Inhibition of capacitative calcium entry is not obligatory for relaxation of the mouse anococcygeus by the NO/cyclic GMP signalling pathway

1 The object of this study was to determine whether inhibition of capacitat ive calcium entry is essential for relaxation of the mouse anococcygeus via the NO/cyclic GMP signalling pathway. 2 In intact muscles, thapsigargin (Tg; 100 nM)-induced tone was relaxed by NO, sodium nitroprusside (SNP), 8-Br-cyclic GMP, and nitrergic field stimul ation. The relaxations were similar in magnitude to those observed against carbachol (50 muM) tone and, with the exception of those to 8-Br-cyclic GMP , were reduced by the soluble guanylyl cyclase inhibitor 1H-[1.2,4]oxodiazo lo[4,3-a]quinoxalin-1-one (ODQ, 5 muM). 3 In single smooth muscle cells, loaded with Fura-2, both carbachol and Tg produced sustained elevations in cytoplasmic calcium levels ([Ca2+](i)). SN P inhibited the rise in [Ca2+](i) produced by carbachol, an effect attenuat ed by ODQ. In contrast. neither SNP nor 8-Br-cyclic GMP reduced the elevate d [Ca2+](i) associated with Tg. 4 In beta -escin skinned preparations, NO had no effect on tone induced by calcium (1 muM in the presence of 100 muM GTP). Carbachol and Tg produced f urther increases in calcium/GTP-induced tone and, in both cases, this addit ional tone was relaxed by NO and 8-Br-cyclic GMP. 5 The results support the hypothesis that the NO/cyclic GMP pathway inhibit s capacitative calcium entry by refilling the internal stores, since reduct ion in [Ca2+](i) was not observed in the presence of Tg. However, as muscle relaxation was still observed, impairment of capacitative calcium entry ca nnot be considered obligatory for relaxation. Results from skinned tissues suggest that inhibition of calcium sensitization processes, perhaps associa ted with store-depletion, may be an important mechanism of NO/cyclic GMP-in duced relaxation.