Does vasopressin sustain blood pressure in conscious spontaneously hypertensive rats?

To investigate the possible role of arginine vasopressin in maintaining high blood pressure of spontaneously hypertensive rats (SHR), the effect of two arginine vasopressin pressor antagonists on mean arterial pressure and the pressor responsiveness to exogenous arginine vasopressin were studied in conscious, freely moving SHR and in Wistar-Kyoto rats (WKY). Intravenous injections of either d(CH2)5Tyr(Me)arginine vasopressin, 10 micrograms/kg, or dPTyr(Me)arginine vasopressin, 20 micrograms/kg, had no effect on mean arterial pressure or heart rate of normohydrated SHR, although both antagonists almost completely abolished the pressor response to exogenous arginine vasopressin. Furthermore, dPTyr(Me)arginine vasopressin was ineffective in eliciting a depressor response, even after 24 or 48 hours of water deprivation. During converting enzyme inhibition with SQ 20881, mean arterial pressure and heart rate remained unchanged following arginine vasopressin blockade in both normohydrated and fluid-restricted animals. alpha-Adrenergic receptor blockade reduced the blood pressure of normohydrated SHR, from 160 +/- 7 to 81 +/- 8 mm Hg. When dPTyr(Me)arginine vasopressin was given during alpha-adrenergic receptor blockade there was a small, transient fall in mean arterial pressure. The pressor responsiveness to exogenous arginine vasopressin was similar in hypertensive and normotensive rats. These results suggest that arginine vasopressin does not function as an important pressor hormone in conscious SHR.