Abstract
This study deals with the effect of 5-hydroxytryptamine (5-HT; serotonin) on glucose transport in isolated rat cardiac myocytes. In these cells, 5-HT (10-300 microns), as well as tryptamine, 5-methoxytryptamine and dopamine, elicited a 3-5 fold increase in glucose transport, as compared with control. This effect was maximal after 90 min, and was concomitant with a 1.8- and 1.5-fold increase in the amounts of glucose transporters GLUT1 and GLUT4 at the cell surface of the cardiomyocytes, as determined by using the photoaffinity label 3H-2-N-[4-(1-azi-2,2,2-trifluoroethyl)benzoyl]-1,3-bis-(D-manno s-4-yl) propyl-2-amine (3H-ATB-BMPA). In contrast, 3-3000 microM of the selective 5-HT receptor agonists 5-carboxyamido-tryptamine, alpha-methyl-serotonin, 2-methyl-serotonin or renzapride failed to stimulate glucose transport. The effect of 5-HT was not affected by (i) the 5-HT receptor antagonists methysergide (1 microM), ketanserin (1 microM), cyproheptadine (1 microM), MDL 72222 (1 microM) or ICS 205-930 (3 microM), nor by (ii) the adrenergic receptor antagonists prazosin (1 microM), yohimbine (1 microM) or propranolol (5 microM), nor by (iii) the dopaminergic antagonists SCH 23390 (1 microM) or haloperidol (1 microM). The monoamine oxidase inhibitors clorgyline (1 microM) and tranylcypromine (1 microM) completely suppressed the effect of 5-HT, whereas the control and insulin-stimulated rates of glucose transport were unaffected. Addition of catalase or glutathione diminished the 5-HT-dependent stimulation of glucose transport by 50%; these two factors are known to favour the degradation of H2O2 (which can be formed during the deamination of amines by monoamine oxidases). Glutathione also depressed the stimulatory action of exogenously added H2O2 (20 microM) by 30%. Furthermore, in cells treated with 5_HT, a time-dependent accumulation of 5-hydroxy-1H-indol-3-ylacetic acid (a product of 5-HT metabolism via monoamine oxidases) was observed, which paralleled the changes in glucose transport. In conclusion, the stimulation of glucose transport by 5-HT in cardiomyocytes is not mediated by a 5-HT1, 5-HT2, 5-HT3 or 5-HT4 receptor, nor by an adrenergic or dopaminergic receptor, but is likely to occur through the degradation of by a monoamine oxidase and concomitant formation of H2O2.