Myocardial action potential prolongation by calcium channel activation under calcium-free EGTA condition in guinea pigs and rats.
1. Prolongation of action potentials upon the addition of isoproterenol or dbcAMP under Ca-free EGTA condition were observed in isolated myocardial preparations from both the guinea pig and the rat, whose action potential configuration greatly differ. The degree of prolongation was greater in the rat than in the guinea pig. 2. The prolongation of the action potential was rapidly reversed upon the addition of calcium ion and was dose-dependently suppressed by the addition of calcium antagonists. The sensitivity to nicardipine of this action potential was tenfold higher than of the so-called slow response action potentials. The duration of the prolonged action potential was dependent on the external sodium concentration, but was not affected by tetrodotoxin. 3. Thus, it was demonstrated in intact myocardia that sodium ion may persistently pass through the calcium channel to prolong the action potential when it is activated under the condition where the calcium-mediated inactivation of calcium channels is removed. 4. Contribution of calcium-mediated inactivation of calcium channels to the repolarization of normal myocardium may be larger in the rat than in the guinea pig.
Tanaka
H
H
Department of Pharmacology, Toho University School of Pharmaceutical Sciences, Chiba, Japan.
Noguchi
K
K
Shigenobu
K
K
D000200:Action Potentials / Q000187:drug effects; D000818:Animals; D003994:Bucladesine / Q000494:pharmacology*; D002118:Calcium / Q000502:physiology; D002120:Calcium Channel Agonists / Q000494:pharmacology*; D004533:Egtazic Acid / Q000494:pharmacology*; D006168:Guinea Pigs; D006321:Heart / Q000187:drug effects* / Q000502:physiology; D066298:In Vitro Techniques; D007545:Isoproterenol / Q000494:pharmacology*; D051381:Rats; D017208:Rats, Wistar; D013045:Species Specificity; D013997:Time Factors
D002120:Calcium Channel Agonists; D004533:Egtazic Acid; D003994:Bucladesine; D007545:Isoproterenol; D002118:Calcium