Dual effects of 5-hydroxytryptamine on the heart of a pulmonate, Achatina fulica Férussac

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1. The effects of 5-hydroxytryptamine (serotonin) were examined on the isolated ventricle from the African giant snail, Achatina fulica Ferussac. 2. Serotonin at concentrations above 10−9M enhanced the frequency and amplitude of heart beat, while at high concentrations (∼10−5M) it produced a periodic arrest of beating after potentiation. 3. Methysergide inhibited the potentiation of heart beat and cinanserin strongly antagonized the periodic arrest of beating, suggesting that the two actions of serotonin are mediated through separate receptors. 4. The inhibitory actions of acetylcholine (ACh) on heart beat were also demonstrated. Strychnine partially blocked the responses to ACh. 5. Strychnine did not significantly inhibit the periodic arrest of beating caused by serotonin. High magnesium solution was also ineffective in blocking the two serotonin actions.

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