Positive and negative inotropic effects of NO donors in atrial and ventricular fibres of the frog heart

The cardiac effects of the NO donors sodium nitroprusside (SNP), S -nitroso- N -acetyl-penicillamine (SNAP) and 3-morpholino-sydnonimine (SIN-1) were studied in frog fibres to evaluate the contribution of cyclic GMP-dependent mechanisms. SNP and SNAP (0·1-100 μM) reduced the force of contraction in a concentration-dependent manner in atrial and ventricular fibres. This effect was associated with a reduction in the time to peak ( T TP ) and the time for half-relaxation of contraction ( T ½ ). SIN-1 (100 μM) also reduced the force of contraction in two-thirds of the atrial fibres. However, it exerted a positive inotropic effect in the remaining atrial fibres, as well as in most ventricular fibres. The guanylyl cyclase inhibitor 1 H -[1,2,4]oxidiazolo[4,3-a]quinoxaline-1-one (ODQ, 10 μM) antagonized the negative inotropic effects of SIN-1 (50 μM) and SNAP (25 μM) but had no effect on the positive inotropic response to SIN-1 (100 μM). In the presence of SIN-1, superoxide dismutase (SOD, 50-200 U ml −1 ) either potentiated the negative inotropic effect or turned the positive inotropic effect of the drug into a negative effect. SOD had no effects when applied alone or in the presence of SNAP. 6-Anilino-5,8-quinolinedione (LY 83583, 3-30 μM), a superoxide anion generator also known as a cyclic GMP-lowering agent, exerted a positive inotropic effect, which was antagonized by SOD (200-370 U ml −1 ) but not by ODQ (10 μM). We conclude that SNP, SNAP and SIN-1 exert cyclic GMP-dependent negative inotropic effects, which are attributed to the generation of NO. In addition, SIN-1 and LY 83583 exert cyclic GMP-independent positive inotropic effects, which require the generation of superoxide anion.