Inhibition by ethacrynic acid of no‐mediated relaxations of the rat anococcygeus muscle

1. The effects of ethacrynic acid were studied on relaxations elicited by nitric oxide (NO), the NO‐donors sodium nitroprusside (SNP) and glyceryl trinitrate (GTN), nitrergic nerve stimulation and the NO‐independent agent papaverine in isolated preparations of rat anococcygeus muscles. 2. Ethacrynic acid (100 μmol/L) produced complete relaxation of partially contracted anococcygeus muscles, but the tone recovered after the ethacrynic acid was washed out. Following exposure to ethacrynic acid, the relaxant responses to NO, SNP, GTN and nitrergic nerve stimulation were abolished or markedly reduced; however, the response to papaverine was only slightly reduced. 3. The presence of 3 mmol/L l‐cysteine during the period of exposure to ethacrynic acid prevented the inhibition of the relaxing effects of SNP, GTN and nitrergic nerve stimulation almost completely, but did not affect the slight reduction in responses to papaverine. 4. The addition of l‐cysteine (3 mmol/L) after incubation with ethacrynic acid did not significantly affect the inhibited responses to SNP and GTN; however, the inhibited responses to nitrergic nerve stimulation were slightly but significantly increased. 5. The results suggest that endogenous sulphydryl groups are required for the actions of NO, NO‐donating drugs and the nitrergic transmitter in the rat anococcygeus muscle and possibly for the synthesis or release of the nitrergic transmitter.