An ATP-sensitive conductance in cultured smooth muscle cells from pregnant rat myometrium

The whole cell voltage-clamp technique was used to study the effects of extracellular ATP in cultured smooth muscle cells isolated from pregnant rat myometrium. An inward current was elicited by ATP (IATP) in cells held at -70 mV under voltage clamp. The amplitude of IATP was reduced by estrogen pretreatment and by the end of pregnancy. IATP not only did not undergo any desensitization but showed facilitation. The current-voltage relationship of IATP was linear and reversed close to 0 mV. Changing the sodium electrochemical gradient by decreasing extracellular or intracellular sodium resulted in a linear relationship between the reversal potential of IATP and Na equilibrium potential that, however, differed from the predicted curve for a purely sodium conductance. The conductance activated by ATP was monovalent cation selective with little discrimination between potassium, cesium, and sodium ions. IATP was depressed by divalent cations, and the rank order of potency was Co greater than Mg greater than Ca greater than Ba, suggesting that the free-acid form of ATP was the effective ligand. Adenosine, AMP, and ADP were ineffective in eliciting IATP, whereas ATP gamma S and alpha,beta-methylene ATP were capable of mimicking the effects of ATP, although they were less potent. These results are consistent with the free-acid form of ATP activating a monovalent cation-selective and estrogen-sensitive conductance in myometrium.