Evidence against a vasopressor role of ADH in malignant DOC-salt hypertension

Role of endothelin and vasopressin in DOCA-salt hypertension

1. The relative roles of endothelin (ET) and vasopressin (AVP) in the regulation of blood pressure (BP), cardiac output (CO) and total peripheral resistance (TPR) were investigated in the early stages (24 - 31 days) of development of hypertension in the conscious deoxycorticosterone acetate (DOCA)-salt hypertensive rat model. 2. BP was recorded with radiotelemetry devices and CO with ultrasonic transit-time probes. TPR was calculated from the BP and CO recordings. The contributions of endogenous ET and AVP were studied by infusing [d(CH(2))(5)(1),O-Me_Tyr(2),Arg(8)]-vasopressin, a V(1)-receptor antagonist, and bosentan, a mixed ET(A)/ET(B) receptor antagonist (Study 1). Vascular responsiveness was estimated from the changes in TPR evoked by i.v. infusions of ET-1 and AVP (Study 2). 3. In study 1, infusion of bosentan reduced TPR and BP dramatically in DOCA-salt hypertensive rats but not in SHAM control rats, and this effect was greater when the AVP system had been blocked. In contrast, the V(1) receptor antagonist alone failed to change TPR and BP in DOCA-salt hypertensive rats. However, subsequent infusion of the V(1) receptor antagonist during the plateau phase of the response in bosentan pretreated DOCA-salt hypertensive rats led to significant decreases in both BP and TPR. 4. In study 2, TPR and BP responses to ET-1, but not AVP, were greater in DOCA-salt rats than in control rats. CO responses to ET-1 or AVP were similar in the two groups. 5. The results suggest that both ET and AVP play a role in the maintenance of TPR and BP; when one system is blocked the other compensates. However, the magnitude of the contribution to the hypertensive state appears greater for ET than for AVP. Enhanced vascular responses to ET appear to contribute to this greater role.

Changes in vascular vasopressin receptors and responsiveness in DOCA/NaCl-treated rats

This study investigated the effects of treatment with desoxycorticosterone acetate and salt (DOCA/NaCl) on blood pressure, aortic vascular vasopressin receptors and in vitro vascular responsiveness in male rats. Four groups of animals were utilized in the study: a control group on normal tap water, a control group drinking a 1% NaCl solution; a DOCA/NaCl-treated group (1% NaCl in the drinking water); and a uninephrectomized (1K) DOCA/NaCl-treated group. DOCA/NaCl treatment for 4 weeks resulted in a significant elevation in blood pressure which was more pronounced in the uninephrectomized animal. The concentration of specific binding sites for vasopressin on the aorta were reduced in both the DOCA/NaCl-treated groups. However, the vascular responsiveness of the aorta to vasopressin was significantly reduced only in the hypertensive, uninephrectomized-DOCA/NaCl-(1K DOCA/NaCl) treated rat. Both the maximal contraction and the sensitivity was reduced in the 1K DOCA/NaCl group. The results of this study would suggest that the vascular alterations to vasopressin are probably post receptor mediated and result from the DOCA/NaCl-induced hypertension.

Contribution of elevated peripheral resistance to acute volume loaded hypertension in dogs

The role of peripheral resistance in volume loaded hypertension was examined by measuring changes in hemodynamic and humoral factors during dextran infusion for one hour, and for three hours after stopping the infusion in 30 anesthetized dogs. The mean blood pressure was elevated significantly (118%, p less than 0.01), accompanied by increased cardiac output (170%, p less than 0.01) during volume loading. In the recovery period, cardiac output returned to the basal level while the total peripheral resistance gradually increased (118%, p less than 0.01) contributing to the maintenance of high blood pressure (108%, p less than 0.01). There was no significant correlation between the hematocrit change and the increase in peripheral resistance in the recovery period. The levels of plasma norepinephrine, epinephrine, renin activity and vasopressin were suppressed by volume loading. These results demonstrated the importance of increase in peripheral resistance for maintaining high blood pressure in this acute volume excess model. The participation of the renin-angiotensin system, the sympathetic nervous system and vasopressin were all excluded and vascular structural changes were most unlikely to be involved in this acute experiment. It remains unknown whether this conversion is attributable to the summation of local autoregulation or to some other factors.