Nitric oxide donors inhibit spontaneous depolarizations by L-type Ca2+ currents in alveolar epithelial cells

L2 cells, a cloned pneumocyte-derived cell line, express voltage-dependent L-type Ca2+ channels, causing transient depolarizing spikes of the membrane potential (V-m) [P. Dietl, T. Haller, B. Wirleitner, H. Volkl, F. Friedrich, and J. Striessnig. Am. J. Physiol. 269 (Lung Cell. Mot. Physiol. 13): L873-L883, 1995]. In this study, we examined the effect of nitric oxide (NO)- and guanosine 3',5'-cyclic monophosphate (cGMP)-dependent cell signaling on the activity of L-type Ca2+ channels. Using conventional microelectrodes, spontaneous depolarizations (SD) of V-m by activation of these channels are regularly seen in the presence of 10 mM bath Sr2+. Th, NO donors sodium nitroprusside (SNP; 1 mM), 3-morpholinosydnonimine (SIN-1; 100 mu M), as well as S-nitroso-N-acetyl-D,L-penicillamine (SNAP; 10 mu M) caused a significant reduction of the frequency of Sr2+-induced SD. These effects were completely reversed by 6-anilino-5,8-quinolinequinone (10 mu M), an inhibitor of the soluble guanylyl cyclase, and could be mimicked by 8-bromoguanosine 3',5'-cyclic monophosphate (8-BrcGMP; 100 mu M). Perforated patch-clamp experiments revealed that 8-BrcGMP exerted a significant decrease of the depolarization-induced L-type Sr2+ current in the majority of tested cells. Consistent with the dependency of these NO-mediated effects on cGMP, incubation of L2 cells with SNP, SIN-1, and SNAP lead to a pronounced increase of cellular cGMP concentration. We conclude that the NO donors inhibit the activity of L-type Ca2+ channels in L2 cells via a cGMP-dependent pathway. In the alveoli, this might occur under conditions associated with the release of NO.