Correlation between decreased heart rate and central inhibition of sympathetic discharge after prazosin administration in the spontaneously hypertensive rat.

Mean arterial blood pressure, heart rate and splanchnic sympathetic discharge were recorded in conscious and anaesthetized spontaneously hypertensive rats (SHR). Administration of prazosin (1 mg/kg i.v.) reduced heart rate in most rats and this effect was closely parallelled by a decrease in sympathetic discharge. In contrast, 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. It is suggested that the bradycardia obtained after prazosin administration 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.