VASOPRESSIN COMPENSATES FOR ACUTE LOSS OF SYMPATHETIC PRESSOR TONE INSPONTANEOUSLY HYPERTENSIVE RATS

1. The aim of this study was to examine the pressor response of vasopr essin (AVP) to an acute fall in blood pressure induced by ganglion blo ckade. 2. Aortic catheters were implanted in spontaneously hypertensiv e rats (SHR), stroke-prone SHR (SHRSP), normotensive Wistar-Kyoto (WKY ), black-hooded Wistar (BHW) and Sprague-Dawley (SD) rats, aged 5-7 we eks and 7-9 months, for direct measurement of mean arterial pressure ( MAP) under conscious, resting conditions. The ganglion blocking agent pentolinium was administered intra-arterially, followed by an AVP rece ptor antagonist specific for the pressor effect of AVP. The basal leve l of MAP attained with each drug was recorded. 3. In the adult SHR and SHRSP with established hypertension, acute ganglion blockade caused M AP to fall to a similar extent as in WKY, suggesting that the level of sympathetic pressor tone was similar in all three strains. Administra tion of the AVP antagonist alone did not affect resting MAP. During ga nglion blockade, however, it caused a further reduction of MAP in WKY, SHR and SHRSP, the magnitude of which was greater in the hypertensive strains. After both drugs, the total fall in MAP and the residual MAP were significantly greater in the hypertensive rats. 4. In young rats , AVP had little effect on MAP, even during ganglion blockade. The res idual level of MAP after both drugs was greater in the hypertensive st rains. 5. The extent to which AVP can compensate for an acute fall in MAP increases with age and the development of hypertension. This tends to mask the loss of sympathetic mediated pressor tone after ganglion blockade. By preventing this compensation we have shown that the sympa thetically mediated component of blood pressure is elevated in SHRSP w ith established hypertension.