Effect of changes of pHi on intracellular calcium in a smooth muscle-like cell line

The effect of changes of pHi on Cai were studied using fluorescent dyes in cells of the cultured smooth muscle-like line, BC3H-1. Resting Cai in these cells was 182 +/- 12 nM (n = 74) at pHo of 7.4. Upon exposure to NH4Cl, which rapidly alkalinized cells, a transient increase of Cai to 394 +/- 55 nM (n = 29) was observed. The peak of the transient occurred within 30 s of exposure to NH4Cl and returned to baseline within 1 minute. Two other procedures which resulted in rapid cellular alkalinization also caused a transient rise in Cai: exposure to and then removal of CO2 (Cai increased from 182 +/- 22 to 248 +/- 28 nM; n = 8); and exposure to and then removal of Na propionate (Cai increased from 242 +/- 32 to 456 +/- 71 nM; n = 9). The NH4Cl-induced Cai transient was eliminated by exposure to 0.2 mM TMB8 and to Ca-free solutions, but not by exposure to 0.5 mM LaCl3. Sustained changes of pHi can be induced by varying pHo. When pHo was lowered to 6.9, Cai fell by 49 +/- 11 nM but increased by 203 +/- 51 nM (n = 6) when pHo was raised to 7.9. These data indicate that rapid alkalinization of BC3H-1 cells results in a rapid transient rise of Cai. 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 pHi. 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 pHi in cells from hypertensive animals.