Effect of cAMP on intracellular and extracellular ATP content of Cl- -secreting epithelia and 3T3 fibroblasts.

Abstract

ATP may play an important role in the regulation of Cl- secretion in epithelial cells. Previous studies have suggested that ATP may regulate Cl- secretion in at least two ways. First, cytoplasmic ATP is required for the opening of cystic fibrosis transmembrane conductance regulator (CFTR) Cl- channels. Thus, an increase in metabolic activity associated with cAMP-stimulated Cl-secretion might decrease cellular levels of ATP, thereby decreasing the activity of CFTR Cl- channels. Second, cAMP-dependent opening of CFTR channels might allow the efflux of ATP through the channels into the mucosal solution. An increase in extracellular ATP could in turn activate apical membrane receptors that stimulate other Cl- channels. We tested the hypothesis that acute stimulation of Cl- secretion would produce changes in cell ATP concentration and/or in ATP concentration in the mucosal fluid. We found that in an intestinal epithelial cell line (T84) and in primary cultures of airway epithelia, cAMP-dependent stimulation of Cl- secretion did not alter cell ATP content. Moreover, cAMP agonists did not increase the ATP or nucleotide content of the mucosal fluid. In contrast, when 3T3 fibroblasts stably expressing CFTR were stimulated with cAMP agonists, cell ATP levels decreased. In addition, 3T3 cells appear to have a pathway that allows leakage of ATP into the extracellular fluid; this pathway is not affected by the presence or activation of CFTR. We interpret the data to suggest that changes in intracellular ATP probably do not regulate CFTR in Cl- -secreting epithelia under physiologic conditions and that secretion of ATP probably plays a minor role in control of transepithelial Cl-transport.