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The effects of endothelin (0.1 and 1 nmol/kg into the left ventricle) on mean blood pressure (MAP), heart rate (HR), central venous pressure (CVP), and mean circulatory filling pressure (MCFP) were investigated in conscious and unrestrained WKY rats before and during continuous infusion of atriopeptide (1 mu mk/kg/min i.v.). In intact rats, bolus injection of endothelin into the left ventricle caused a transient hypotension followed by a long-term increase in MAP. In the rats receiving atriopeptide infusion constantly, the hypotensive phase of endothelin effects was augmented whereas the hypertensive phase was reduced. Low doses of endothelin did not affect CVP and MCFP while the highest dose given to intact rats and atriopeptide-treated rats caused small, but significant decreases in CVP and MCFP. Our results suggest that endothelin has small effects on total body's venous tone in contrast to its effectiveness by increasing blood pressure.

Changes in the arterial pressure (AP), heart rate (HR) and sensitivity of chronotropic baroreceptor reflex (SBR) evoked by i. v. infusion of alpha-ANP, AP-III, AP-II, and AP-I (1 mg/kg/min) were studied in alert Wistar rats. The SBR was tested by infusion of phenylephrine or sodium nitroprusside. All the ANP analogues, except AP-1, were shown to decrease the AP and evoke differential changes in the SBR. The SBR evoked by phenylephrine was increased by 60-80%, whereas SBR evoked by sodium nitroprusside was decreased by 30-60%. The specific changes in tachycardic and bradycardic components of the baroreceptor reflex after ANP analogues infusions seem to reflect participation of the baroreceptor reflex in the mechanism of ANP-induced hypotension.