Serotonin and its requirement for maintenance of contractility in muscle fibres isolated fromSchistosoma mansoni

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Abstract
SUMMARY: Muscle fibres isolated from adultSchistosoma mansonicontracted in a dose-dependent manner when exposed to elevated K+with a maximum response obtained with 25 mM K+. These contractions were dependent on extracellular Ca2+since Co2+(5 mM) or nicardipine (1 μM) blocked the high K+contractions. Serotonin (300 nM or higher) was required for maintenance of high K+contractions. With concentrations of serotonin less than 300 nM the response was dose dependent. 5-Methoxytryptamine or α-methylserotonin at 1 μM as well as 10 μM tryptamine were able to substitute for serotonin, but 1 μM 5-carboxyamidotryptamine was ineffective. The order of potency for antagonists (10 μM) was: methiothepin > metergoline > Ly-278,584 = ketanserin. This pattern of responsiveness does not fit well with any known mammalian serotonin receptor subtype. Since forskolin, an adenylate cyclase activator, is able to mimic the action of serotonin and H89, a protein kinase inhibitor, is able to block the effect of serotonin, the effect of serotonin on contractility of the muscle may beviaa cAMP-dependent pathway.