Effect of Changes of pH_i on Intracellular Calcium in a Smooth Muscle-Like Cell Line

The effect of changes of pH _i on Ca _i were studied using fluorescent dyes in cells of the cultured smooth muscle-like line, BC3H-1. Resting Ca _i in these cells was 182 ± 12 nM (n = 74) at pH _o of 7.4. Upon exposure to NH ₄ Cl, which rapidly alkalinized cells, a transient increase of Cai to 349 ± 55 nM (n = 29) was observed. The peak of the transient occurred within 30 s of exposure to NH ₄ Cl and returned to baseline within 1 minute. Two other procedures which resulted in rapid cellular alkalinization also caused a transient rise in Ca _i : exposure to and then removal of CO ₂ (Ca _i increased from 182 ± 22 to 248 ± 28 nM; n = 8); and exposure to and then removal of Na propionate (Ca _i increased from 242 ± 32 to 456 ± 71 nM; n = 9). The NH4Cl-induced Ca _i transient was eliminated by exposure to 0.2 mM TMB8 and to Ca-free solutions, but not by exposure to 0.5 mM LaCl ₃ . Sustained changes of pH _i can be induced by varying pH _o . When pH _o was lowered to 6.9, Ca _i fell by 49 ± 11 nM but increased by 203 ± 51 nM (n = 6) when pH _o was raised to 7.9. These data indicate that rapid alkalinization of BC3H-1 cells results in a rapid transient rise of Ca _i . This transient is most likely due to the release of Ca from intracellular stores but may also involve an increase of Ca influx. Steady state values of Cai are positively correlated with steady state pH _i . These data may have implications for the contractile state of smooth muscle during periods of acid/base disturbances and relate to the role of elevated pH _i in cells from hypertensive animals.