Blood pH and the actions of nifedipine on cardiac excitation-contraction coupling.

Hearts from male Sprague-Dawley rats were perfused, by the Lagendorff method, with calcium-free Krebs solution (containing either adrenaline or a high level of potassium) at pH 7.48 (control), 7.26 (acidosis) or 7.69 (alkalosis). When the hearts stopped contracting, a dose of nifedipine or its vehicle was given before measuring the force of contraction, coronary perfusion pressure and heart rate in response to graded doses of calcium. The calcium-antagonising efficacy of nifedipine was reduced during acidosis in both adrenaline- and potassium-stimulated hearts, but the reduction was greater in the adrenaline-stimulated hearts. Alkalosis led to a small increase in the efficacy of nifedipine on adrenaline- and potassium-stimulated contractions.