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

1. Simultaneous recordings of tension and [Ca2+](i) during NANC-mediat ed relaxation were made in the rat anococcygeus muscle under various c onditions. 2. In muscles precontracted with guanethidine, nitrergic st imulations at 2 Hz produced a rapid decrease in both the tension and [ Ca2+](i). 3. The nitric oxide synthase inhibitor, N-G-nitro-L-Arginine (NOLA, 100 mu mol/L) completely abolished the decreases in the [Ca2+] (i) and force response of the NANC-mediated relaxation, 4. Noradrenerg ic-mediated contractions elicited by electrical field stimulation were potentiated by the addition of NOLA. In the absence of NOLA, the moto r responses were larger in magnitude at 10 Hz stimulation than at 2 Hz . After NOLA, both the force response and the associated rise in [Ca2](i) were substantially increased in comparison to the control stimula tions. Proportionately the potentiation of the 2 Hz response was of a far greater magnitude than that of the 10 Hz response. 5. The guanylat e cyclase inhibitor methylene blue (10 mu mol/L), partially inhibited the force and [Ca2+](i) response of the NANC relaxation. 6. Following exposure of the muscles to the sarcoplasmic reticulum Ca2+-ATPase inhi bitor, cyclopiazonic acid, (10 mu mol/L) the responses to NANC stimula tion were inhibited. The attenuated relaxation response displayed a bi -phasic timecourse and the Ca2+ change in comparison to that of the co ntrol was markedly smaller. In some cases, a relaxation was observed w ith no detectable change in the [Ca2+](i). 7. The results suggest that part of the relaxation response observed with NANC-mediated relaxatio n in the rat anococcygeus is dependent on Ca2+ sequestration into the sarcoplasmic reticulum. However, other Ca2+ lowering mechanisms and po ssible Ca2+ independent mechanisms may also contribute to the NANC rel egation response.