title: Sodium-calcium exchange in Purkinje fibers: electrical and mechanical effects.
abstract: 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.
mesh_terms: D000200:Action Potentials; D000818:Animals; D002110:Caffeine / Q000494:pharmacology; D002118:Calcium / Q000378:metabolism*; D002121:Calcium Channel Blockers / Q000494:pharmacology; D003470:Culture Media; D046508:Culture Techniques; D004285:Dogs; D006329:Heart Conduction System / Q000502:physiology*; D009200:Myocardial Contraction / Q000187:drug effects; D010477:Perfusion; D011690:Purkinje Fibers / Q000502:physiology*; D012964:Sodium / Q000378:metabolism*
chemical_list: D002121:Calcium Channel Blockers; D003470:Culture Media; D002110:Caffeine; D012964:Sodium; D002118:Calcium