cGMP-mediated Ca²⁺release from IP₃-insensitive Ca²⁺stores in smooth muscle

Recent studies on the role of nitric oxide (NO) in gastrointestinal smooth muscle have raised the possibility that NO-stimulated cGMP could, in the absence of cGMP-dependent protein kinase (PKG) activity, act as a Ca 2+ -mobilizing messenger [K. S. Murthy, K.-M. Zhang, J.-G. Jin, J. T. Grider, and G. M. Makhlouf. Am. J. Physiol. 265 ( Gastrointest. Liver Physiol. 28): G660–G671, 1993]. This notion was examined in dispersed gastric smooth muscle cells with 8-bromo-cGMP (8-BrcGMP) and with NO and vasoactive intestinal peptide (VIP), which stimulate endogenous cGMP. In muscle cells treated with cAMP-dependent protein kinase (PKA) and PKG inhibitors (H-89 and KT-5823), 8-BrcGMP (10 μM), NO (1 μM), and VIP (1 μM) stimulated 45 Ca 2+ release (21 ± 3 to 30 ± 1% decrease in 45 Ca 2+ cell content); Ca 2+ release stimulated by 8-BrcGMP was concentration dependent with an EC 50 of 0.4 ± 0.1 μM and a threshold of 10 nM. 8-BrcGMP and NO increased cytosolic free Ca 2+ concentration ([Ca 2+ ] i ) and induced contraction; both responses were abolished after Ca 2+ stores were depleted with thapsigargin. With VIP, which normally increases [Ca 2+ ] i by stimulating Ca 2+ influx, treatment with PKA and PKG inhibitors caused a further increase in [Ca 2+ ] i that reverted to control levels in cells pretreated with thapsigargin. Neither Ca 2+ release nor contraction induced by cGMP and NO in permeabilized muscle cells was affected by heparin or ruthenium red. Ca 2+ release induced by maximally effective concentrations of cGMP and inositol 1,4,5-trisphosphate (IP 3 ) was additive, independent of which agent was applied first. We conclude that, in the absence of PKA and PKG activity, cGMP stimulates Ca 2+ release from an IP 3 -insensitive store and that its effect is additive to that of IP 3 .