Activation of cardiac adenylate cyclase: horminal modification of the magnesium ion requirement.

Histamine and epinephrine stimulate the activity of guinea pig heart adenylate cyclase [ATP pyrophosphate-lyase (cyclizing) EC 4.6.1.1], in part, by decreasing the requirement for Mg2+ as an activator. This effect may represent an increase in affinity for Mg2+ and/or a decrease in sensitivity of the enzyme towards inhibition by free ATP. Both of these inotropic hormones also increase maximum velocity. Pretreatment of the membrane-bound enzyme with EDTA, to remove available divalent cations, almost eliminates persistent stimulation by guanyl-5'-yl imidodiphosphate [Gpp(NH)p]. Addition of Mg2+ to the preincubation medium restores the capacity of Gpp(NH)p to acutely activate the enzyme. These results indicate that Mg2+ interacts with the nucleotide (GTP) regulatory site. Persistent stimulation of the enzyme by either Gpp(NH)p or fluoride ion also involves a decrease in the requirement for Mg2+ and an increase in maximum velocity.