Drug effects on myocardial 45Ca uptake in conscious rats.

The myocardial content of 45Ca++ in conscious rats is increased by single s.c. injections of sympathomimetics. A dose dependent inhibition of this effect is achieved by simultaneous administration of calcium antagonists or beta-receptor blocking agents. The myocardial 45Ca++ content of conscious rats is increased by i.v. administration of dibutyrylcycloadenosinemonophosphate (DBcAMP). The effect of the cyclic nucleotide is suppressed only by a calcium antagonist but not by a beta-receptor blocker. The following conclusions may be drawn: 1. Calcium antagonists and beta-sympatholytics have different sites of action in the heart. 2. The lack of DBcAMP-antagonism of the beta-sympatholytics permits a simple discrimination between both types of substances. After pretreatment with sympathomimetics for 7 days (0.3 mg/kg isoprenaline s.c.), neither high doses of isoprenaline nor doses of DBcAMP and aminophylline increased the myocardial 45Ca content in rats. This effect only lasted briefly (for about two weeks); the site of its action is so far unknown.