Contribution of elevated intracellular calcium to pulmonary arterial myocyte alkalinization during chronic hypoxia

Abstract

In the lung, exposure to chronic hypoxia (CH) causes pulmonary hypertension, a debilitating disease. Development of this condition arises from increased muscularity and contraction of pulmonary vessels, associated with increases in pulmonary arterial smooth muscle cell (PASMC) intracellular pH (pHi) and Ca2+ concentration ([Ca2+]i). In this study, we explored the interaction between pHi and [Ca2+]i in PASMCs from rats exposed to normoxia or CH (3 weeks, 10% O2). PASMC pHi and [Ca2+]i were measured with fluorescent microscopy and the dyes BCECF and Fura-2. Both pHi and [Ca2+]i levels were elevated in PASMCs from hypoxic rats. Exposure to KCl increased [Ca2+]i and pHi to a similar extent in normoxic and hypoxic PASMCs. Conversely, removal of extracellular Ca2+ or blockade of Ca2+ entry with NiCl2 or SKF 96365 decreased [Ca2+]i and pHi only in hypoxic cells. Neither increasing pHi with NH4Cl nor decreasing pHi by removal of bicarbonate impacted PASMC [Ca2+]i. We also examined the roles of Na+/Ca2+ exchange (NCX) and Na+/H+ exchange (NHE) in mediating the elevated basal [Ca2+]i and Ca2+-dependent changes in PASMC pHi. Bepridil, dichlorobenzamil, and KB-R7943, which are NCX inhibitors, decreased resting [Ca2+]i and pHi only in hypoxic PASMCs and blocked the changes in pHi induced by altering [Ca2+]i. Exposure to ethyl isopropyl amiloride, an NHE inhibitor, decreased resting pHi and prevented changes in pHi due to changing [Ca2+]i. Our findings indicate that, during CH, the elevation in basal [Ca2+]i may contribute to the alkaline shift in pHi in PASMCs, likely via mechanisms involving reverse-mode NCX and NHE.