The radioactive microsphere technique was used to study hemodynamic mechanisms of action of 2 calcium antagonists--foridon and nifedipine in conscious rabbits with acute hypertension evoked by immobilization. Microspheres were injected before and 5 min after bolus i.v. injection of equihypotensive (10% blood pressure drop) doses of nifedipine (30 mcg/kg) and foridon (50 mcg/kg). Nifedipine in most cases decreased cardiac index by 7% without significant changes in regional hemodynamic. After foridon injection total peripheral resistance decreased by 19% (P less than 0.05) and cardiac index increased by 18%. There were increase in blood flow: in the heart by 34% (P less than 0.05), in the skeletal muscle by 41% (P less than 0.05) and in testes by 12% (P less than 0.05). The results show that in acute stress-induced hypertension these dihydropyridine-derivatives have different hemodynamic mechanisms of action: nifedipine preferentially depress myocardial contractility, whereas foridon dilates blood vessels.
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