Nitric oxide inhibits calcium release from sarcoplasmic reticulum of porcine tracheal smooth muscle cells

In the present study, effects of the nitric oxide donor, S-nitroso-N- acetylpenicillamine (SNAP), on sarcoplusmic reticulum (SR) Ca²⁺ release were examined in freshly dissociated porcine tracheal smooth muscle (TSM) cells. Fura 2-loaded TSM cells were imaged using video fluorescence microscopy. SR Ca²⁺ release was induced by acetylcholine (ACh), which acts principally through inositol 1,4,5-trisphosphate (IP₃) receptors, and by caffeine, which acts principally through ryanodine receptors (RyR). SNAP inhibited ACh-induced SR Ca²⁺ release at both 0 and 2.5 mM extracellular Ca²⁺. Degraded SNAP had no effect on ACh-induced SR Ca²⁺ release. SNAP also inhibited caffeine-induced SR Ca²⁺ release. ACh-induced Ca²⁺ influx was not affected by SNAP when SR reloading was blocked by thapsigargin. SNAP also did not affect SR Ca²⁺ reuptake. The membrane-permeant analogue of guanosine 3',5'-cyclic monophosphate (cGMP), 8-bromo-cGMP, mimicked the effects of SNAP. These results suggest that, in porcine TSM cells, SNAP reduces the intracellular Ca²⁺ response to ACh and caffeine by inhibiting SR Ca²⁺ release through both IP₃ and RyR, but not by inhibiting influx or repletion of the SR Ca²⁺ stores. These effects are likely mediated via cGMP-dependent mechanisms.