Cytochemical localization of adenylate cyclase in cultured renal epithelial (A6) cells

To characterize the vasopressin-adenylate cyclase (AC) signaling pathway in control of Na+ reabsorption in cultured renal (A6) cells, we determined the distribution of AC with a cytochemical technique using 5'-adenylylimidodiphosphate as substrate and cerium chloride as capturing agent. The addition of forskolin to the medium to stimulate AC activity increased the production of reaction deposits at the enzyme sites. To ensure that the cells were close to their physiological states, cytochemical reactions were performed on unfixed tissues. Subsequent postfixation adequately preserved the morphological features of the cells. AC was mainly restricted to the lateral folds of the cells while the apical membranes were devoid of any deposits. This result provided evidence that the V2-AC pathway is not present in the apical membrane and, hence, any vasopressin action on apical Na+ channels from the luminal side of the cell must involve other signaling pathways. The cytochemical results provided further morphological evidence of the functional coupling between the basolateral and apical membranes of renal cells. We examined the idea that highly variable basal rates of Na+ transport in young differentiating cell cultures may be related to the degree of AC activity. Cytochemical results apparently revealed highly variable amounts of deposits in these cells, but by quantitative analysis of AC activity we could find no significant differences between cells of 6, 14, and 21 days.