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

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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.