1. The nitric oxide (NO) donor S-nitro-N-acetyl-penicillamine (SNAP) inhibits Helix aspersa heart activity and relaxes muscles. 2. K-free saline and ouabain both depress SNAP-induced relaxation in most experiments, but in a few preparations they either had no effect or potentiated SNAP-induced relaxation. 3. Na-K pump reactivation following preincubation in K-free saline leads to the pronounced transient relaxation of heart muscle, the magnitude of which depends on the duration of preincubation. 4. 0.1 mM SNAP inhibited the ouabain sensitive part of 86Rb uptake, which reflects Na-K pump activity. This inhibition is potentiated by phospholipase C. 5. SNAP increased cGMP levels in the heart. 6. These results indicate that SNAP-induced relaxation depends on Na and Ca gradients across the membrane, which suggests that Na:Ca exchange is involved in the mechanisms of SNAP-induced relaxation. It is postulated that SNAP elicits its inhibitory effect on the heart through a cGMP-dependent Na:Ca exchange.
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