Effect of Serotonin on Glycogen Metabolism in Isolated Rat Liver*

In the invertebrate liver fluke, Fasciola hepatica, there is evidence that 5-hydroxytryptamine (serotonin) increases glycolysis (1, 2), glycogenolysis (1), and phosphorylase activity (3). However, previous studies in mammalian liver, in vitro and in vivo, have yielded conflicting results concerning the effect of serotonin on hepatic glycogen content, hepatic phosphorylase, and blood glucose. Some investigators have reported a hyperglycemic action that they have attributed either to liberation of epinephrine from the adrenal medulla (4, 5) or to a direct glycogenolytic effect in the liver (6, 7). Others have found that serotonin either has no effect on blood glu- cose or produces hypoglycemia (8-11).