Pancreatic Islets: A Tissue Rich in Serotonin

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Abstract
Despite the fact that exogenous serotonin is an inhibitor of insulin secretion in a variety of species, many investigators doubt that endogenous serotonin plays a role in the regulation of insulin secretion. This is because, with the exception of the guinea pig, most animals do hot demonstrate histochemical evidence of serotonin in their pancreatic islets. In this study, the serotonin content of collagenase-isolated golden hamster islets was compared with the serotonin content of other hamster tissues known to contain serotonin. In the serotonin assay, serotonin was converted to N-acetyl serotonin by N-acetyltransferase; the latter is converted to tritiated melatonin by hydroxyindole-o-methyltransferase and tritiated S-adenosylmethionine. In control hamsters, the mean ± SEM serotonin concentration in μmol/kg wet weight of islets (26.3 ± 9.00) was greater than that of pancreas (4.0 ± 0.84), anterior pituitary (6.3 ± 1.53), median eminence (2.5 ± 0.69), and cerebral cortex (1.9 ± 0.30). There was no significant increase in tissue serotonin after the administration of the monoamine oxidase inhibitor tranylcypromine. When the serotonin precursor 5-hydroxytryptophan alone was given to hamsters, there was a 22-, 31-, and fivefold increase in islet, pancreatic, and anterior pituitary serotonin, respectively. When 5-hydroxytryptophan was given to hamsters after tranylcypromine administration, there was a 58-, 73-, 14-, and 10-fold increase in islet, pancreatic, anterior pituitary, and median eminence serotonin, respectively. Tissue serotonin decreased after administration of reserpine but not after administration of p-chiorophenylalanine. Exposure to collagenase did hot alter the serotonin concentration of pancreas or cerebral cortex in control or drug-treated hamsters. Hamsters receiving tranylcypromine plus 5-hydroxytryptophan had a higher plasma glucose concentration (266 ± 19 versus 116 ± 8 mg/dl), lower plasma insulin concentration (19 ± 3 versus 46 ± 8 μU/ml), and lower plasma insulin/glucose ratio (0.06 ± 0.01 versus 0.40 ± 0.06) than control hamsters. The present study demonstrates that animals whose islets do not have histochemical evidence of serotonin may have a high concentration of serotonin by chemical analyse that large increases in islet serotonin concentration probably inhibit pancreatic insulin secretion.