Role of Na + /Ca 2+ exchange in maintenance of elevated pulmonary arterial smooth muscle cell (PASMC) intracellular Ca 2+ concentration ([Ca 2+ ] i ) and intracellular pH (pHi) during chronic hypoxia (CH)

Abstract

In the lung, exposure to CH may result in pulmonary hypertension. Development of this condition is due to increased muscularity and contraction of the pulmonary arteries. These processes are associated with an increase in both pHi and [Ca2+]i. We previously demonstrated that the alkaline shift in pHi in PASMCs isolated from chronically hypoxic rats was partially dependent on the increase in [Ca2+]i. In this study, we examined the role of Na+/Ca2+ exchange (NCX) and Na+/H+ exchange (NHE) in mediating the elevated basal [Ca2+]i and Ca2+-dependent changes in pHi in PASMCs isolated from rats exposed to CH (3weeks, 10% O2). Ratiometric fluorescent microscopy was used to measure PASMC pHi and [Ca2+]i with the pH-sensitive dye, BCECF-AM, and the Ca2+-sensitive dye, Fura-2-AM. At baseline, both bepridil, a general Na+/Ca2+ exchange inhibitor, and KB-R7943, an inhibitor selective for reverse mode NCX (Ca2+ entry), caused a small but significant decrease in resting [Ca2+]i. In the presence of KB-R7943, the changes in pHi induced by increasing [Ca2+]i with KCl or decreasing [Ca2+]i with Ca2+-free extracellular solution or NiCl2 were markedly reduced. Exposure to ethyl isopropyl amiloride (EIPA), a NHE inhibitor, decreased resting pHi. In the presence of EIPA, no further changes in pHi were observed in response to changing [Ca2+]i. These results suggest that during CH, Na+ efflux through reverse mode NCX facilitated increased activation of NHE activity and an alkaline shift in pHi, while Ca2+ entry contributed to regulation of increased basal [Ca2+]i.