ACTIVATION OF L-TYPE CA2-CELL (L2)( CHANNELS AFTER PURINOCEPTOR STIMULATION BY ATP IN AN ALVEOLAR EPITHELIAL)

In the alveolar epithelium, ATP increases the intracellular Ca2+ conce ntration ([Ca2+](i)) and stimulates the secretion of surfactant. We in vestigated the effects of extracellular ATP on the membrane potential (V-m), the whole cell current, and [Ca2+](i) in a cloned rat alveolar epithelial cell line (L2). In microelectrode experiments, ATP caused a sustained depolarization of V-m, resulting from the activation of cat ion and Cl- conductances, as revealed by ion replacements. The depolar izing phase of the V-m shift was superimposed by Ca2+-dependent depola rizing spikes. Spikes were also induced by depolarizing V, with charyb dotoxin or maitotoxin. Replacement of bath Ca2+ with Ba2+ or Sr2+ also evoked repetitive spikes. Ca2+ (Ba2+ Sr2+)-induced spikes were unaffe cted by pretreatment with ionomycin or thapsigargin. They were, howeve r, completely abolished by (+)-isradipine (100 nM) and stimulated by B AY K 8644 (100 nM). Whole cell L-type Ca2+ (Ba2+, Sr2+) currents were similarly abolished by (+)-isradipine and enhanced by BAY K 8644. L-ty pe Ca2+ channels were further confirmed by demonstrating high-affinity dihydropyridine receptors stereoselectively labeled by (+)-[H-3]-isra dipine, apparent dissociation constant <1 nM. In fura 2 experiments, A TP evoked a transient elevation of [Ca2+](i) in the absence of Ca2+ an d a biphasic sustained elevation in the presence of Ca2+, indicating i ntracellular Ca2+ release and Ca2+ entry. The ATP-induced fura 2 signa ls were unaffected by (+)-isradipine. We conclude that in L2 cells, L- type Ca2+ channels are activated after purinoceptor stimulation by ATP . The overall [Ca2+](i) response is, however, mediated by Ca2+ entry t hrough an (+)-isradipine-insensitive mechanism and by intracellular Ca 2+ release.