Activation of l-type ca2+ channels following purinoceptor stimulation by atp in an alveolar epithelial cell (l2). Dietl, Paul, Thomas Haller, Barbara Wirleitner, Harald V[diaeresis]olkl, Franz Friedrich, and J[diaeresis]org Striessnig. Departments of Physiology, Anaesthesia and Intensive Care Medicine and Biochemical Pharmacology, University of Innsbruck, A-6020 Innsbruck, Austria
APStracts 2:0134L, 1995.
In the alveolar epithelium, ATP increases the intracellular Ca2+ concentration ([Ca2+]i) and stimulates the secretion of surfactant. We investigated the effects of extracellular ATP on the membrane potential (VM), 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 VM, resulting from the activation of cation and Cl- conductances as revealed by ion replacements. The depolarizing phase of the VM shift was superimposed by Ca2+-dependent depolarizing spikes. Spikes were also induced by depolarizing VM with charybdotoxin (ChTX) or maitotoxin (MTX). Replacement of bath Ca2+ with Ba2+ or Sr2+ also evoked repetitive spikes. Ca2+ (Ba2+, Sr2+)-induced spikes were unaffected by pretreatment with ionomycin or thapsigargin. They were, however, completely abolished by (+)-isradipine (100 nM) and stimulated by BayK 8644 (100 nM). Whole cell L-type Ca2+ (Ba2+, Sr2+) currents were likewise abolished by (+)-isradipine and enhanced by BayK 8644. L -type Ca2+ channels were further confirmed by demonstrating high affinity dihydropyridine receptors stereoselectively labeled by (+) -[3H]-isradipine (Kd &LT 1 nM). In fura-2 experiments, ATP evoked a transient elevation of [Ca2+]i in the absence of Ca2+ and a biphasic sustained elevation in the presence of Ca2+, indicating intracellular Ca2+ release and Ca2+ entry. The ATP-induced fura-2 signals were unaffected by (+)-isradipine. We conclude that in L2 cells, L-type Ca2+ channels are activated following purinoceptor stimulation by ATP. The overall [Ca2+]i response is, however, mediated by Ca2+ entry through an (+)-isradipine-insensitive mechanism and by intracellular Ca2+ release. 

Received 3 May 1995; accepted in final form 12 July 1995.
APS Manuscript Number L130-5.
Article publication pending Am. J. Physiol. (Lung Cell. Mol.
Physiology).
ISSN 1080-4757 Copyright 1995 The American Physiological Society.
Published in APStracts on 14 August 1995.