FORCE AND INTRACELLULAR Ca2+ DURING NANC‐MEDIATED RELAXATION OF RAT ANOCOCCYGEUS MUSCLE AND THE EFFECTS OF CYCLOPIAZONIC ACID

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Abstract
1. Simultaneous recordings of tension and [Ca2+]i during NANC-mediated relaxation were made in the rat anococcygeus muscle under various conditions. 2. In muscles precontracted with guanethidine, nitrergic stimulations at 2 Hz produced a rapid decrease in both the tension and [Ca2+]i. 3. The nitric oxide synthase inhibitor, NG-nitro-L-Arginine (NOLA, 100 μmol/L) completely abolished the decreases in the [Ca2+]i and force response of the NANC-mediated relaxation. 4. Noradrenergic-mediated contractions elicited by electrical field stimulation were potentiated by the addition of NOLA. In the absence of NOLA, the motor 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 stimulations. Proportionately the potentiation of the 2 Hz response was of a far greater magnitude than that of the 10 Hz response. 5. The guanylate cyclase inhibitor methylene blue (10 μmol/ 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 inhibitor, cyclopiazonic acid, (10 μmol/ L) the responses to NANC stimulation were inhibited. The attenuated relaxation response displayed a bi-phasic timecourse and the Ca2+ change in comparison to that of the control was markedly smaller. In some cases, a relaxation was observed with no detectable change in the [Ca2+]i. 7. The results suggest that part of the relaxation response observed with NANC-mediated relaxation in the rat anococcygeus is dependent on Ca2+ sequestration into the sarcoplasmic reticulum. However, other Ca2+ lowering mechanisms and possible Ca2+ independent mechanisms may also contribute to the NANC relaxation response.