Regulation of canine heart sarcolemmal Ca2+-pumping ATPase by cyclic GMP

Cyclic nucleotide modulation of the sarcoplasmic reticulum calcium pump has been recognized for some time. Little is known, however, of cyclic nucleotide effects on the sarcolemmal Ca2+-pump. In sarcolemmal vesicles prepared from ventricular muscle by a recent technique (Jones, L.R., Maddock, S.W. and Besch, H.R. (1980) J. Biol. Chem. 255, 9971-9980) we have demonstrated via Millipore filtration that 10(-8) M and 10(-9) M cyclic GMP depressed the rate of ATP- and Mg2+-dependent 45Ca2+ uptake by 34% and 52%, respectively. Only at millimolar levels did cyclic AMP have any effect and the respective 5'-nucleotides had no effect at all. Parallel measurement of the associated (Ca2+ + Mg2+)-ATPase in the presence of either cyclic or 5'-nucleotides, however, revealed no concomitant depression in ATP hydrolysis. In another series of experiments, the cyclic GMP effect on 45Ca2+ uptake was associated with a significant decrease in the pump Vmax, and at the most effective concentration of cyclic GMP increased the apparent Km for Ca2+. These results suggest that cyclic GMP may depress ventricular Ca2+ efflux by decreasing the enzyme turnover and to a limited extent, decreasing pump affinity for Ca2+. This supports a hypothesis whereby cyclic GMP might modulate both local biochemical and electrophysiological events by an effect on a discrete, regional pool of intracellular Ca2+.