Small fluorescent cells in the rat kidney contain 5-hydroxytryptamine not a catecholamine.

The rat kidney contains and excretes more dopamine than can be attributed solely to its originating from intrarenal noradrenergic nerves and plasma-free dopamine. It has recently been suggested that the source of this excess dopamine may be a population of small cells that are present in the renal medulla, and which by fluorescence and electron microscopy appear to contain high concentrations of a monoamine. We have therefore further investigated these cells. After formaldehyde treatment the fluorescence of the cells was characteristic of indoleamines rather than of catecholamine-containing structures. They did not form a visible precipitate with chromium salts in the classic chromaffin reaction. DOPA decarboxylase-like immunoreactivity was present in the adrenal medulla but not in the intrarenal cells or in mesenteric mast cells. 5-hydroxytryptamine (5-HT)-like immunoreactivity was seen in the intrarenal cells and in mesenteric mast cells, but was not evident in adrenal medullary cells. These results indicate that the intrarenal cells are 5-HT-containing mast cells and do not contribute to renal dopamine production.