Specificity of arginine vasopressin and angiotensin II for microvessels in the hamster cheek pouch after the induction of renovascular hypertension

Characterization of vasopressin actions in isolated submucosal arterioles of the intestinal microcirculation

Submucosal arterioles are the final resistance vessels of the mesenteric circulation; they supply intestinal mucosa and smooth muscle and contribute significantly to total mesenteric resistance. Characterization of receptors present on submucosal arterioles has not been carried out, because these vessels have not been accessible to study by previous methods. We have used a novel optical method for on-line tracking of outside diameter from in vitro preparations of submucosal arterioles in the ileum and colon of guinea pigs, rabbits, and humans to characterize the vasoconstrictor responses to vasopressin as well as other vasopressor agents along the gastrointestinal tract. All ileal submucosal arterioles showed smoothly graded constrictor responses, whereas colonic arterioles from each species exhibited rhythmic vasoconstrictions. Vasopressin constricted guinea pig and human submucosal arterioles (EC50, 1 nM) by activating classical V1 receptors; dissociation equilibrium constants (Kd) for the V1 antagonist d(CH2)5 Tyr (Me) arginine vasopressin were 1-3 nM. This antagonist was 10-50-fold more potent in inhibiting vasopressin constrictions in rabbit submucosal arterioles (Kd = 0.05-0.1 nM). No evidence for the presence of V2 receptors was obtained in any arteriole, and no significant differences in the alpha 1-adrenoceptor-mediated constrictions were observed in these vessels. Results from this study suggest the presence of heterogeneity of V1 receptors in submucosal arterioles; these differences appear to be species dependent. Our results also suggest that intrinsic vasoconstrictor properties of submucosal arterioles differ along the length of the gastrointestinal tract; these differences appear to be species independent.

Interaction of arginine vasopressin and angiotensin II on Ca2+ in vascular smooth muscle cells

The non-osmotic release of arginine vasopressin (AVP) is associated with the concomitant activation of the renin-angiotensin and sympathetic nervous systems. In vivo studies suggest that a positive interaction may occur between AVP and angiotensin II (Ang II), and other Ca2+ mobilizing hormones. In the present study, the cellular mechanisms of this interaction between AVP and Ang II in vascular smooth muscle cell (VSMC) were examined. These results support the existence of a positive interaction between AVP and Ang II on Ca2+ mobilization in VSMC. In fact, the challenge of VSMC with combined AVP and Ang II, in a range from 5 x 10(-11) to 10(-8) M, enhanced cytosolic free Ca2+ ([Ca2+]i) and 45Ca2+ efflux in a more than additive manner. This potentiation, which was not dependent of the presence of extracellular calcium, correlated with an increased VSMC shape change. Moreover, the combination of subthreshold doses of AVP and Ang II (5 x 10(-11) M), which do not release Ca2+ alone, evoked a Ca2+ mobilizing response. A subthreshold dose of Ang II also shifted to the left the concentration-response curve of the AVP-mediated 45Ca2+ efflux. Since there were no changes in receptor binding of either hormone by the other hormone and the interaction of the two hormones on the production of inositol phosphatides was additive, the AVP and AII positive interaction on Ca2+ mobilization on VSMC may occur at the level of the intracellular Ca2(+)-releasing mechanism itself. Such an interaction can occur at hormone concentrations below the Ca2+ release threshold and may explain an increased functional response to the combination of pressor hormones compared to that of each hormone alone.