Evidence for uptake and synthesis of 5-hydroxytryptamine by a subpopulation of intrinsic neurons in the guinea-pig heart.

Using an indirect immunofluorescence technique, a subpopulation of 5-hydroxytryptamine-like immunoreactive neurons was observed in cell cultures dissociated from the atria and interatrial septum of newborn guinea-pig heart maintained in fetal calf serum-supplemented medium. 5-Hydroxytryptamine has not been demonstrated in intracardiac neurons in situ, and since 5-hydroxytryptamine has been previously shown to be a constituent of fetal calf serum, the 5-hydroxytryptamine-like immunoreactivity seen in culture may have been the result of neuronal uptake of 5-hydroxytryptamine from the growth medium. This was examined by growing the cultures in a serum-free, hormone-supplemented, defined medium. Under these conditions, 5-hydroxytryptamine-like immunoreactive neurons were not present. When cultures were grown in hormone-supplemented, defined medium containing 10(-4) to 10(-6) M 5-hydroxytryptamine, some intracardiac neurons accumulated 5-hydroxytryptamine. This type of neuron also developed 5-hydroxytryptamine-like immunoreactivity after incubation with 5 X 10(-5) M 5-hydroxytryptophan, indicating that the subpopulation of intracardiac neurons which can take up exogenous 5-hydroxytryptamine can also synthesize it from 5-hydroxytryptophan. However, no 5-hydroxytryptamine-like immunoreactive neurons were observed after incubation with L-tryptophan, the other 5-hydroxytryptamine precursor molecule. Under all of the conditions described, some small, 5-hydroxytryptamine-like immunofluorescent cells, very similar to the catecholamine-containing, small intensely fluorescent cells of the heart, were observed in culture. Bright, 5-hydroxytryptamine-like immunoreactive endothelial cells were seen only in cultures maintained in defined medium and loaded with 5-hydroxytryptamine. The present study shows that some intracardiac neurons are amine-handling, and also raises the possibility that 5-hydroxytryptamine is utilized as a neurotransmitter or neuromodulator by these neurons in the mammalian heart. Further, there is evidence to suggest that two populations of small intensely fluorescent cells, one containing 5-hydroxytryptamine, the other a catecholamine, are present in the heart; and to indicate that atrial endothelial cells can take up 5-hydroxytryptamine.