DISCREPANT EFFECTS OF ANGIOTENSIN-II AND PHENYLEPHRINE ON PLASMA-VOLUME IN CONSCIOUS GOATS

Presser doses of angiotensin II induced haemodilution in goats despite renal fluid losses. This study was undertaken to determine if this re sponse is dose-dependent and correlated to the vasoconstrictor action of angiotensin II, Angiotensin II at the doses 0.025, 0.05 and 0.1 mu g min(-1) was given intravenously to five goats. Mean arterial blood p ressure increased by 3, 10 and 20 mmHg, respectively, and the renal Na excretion rose. The haematocrit decreased by 7, 10, and 9% (percentag e of control values) and the plasma protein concentration by 1% (n.s.) , 4.5, and 3.5%, respectively. Infusions of phenylephrine (40 mu g min (-1); n = 6) caused an equivalent increase of blood pressure and renal Na excretion as angiotensin II (0.1 mu g min(-1)), but the haematocri t increased by 16% and the plasma protein concentration by 6.5%. Infus ions of atrial natriuretic peptide (1 mu g min(-1)) alone or together with angiotensin II (0.1 mu g min(-1)), or phenylephrine were also giv en (n = 6). Infusions of atrial natriuretic peptide alone did not chan ge blood pressure, but renal Na excretion increased. The haematocrit r ose by 10.5% and the plasma protein concentration by 7.6%. Adding atri al natriuretic peptide to the angiotensin II solution attenuated the r ise of MAP and counteracted the haemodilution, but did not decrease th e natriuresis. Infusions of phenylephrine plus atrial natriuretic pept ide caused similar elevations of blood pressure and renal Na excretion as phenylephrine alone. The haematocrit rose by 24%, and the plasma p rotein concentration by 13%. These results show that in the intact con scious 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 arteria l blood pressure or renal Na excretion.