+ increased. One explanation of the effect of fluoride, consistent with the idea that free Mg2+ activates adenylate cyclase and free ATP is not an inhibitor, is that fluoride increases the affinity of the enzyme for Mg2+. Weak inhibition of adenylate cyclase by ATP4- in the presence of fluoride cannot be excluded.