The effect of menadione on sarcoplasmic reticulum Ca2plus and contractions of single guinea-pig cardiomyocytes

We investigated the effect of 2-methyl-1,4-naphtoquinone (Menadione) on sarcoplasmic reticulum (SR) Ca2²⁺ content and electrically stimulated contractions (ESCs) of single isolated myocytes of guinea-pig ventricular myocardium. The contractures initiated by means of microinjections of caffeine into the close vicinity of the cell were used as an indirect index of the SR Ca²⁺ content. Superfusion of the cells for 45 min with Menadione resulted in gradual disappearance of contractile respones to caffeine, prolongation of time to peak amplitude of ESCs by 48±15% and complete inhibition of postrest and postextrasystolic potentiation. These results are consistent with those of Floreani and Caipenedo (7) who found that Menadione strongly inhibits the SR Ca²⁺ ATPase. Despite depletion of the SR Ca²⁺ the amplitude of ESCs did not change which suggests that contractions were initiated in the cells treated with Menadione by Ca²⁺ derived from the sources other than the SR.