Correlation Between Decreased Heart Rate and Central Inhibition of Sympathetic Discharge After Prazosin Administration in the Spontaneously Hypertensive Rat

Abstract

Mean arterial blood pressure, heart rate and splanchnic sympathetic discharge were recorded in conscious and anesthetized spontaneously hypertensive rats (SHR). Administration of prazosin (1 mg/kg i.v.) reduced heart rate in most rats and this effect was closely paralleled by a decrease in sympathetic discharge. Administration of hydralazine (1 mg/kg i.v.) caused a sustained tachycardia and increased sympathetic nervous activity. The cardiovascular effects of prazosin (0.2-0.3, 1 and 3 mg/kg i.v.) were examined in different rat strains. Sprague-Dawley and Wistar-Kyoto rats responded with tachycardia, whereas a significant bradycardia was observed in SHR following the higher doses of prazosin. The bradycardia obtained after prazosin administration apparently is due to a central inhibition of sympathetic outflow, an effect possibly caused by blockade of .alpha.-adrenoreceptors. The different heart rate responses in different rat strains may be interpreted to reflect differences in sensitivity of central .alpha.-adrenoreceptors.