FORCE AND INTRACELLULAR CA2-MEDIATED RELAXATION OF RAT ANOCOCCYGEUS MUSCLE AND THE EFFECTS OF CYCLOPIAZONIC ACID( DURING CYCLIC NUCLEOTIDE)

1 Simultaneous recordings of tension and [Ca2+](i) were made in rat an ococcygeus muscle strips to investigate possible mechanisms involved d uring cyclic nucleotide-mediated relaxation. Relaxation of pre-contrac ted muscles was induced by sodium nitroprusside (SNP) or forskolin and the effects of cyclopiazonic acid (CPA) on these responses were exami ned. 2 In muscles pre-contracted with 0.2 mu M phenylephrine addition of SNP (10 mu M) caused a rapid and near complete relaxation of force. This was accompanied by a decrease in [Ca2+](i), however, this was no t of a comparable magnitude to the decrease in force. The level of [Ca 2+](i) in muscles relaxed with SNP was shown to be associated with sub stantially higher force levels in the absence of SNP. Forskolin (10 mu M) caused a slower, essentially complete relaxation which was associa ted with a proportional decrease in [Ca2+](i). 3 In muscles pretreated with SNP or forskolin subsequent responses to phenylephrine were atte nuated with both force and [Ca2+](i) rising slowly to attain eventuall y levels similar to those observed when the relaxant was applied to pr e-contracted muscles. 4 Exposure of the muscles to the sarcoplasmic re ticulum Ca2+-ATPase inhibitor, CPA (10 mu M), resulted in a sustained increase in [Ca2+](i) which, in most cases, was not associated with an y force development. The relaxation and decrease in [Ca2+](i) in respo nse to both SNP and forskolin were attenuated and substantially slowed in the presence of CPA. Overall the extent of this attenuation was gr eater for SNP. For both SNP and forskolin, CPA attenuated the decrease in [Ca2+](i) to a greater extent than the decrease in force. In some cases, SNP-mediated relaxation in the presence of CPA was observed wit h almost no detectable change in [Ca2+](i). 5 The results suggest that , in the rat anococcygeus muscle under normal circumstances, a lowerin g of [Ca2+](i) can fully account for the relaxation induced by forskol in but not for that induced by SNP, where mechanisms independent of ch anges in [Ca2+](i) appear to contribute. Whilst Ca2+ sequestration int o the sarcoplasmic reticulum plays a role in the relaxation mediated b y both SNP and forskolin other Ca2+ lowering mechanisms may also be in volved, especially in the response to forskolin.