CAMP CAN RAISE OR LOWER CARDIAC ACTOMYOSIN ATPASE ACTIVITY DEPENDING ON ALPHA-ADRENERGIC ACTIVITY

Adenosine 3',5'-cyclic monophosphate (cAMP) or beta-adrenergic stimula tion has been shown to increase actomyosin adenosinetriphosphatase (AT Pase) activity in cardiac muscle. Because the major catecholamine tran smitters have both alpha- and beta-adrenergic activity, the possibilit y of a role for alpha-adrenergic stimulation in the regulation of ATPa se activity has been investigated. Histochemical measurement of actomy osin ATPase activity in quickly frozen rat hearts has been used as the assay of enzymatic function of the contractile proteins. The dose-res ponse curve of ATPase activity to cAMP shows an increase in ATPase act ivity at a threshold concentration of 0.01 mu M, a peak effect at 0.5- 1.0 mu M, and a decline beyond 1.5 mu M to a level below control at 10 mu M cAMP. Kinetic studies varying ATP concentration from 0.5 to 10 m M indicated the existence of multiple forms of actomyosin ATPase activ ity in the absence of cAMP and only one form with a higher maximum vel ocity in the presence of 1 mu M cAMP. Apparently cAMP raises the enzym atic activity of the individual actomyosin molecule rather than increa sing the number of active molecules. The addition of an a-adrenergic b locker had no significant effect in the absence of added cAMP, but in the presence of the cyclic nucleotide, 1 mu M prazosin always produced a negative effect on ATPase activity. Over the entire range of 0.01-1 0 mu M, cAMP lowered ATPase activity when the alpha-adrenergic antagon ist was present. The integrity of the cAMP regulatory system was sensi tive to the tissue oxygen tension at the time the heart was quickly fr ozen. At certain oxygen tension, the stimulatory component of the cAMP regulation was observed without any inhibitory component, suggesting that there are two relatively independent parts of the regulatory mech anism, an inhibitory and a stimulatory. In the presence of gamma-label ed [P-32]ATP, P-32 was incorporated into several proteins, including t he inhibitory subunit of troponin (TNI), C protein, and the regulatory light chain of myosin. cAMP (1 mu M) caused an increase in P-32 label ing of TNI and C protein. The addition of prazosin with cAMP caused a decrease in the overall level of phosphorylation with specific dephosp horylation of C protein and TNI, the former to a degree similar to the decrease in actomyosin ATPase activity, the latter to a greater degre e. These results indicate that a-adrenergic activity modulates the bal ance between kinase and phosphatase activity in the presence of cAMP, probably by inhibiting phosphatase activity. A model to integrate thes e data involving protein kinase A phosphorylation of C protein, a guan osine 3',5'-cyclic monophosphate-activated phosphatase that is also ac tivated by higher concentrations of cAMP, and a protein kinase C-depen dent inhibition of phosphate activity controlling the alpha-adrenergic system is proposed.