1. Male Wistar rats were either bilaterally adrenalectomized or sham-operated, and given 1% sodium chloride solution instead of tap water to drink. Seven days later, arterial blood pressures were recorded directly from conscious freely moving rats.
2. Systolic and diastolic blood pressures were significantly lower in the adrenalectomized rats, whereas heart rates were significantly higher than in sham-operated animals. The tachycardia was due to a combination of sympathetic hyperactivity and reduced vagal tone, which may have been reflex responses to a reduction in effective blood volume.
3. Baroreflex control of the sinus node was assessed from the pulse interval responses to rises (induced by methoxamine) or falls (induced by glyceryl trinitrate or sodium nitroprusside) in systemic arterial blood pressure. The relation between pulse interval and systolic blood pressure was described by the same curve in sham-operated and adrenalectomized rats, indicating that, in the latter, there was no change in baroreflex setting or sensitivity.
4. Intravenous administration of naloxone (2mg/kg) had no effect on systemic arterial blood pressure in adrenalectomized rats, suggesting that endogenous opiates were not contributing to the hypotension.