Size Dependence of Functional Alterations in Mesenteric Arteries from the Aldosterone-Salt Hypertensive Rat

We tested the hypothesis that an elevated potassium-42 (⁴²K⁺) efflux (highly dependent on Ca²⁺) and an increase in the sensitivity of contraction and ⁴²K⁺ efflux to norepinephrine (NE) in conduit arteries of aldosterone-salt hypertensive rats (AHR) exended to smaller, distributing arteries. Functional endpoints were compared in two sizes of arteries from the mesenteric bed: second-order branches of the superior mesenteric artery (SMA branches) and the SMA. Contraction and free cytosolic Ca²⁺ concentration ([Ca²⁺]c; fura-2 microfluorometry) were measured simultaneously and ⁴²K⁺ efflux was measured separately in SMA branches. Contraction and ⁴²K⁺ efflux were measured separately in SMA. Basal tone, [Ca²⁺]c, and ⁴²K⁺ efflux were similar in SMA branches from AHR and control-salt rats (CSR). However, basal ⁴²K⁺ efflux was elevated in SMA from AHR compared to CSR. The sensitivity of the contractile, [Ca²⁺]c, and ⁴²K⁺ efflux responses to NE was similar in SMA branches. In contrast, the sensitivity of the contractile and ⁴²K⁺ efflux responses to NE was enhanced in SMA from AHR compared to CSR. Inhibiton of endothelium-derived vasoactive substances by pretreatment with Nω-nitro-L-arginine methyl ester and indomethacin significantly shifted the NE concentration-response relationships to the left for contraction, [Ca²⁺]c, and ⁴²K⁺ efflux in both SMA branches and SMA from CSR. A similar shift to the left was observed in AHR for contraction but not consistently for [Ca²⁺]c and ⁴²K⁺ efflux. We conclude that SMA branches from AHR demonstrate neither the elevated basal ⁴²K⁺ efflux, nor the NE supersensitivity exhibited by SMA. Endothelial function was not impaired both in SMA and SMA branches.