Modulation of vascular and cardiac contractile protein regulatory mechanisms by calmodulin inhibitors and related compounds.

The abilities of several calmodulin antagonists and other compounds belonging to different pharmacological classes to modulate Ca2+ X calmodulin mediated arterial myosin light chain phosphorylation and Ca2+-troponin C regulated cardiac myofibrillar ATPase activity have been quantitated in Triton X-100 purified preparations of bovine aortic actomyosin and canine ventricular myofibrils. At submaximal free Ca2+ concentrations, all calmodulin antagonists inhibited myosin phosphorylation; however, some (calmidazolium, trifluoperazine, chlorpromazine, pimozide) stimulated myofibrillar ATPase activity, some (compound 48/80, W-5) had no effect on activity, while others (W-7, haloperidol, mastoparan) inhibited ATPase activity. The relative order of potency for several agents in both preparations was the same, as IC50 values for inhibition of arterial myosin phosphorylation were: calmidazolium, 0.5 microM; trifluoperazine, 22 microM; perhexiline, 35 microM; and concentrations which stimulated cardiac myofibrillar ATPase activity by 50% were: calmidazolium, 9 microM; trifluoperazine, 45 microM; perhexiline, 90 microM. A common feature of stimulation of cardiac ATPase activity by these agents was a leftward shift in the pCa relationship, although different shape changes in the pCa curves were also apparent. Maximum ATPase activity was either not affected or inhibited (trifluoperazine). Several other agents belonging to diverse pharmacological classes also had differential effects on myosin phosphorylation and ATPase activity. These results show that structurally-distinct calmodulin antagonists and other compounds differentially affect cardiac myofibrillar ATPase activity. Moreover, several agents have been identified which inhibit arterial, and stimulate cardiac, contractile protein regulatory mechanisms. Thus, it may be possible to develop mechanistically novel cardiotonic/vasodilator agents, Ca2+ binding protein modulators, which function primarily by altering the Ca2+ sensitivity of contractile protein interactions.