CENTRALLY MEDIATED INFLUENCES OF HYPERTONIC NaCI AND ANGIOTENSIN II ON REGIONAL BLOOD FLOW AND HEMODYNAMIC RESPONSES TO HYPOTENSIVE HEMORRHAGE IN CONSCIOUS SHEEP

The influence of separate and combined intracerebroventricular (ICV) infusions of hypertonic (.5 M) NaCl (HTNa) at .02 mL min−1 and angiotensin II (ANG II) at 1 pmol kg−1 min−1 on tolerance to hemorrhage, accompanying systemic hemodynamic changes, and regional blood flow was studied in adult conscious sheep. Corresponding measurements during ICV .9% NaCl served as controls. The hemorrhage volume needed to lower the blood pressure to about 50 mmHg was significantly larger during treatment with HTNa and HTNa/ANG II (27.8 ± 2.2 and 28.3 ± 2.5 mL kg−1, respectively; p < .001; about 45% of estimated blood volume) as well as during ANG II (20.1 ± 1.3 mL kg−1; p < .01) compared to controls (15.1 ± .7 mL kg−1; about 25% of estimated blood volume). In spite of a larger hemorrhage volume, the lowering of the cardiac output was not accentuated, and its subsequent recovery was not impaired during ICV infusion of HTNa or HTNa/ANG II. Similarly, the posthemorrhage restoration of the systemic blood pressure was not negatively affected by the more pronounced hypovolemia induced during the ICV treatments compared to controls. In contrast to ANG II, HTNa infusion, alone or in combination with ANG II, was accompanied by a significantly lower renal blood flow, and a higher renovascular resistance, during the posthemorrhage period. The femoral blood flow was maintained or even slightly elevated after hemorrhage in all experiments. The integrated results of the study imply differentiated hemodynamic effects of centrally administered HTNa and ANG II. The cardiovascular responses to HTNa/ANG II were largely the net effect of those obtained by the separate treatments. We conclude that the cardiovascular effects of centrally administered HTNa, but not of ANG II, to a large extent are similar to those obtained with intravascular infusion of hypertonic saline in hemorrhagic shock. However, in contrast to the latter treatment, the renal circulation was impaired in response to centrally administered HTNa.