Vasopressin and the cardiovascular system: physiology or pharmacology?

Anesthetized rats of different strains show a hypotensive response to administration of an antagonist of the V1 receptors for vasopressin [d(CH2)5DAVP]. Such an effect is not seen in conscious, water-replete animals or in Long-Evans rats challenged with a subcutaneous injection of polyethylene glycol (PEG) to cause isosmotic hypovolemia. However, Long-Evans rats experiencing a similar volume reduction due to water deprivation show hyperosmolality and exhibit a small hypotensive response to d(CH2)5DAVP. Inhibition of the renin-angiotensin system following administration of d(CH2)5DAVP causes a greater hypotension in PEG-treated than in water-deprived Long-Evans rats. In both experimental conditions, the fall in blood pressure is greater than when captopril administration precedes that of d(CH2)5DAVP, indicating that prolonged administration of d(CH2)5DAVP may be interfering with mechanisms other than those mediated by peripheral V1 receptors. However, administration of d(CH2)5DAVP and captopril to water-deprived Long-Evans rats rarely causes the profound hypotension seen in water-deprived Brattleboro rats given captopril alone. In some adrenalectomized Wistar rats, following withdrawal of salt supplementation, the hypotensive response to d(CH2)5DAVP is the greatest seen in any experimental model. These results indicate that AVP is overtly involved in the support of blood pressure in various hypotensive states and, more importantly, may be responsible for the maintenance of a "normal" blood pressure in some conditions. However, the involvement of AVP in cardiovascular regulation in the majority of normotensive conditions is intriguingly subtle.