Nitric oxide induces [Ca2+](i) oscillations in pituitary GH(3) cells: involvement of I-DR and ERG K+ currents

The role of nitric oxide ( NO) in the occurrence of intracellular Ca2+ concentration ([Ca2+](i)) oscillations in pituitary GH(3) cells was evaluated by studying the effect of increasing or decreasing endogenous NO synthesis with L-arginine and nitro-L-arginine methyl ester (L-NAME), respectively. When NO synthesis was blocked with L- NAME (1 mM) [Ca2+](i), oscillations disappeared in 68% of spontaneously active cells, whereas 41% of the quiescent cells showed [Ca2+](i) oscillations in response to the NO synthase ( NOS) substrate L- arginine ( 10 mM). This effect was reproduced by the NO donors NOC-18 and S-nitroso-N-acetylpenicillamine ( SNAP). NOC- 18 was ineffective in the presence of the L- type voltage-dependent Ca2+ channels (VDCC) blocker nimodipine (1 mu M) or in Ca2+- free medium. Conversely, its effect was preserved when Ca2+ release from intracellular Ca2+ stores was inhibited either with the ryanodine-receptor blocker ryanodine (500 mu M) or with the inositol 1,4,5-trisphosphate receptor blocker xestospongin C ( 3 mu M). These results suggest that NO induces the appearance of [Ca2+](i) oscillations by determining Ca2+ influx. Patch-clamp experiments excluded that NO acted directly on VDCC but suggested that NO determined membrane depolarization because of the inhibition of voltage-gated K+ channels. NOC-18 and SNAP caused a decrease in the amplitude of slow-inactivating ( IDR) and ether-a-go-go-related gene (ERG) hyperpolarization-evoked, deactivating K+ currents. Similar results were obtained when GH3 cells were treated with L- arginine. The present study suggests that in GH3 cells, endogenous NO plays a permissive role for the occurrence of spontaneous [Ca2+](i) oscillations through an inhibitory effect on IDR and on IERG.