Abstract
1. 1. To obtain a measure of drug effects on myocardial function during diastole, the following experimental protocol was designed: rapid electrical stimulation (5 Hz) at high Ca 2+ o caused an elevated diastolic tension, which could be subjected to drug-induced alterations. 2. 2. Antiarrhythmic drugs (quinidine, propafenone, procainamide, mexiletine) were able to lower diastolic force without appreciably decreasing contraction amplitude. Calcium antagonists (nifedipine, verapamil) lowered both parameters in parallel. 3. 3. Veratridine and Bay K 8644 both enhanced diastolic tension, but only Bay K 8644 concomitantly elevated contraction amplitude. 4. 4. These findings may be explained when taking into account differential actions of sodium- and calcium channel modulating drugs, respectively, on cellular Ca 2+ movements. In quantitative terms, the non-linear dependence of myocardial force on Ca 2+ i also had to be considered.
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