Control of hindlimb vascular resistance and vascular responsiveness in DOCA-salt hypertensive dogs

Increased vasodilator responses to acetylcholine in psychosocial hypertensive mice

Responsiveness to endothelium-dependent (acetylcholine and A23187) and endothelium-independent (nitroprusside and 8-bromo cyclic guanosine 3',5'-monophosphate [cGMP]) vasodilators was examined in two vascular preparations from hypertensive and normotensive mice. CBA Agouti mice were made hypertensive by exposure to social stress in a complex population cage. After 2 months, the hindquarter vascular bed was pump-perfused at a constant flow with plasma substitute to evaluate changes in perfusion pressure, and helical strips of aorta were suspended in muscle baths for measurement of isometric force generation. Tissues were treated with methoxamine to induce contractile tone. Threshold dilator responses to acetylcholine were elicited at a significantly lower dose in the hindquarters of hypertensive mice than in those from normotensive mice, indicating increased vasodilator sensitivity. In contrast, vasodilator responsiveness to nitroprusside in hindquarters of hypertensive mice did not differ from that in hindquarters of normotensive mice. Aortas from hypertensive mice were more sensitive (lower ED50) to the relaxant effects of acetylcholine and A23187 than those from normotensive mice. The relaxant effects of nitroprusside and 8-bromo cGMP on aortas from hypertensive mice were not significantly different from those in normotensive aortas. Aortic strips that had been rubbed on the lumen surface with a wooden stick did not relax to acetylcholine or A23187. In aortas that were not initially contracted with methoxamine, acetylcholine and A23187 caused small contractions from baseline. The magnitude of these contractile responses were potentiated after removal of the endothelium, and the potentiation was greater in aortas from hypertensive mice. These results demonstrate an increased responsiveness to endothelium-dependent vasodilators in this psychosocial model of hypertension.

Central mechanisms in DOC-salt hypertensive rats

These experiments were intended to elucidate the role of central mechanisms in the maintenance of high blood pressure produced by deoxycorticosterone (DOC) and salt in rats. We investigated the central effect of angiotensin (AII) on systemic arterial blood pressure and plasma arginine vasopressin (AVP) in DOC-salt and control rats. There was a pronounced augmentation of the pressor responsiveness to centrally injected AII in DOC-salt hypertensive rats; but there was no difference in AII induced AVP release in DOC-salt hypertensive rats compared to sham controls. The increase in vascular resistance of the perfused hindquarters induced by stimulation of the lumbar sympathetic chains did not change in DOC-salt hypertensive rats although the increases induced by norepinephrine (NE) were potentiated at the higher doses. Pressor responsiveness of the DOC-salt hypertensive rats to i.v. administration of AII, AVP and NE was shown to be augmented by factors of 3.6, 2.5 and 1.8 respectively in DOC-salt rats. Reflex bradycardia to these pressor responses was attenuated indicating impairment of baroreflex function. The potentiation of pressor responses to centrally injected AII in DOC-salt hypertensive rats was greater than could be explained by augmented pressor responsiveness to iv NE and AVP. Neither baroreflex dysfunction, facilitated release of NE at sympathetic terminals, nor augmented release of AVP into the circulation could explain the potentiation. Therefore, our data suggested that selective central amplification of sympathetic vasomotor responses to centrally injected AII stimuli may play a role in the hypertension after 3 weeks of DOC-salt treatment in rats.

Impairment of neurally-mediated vasoconstriction in DOCA-salt hypertensive dogs

Several biochemical studies indicate that hypertension produced by administration of deoxycorticosterone acetate (DOCA) plus saline as the drinking fluid is accompanied by significant increases in catecholamine turnover rates and a decrease in catecholamine content of peripheral tissues. In the present study the functional significance of such biochemical changes was assessed by evaluating vasoconstrictor responsiveness to neurogenic and humoral interventions in DOCA-salt hypertensive dogs. Administration of DOCA, 5 mg/kg subcutaneously twice a week for four weeks to unilaterally nephrectomized dogs given drinking fluid containing 1% sodium chloride produced a significant increase in blood pressure within one week. At the end of 4 week treatment period, the animals were anesthetized with pentobarbital (35 mg/kg) and sympathetic neurotransmission to the hindlimb and kidney was evaluated. Femoral as well as renal vasoconstrictor responses to sympathetic nerve stimulation were significantly inhibited in the hypertensive group in comparison with control animals. Vasoconstriction elicited by exogenous norepinephrine in both these vasculatures was similar in control and hypertensive groups. Femoral vasodilator responses to adenosine, bradykinin and PGE2 were significantly attenuated in the hypertensive group. While indomethacin (10 mg/kg) did not significantly affect renal vasoconstrictor responses to renal nerve stimulation in both the groups of animals, it caused a significant increase in the blood pressure of hypertensive group but not of control group. These results do not support the hypothesis of an increase in neurogenic function contributing to DOCA-salt hypertension. It is suggested that a reduction in sympathetic neurogenic influence may serve to protect against the rise in blood pressure during DOCA-salt administration.