Na+/Ca2+ exchange and its role in intracellular Ca2+ regulation in guinea pig detrusor smooth muscle

The role of Na+/Ca2+ exchange in regulating intracellular Ca2+ concentratio n ([Ca2+](i)) in isolated smooth muscle cells from the guinea pig urinary b ladder was investigated. Incremental reduction of extracellular Na+ concent ration resulted in a graded rise of [Ca2+](i); 50-100 muM strophanthidin al so increased [Ca2+](i). A small outward current accompanied the rise of [Ca 2+](i) in low-Na+ solutions (17.1 +/- 1.8 pA in 29.4 mM Na+). The quantity of Ca2+ influx through the exchanger was estimated from the charge carried by the outward current and was similar to 30 times that which is necessary to account for the rise of [Ca2+](i), after correction was made for intrace llular Ca2+ buffering. Ca2+ influx through the exchanger was able to load i ntracellular Ca2+ stores. It is concluded that the level of resting [Ca2+]( i) is not determined by the exchanger, and under resting conditions (membra ne potential -50 to -60 mV), there is little net flux through the exchanger . However, a small rise of intracellular Na+ concentration would be suffici ent to generate significant net Ca2+ influx.