Sensitivity of cat papillary muscles to verapamil and nifedipine: enhanced effect in acidosis

The effects of acidosis on the myocardial sensitivity to verapamil and nifedipine were examined by measuring the changes in tension in cat papillary muscles perfused in oxygenated physiological saline. Calcium concentration was altered over a range of 2 to 12 mmol·litre−1 and pH adjusted to 7.4, 6.8 or 6.0. Addition of verapamil or nifedipine moved the calcium dose response to the right. At pH 7.4 a Schild plot confirmed competitive interaction between verapamil and calcium, with a calculated dissociation constant (KB) for verapamil of 4.6±0.2 μg·cm−3. At pH 6.0, the slope was less than 1.0 (not significant, P=0.2) but KB was significantly lower (P