Relationship between calcium and hydrogen ions in heart muscle

Developed tension (T), its maximal rate of rise [+dT/dt(max)], and maximal velocity of relaxation [-dT/dt(max)] were measured in cat papillary muscles, after changes in calcium concentration (Ca²⁺) at pH ~7.0, 7.4, and 7.6, produced by altering the PCO₂ of the medium. Only the results obtained at the two extreme pH values were compared. Increasing [Ca²⁺] increased T, +dT/dt(max) and -dT/dt(max). At a given [Ca²⁺] high PCO₂ significantly decreased T and +dT/dt(max). When T and +dT/dt(max) vs. [Ca²⁺] were plotted reciprocally, the ordinate intercepts of the lines, 1/E(max) significantly increased in acidosis (P < 0.01). These data do not support the view of a competitive action between H⁺ and Ca²⁺ ions on the mechanical behavior of cardiac muscle. At a given [Ca²⁺], +dT/dt(max) was depressed significantly more than -dT/dt(max), i.e., at a [Ca²⁺] of 10 mM, the degree of depression of +dT/dt(max) and -dT/dt(max) was 41 ± 4% and 23 ± 5%, respectively (P < 0.01). Furthermore, the slope of the regression line -dT/dt(max) vs. +dT/dt(max) was significantly higher in acidosis (P < 0.01). Similar results were obtained in muscles depleted of catecholamines. These findings suggest that H⁺ ion might act upon the relaxation process.