Regional hemodynamic changes following hypovolemia in conscious rats

Regional hemodynamic responses to subcutaneous injection of hyperoncotic polyethylene glycol (PEG) were assessed in Long-Evans (i.e., normal) and Brattleboro [i.e., arginine vasopressin (AVP)-deficient] rats with chronically implanted, pulsed Doppler probes. The results were compared with saline-injected time controls. PEG injection elicited an early (30 min) selective mesenteric vasoconstriction in Long-Evans rats; 4-5 h after PEG the mesenteric vasoconstriction was greater in Long-Evans than in Brattleboro rats, but the renal and hindquarters vasoconstrictions were similar in both strains. There was a transient mesenteric vasodilatation [with no change in blood pressure (BP)] in Long-Evans rats given an antagonist of the V1-receptor actions of AVP 5 h after PEG. In the presence of the AVP antagonist, Long-Evans rats showed a marked dependence on the vasoconstrictor actions of the renin-angiotensin system for the maintenance of BP, similar to Brattleboro rats. Both strains also showed marked hypotensive responses to inhibition of sympathoadrenal activity (with pentolinium). Hence, AVP has clear-cut hemodynamic effects after PEG administration, but its role is not indispensable.