ALTERATIONS IN NA+-H+ EXCHANGE IN MESENTERIC-ARTERIES FROM SPONTANEOUSLY HYPERTENSIVE RATS

Na+-H+ exchange was examined in segments of the superior mesenteric artery from spontaneously hypertensive (SHR) and Wistar-Kyoto (WKY) rats using the pH-sensitive dye 2',7'-bis(2-carboxyethyl)-5(6)-carboxyfluorescein (BCECF). Basal pH values were identical in the two strains (SHR = 7.18 +/- 0.02, WKY = 7.15 +/- 0.01) and exposure to 20-50 mM NH4Cl produced comparable changes in pH in both strains (tissues acidified to pH 6.8). The recovery of intracellular pH (pH(i)) from acid loading in bicarbonate-free buffer was dependent on extracellular Na+, inhibited by dimethyl amiloride, and thus was presumably mediated by Na+-H+ exchange. Exchange activity was markedly increased in SHR (0.308 vs. 0.182 pH U/min), and the increased activity could not be explained by differences in intracellular buffer capacity or in the Na+ affinity of the transporter. The rate of Na+-H+ exchange was more steeply dependent on pH(i) in SHR vessels (Hill coefficient = 6.6 in SHR vs. 4.1 in WKY), suggesting an alteration in the transport protein. Young SHR (4-6 wk) exhibited kinetic characteristics resembling those of 12- to 14-wk-old WKY rats. Exposure to the Ca2+ ionophore bromo-A23187 significantly increased the Hill coefficient of WKY arteries but had no effect on vessels from SHR. Exposure of vessels from either strain to the phorbol ester phorbol 12-myristate 13 acetate (PMA) had no effect on Na+-H+ exchange.