Sodium-calcium exchange in Purkinje fibers: electrical and mechanical effects.

The role of Na and Ca in the modulation of electrical and mechanical events was studied in canine cardiac Purkinje fibers perfused in vitro. The results show that: 1) A low Na-Ca solution (Na 80.8 mM, Ca 1.08) increased contractile force and shortened the action potential transiently; during the recovery, the force decreased below and increased transiently above control; the action potential lengthened transiently. 2) A low Na solution increased force and transiently shortened the action potential; during the recovery, the force transiently decreased and the action potential lengthened again. 3) A low Ca solution decreased force and prolonged the action potential; during the recovery, the force increased transiently and the action potential shortened. 4) In low Na-Ca solution, verapamil, manganese or a fast driving rate did not prevent the transient increase in force but in the steady state the force was lower. 5) If the low Na-Ca solution was begun during quiescence, resuming activity after one minute already resulted in a higher force. 6) Caffeine and high K decreased force but only caffeine markedly decreased the inotropic effects of low Na. 7) The initial increase in force was reversed in calcium-overloaded fibers. And 8) The Na-Ca ratio necessary to maintain force is smaller for Purkinje than myocardial fibers. It is concluded that a low Na-Ca solution increases force by increasing cellular calcium stores probably in the sarcoplasmic reticulum, that the maintenance of force is critically dependent on a smaller slow inward current, and that there are substantial differences between Purkinje and ventricular muscle fibers.