Hormonal and electrolyte responses to acute isohemic volume expansion in unanesthetized rats

This study was undertaken to explore the time course of the metabolic response to isohemic blood volume expansion (30%) in normotensive, unanesthetized Sprague-Dawley rats. Whole blood, drawn from a femoral artery catheter of conscious donor rats, was infused into the jugular vein of recipient rats. Blood samples were drawn from a carotid artery of recipient rats at time points beginning immediately post-volume expansion (IPVE) up through 5 days post-volume expansion (PVE). To characterize the attendant compensatory mechanisms, the plasma concentrations of electrolytes and fluid regulatory hormones were determined. Hematocrit began to raise IPVE and was significantly elevated above control IPVE 20, 30, 40, 60, and 90 min, and 2, 4, 6, 8, 12, and 24 hr PVE. Consistent with our current understanding of the hormonal response to excess volume, atrial natriuretic factor was significantly increased above the prevolume expansion (control) values 0-30 min PVE. Surprisingly, plasma aldosterone levels were significantly increased above control at 20 and 30 min and 6 hr PVE, whereas plasma renin activity was significantly decreased 30-40 min PVE. Plasma sodium was not changed from control values except for a significant increase at 6 hr post-volume expansion. Plasma potassium, osmolality, and arginine vasopressin levels were not altered by the volume expansion. These studies delineate the physiologic time scheme operative in the regulation of fluid volume during acute ischemic volume expansion.