Discrepant effects of angiotensin II and phenylephrine on plasma volume in conscious goats

Pressor doses of angiotensin II induced haemodilution in goats despite renal fluid losses. This study was undertaken to determine if this response is dose-dependent and correlated to the vasoconstrictor action of angiotensin II. Angiotensin II at the doses 0.025, 0.05 and 0.1 micrograms min-1 was given intravenously to five goats. Mean arterial blood pressure increased by 3, 10 and 20 mmHg, respectively, and the renal Na excretion rose. The haematocrit decreased by 7, 10, and 9% (percentage of control values) and the plasma protein concentration by 1% (n.s.), 4.5, and 3.5%, respectively. Infusions of phenylephrine (40 micrograms min-1; n = 6) caused an equivalent increase of blood pressure and renal Na excretion as angiotensin II (0.1 micrograms min-1), but the haematocrit increased by 16% and the plasma protein concentration by 6.5%. Infusions of atrial natriuretic peptide (1 microgram min-1) alone or together with angiotensin II (0.1 microgram min-1), or phenylephrine were also given (n = 6). Infusions of atrial natriuretic peptide alone did not change blood pressure, but renal Na excretion increased. The haematocrit rose by 10.5% and the plasma protein concentration by 7.6%. Adding atrial natriuretic peptide to the angiotensin II solution attenuated the rise of MAP and counteracted the haemodilution, but did not decrease the natriuresis. Infusions of phenylephrine plus atrial natriuretic peptide caused similar elevations of blood pressure and renal Na excretion as phenylephrine alone. The haematocrit rose by 24% and the plasma protein concentration by 13%. These results show that in the intact conscious goat rapid and marked changes in haematocrit and plasma protein concentration can be provoked by intravenous infusions of vasoactive agents and that these effects are not correlated to changes in arterial blood pressure or renal Na excretion.