title: The negative inotropic effect of calcium overload in cardiac Purkinje fibers.
abstract: The role of calcium in increasing or decreasing the force of contraction was studied in canine cardiac Purkinje fibers perfused in vitro under conditions known to alter cellular calcium. The following results were obtained: (1) increasing calcium load by perfusing a low Na solution increases contractile force in the presence of a low concentration of strophanthidin and decreases it in high strophanthidin; (2) decreasing calcium load by decreasing extracellular calcium leads to opposite effects; (3) the positive inotropic effect of low [Na]0 becomes negative in the presence of high [Ca]0; (4) high K decreases contractile force and reverses the negative inotropic effect of low Na in the presence of high strophanthidin; (5) a low Na-Ca solution increases initially the contractile force: high strophanthidin reverses the effect at normal K and exaggerates at high K, as apparently calcium overload is relieved by high K; (6) a low Na-Ca solution slowly removes calcium overload induced by high strophanthidin; (7) a low [Na]0 solution does not decrease force in the presence of high strophanthidin if extracellular calcium is also low; (8) the rebound increase in force is shifted to lower calcium concentrations after exposure to a low sodium solution with or without strophanthidin, as calcium overload becomes greater. It is concluded that in Purkinje fibers calcium overload induced by different means leads to a decrease in contractile force and that the force decline can be reversed by decreasing cellular calcium either during or after calcium overloading.
mesh_terms: D000818:Animals; D002118:Calcium / Q000494:pharmacology*; D003864:Depression, Chemical; D004285:Dogs; D004305:Dose-Response Relationship, Drug; D005260:Female; D006329:Heart Conduction System / Q000187:drug effects*; D066298:In Vitro Techniques; D008297:Male; D009200:Myocardial Contraction / Q000187:drug effects*; D011188:Potassium / Q000494:pharmacology; D011690:Purkinje Fibers / Q000187:drug effects* / Q000502:physiology; D012964:Sodium / Q000494:pharmacology; D013327:Strophanthidin / Q000494:pharmacology
chemical_list: D013327:Strophanthidin; D012964:Sodium; D011188:Potassium; D002118:Calcium