POTENTIATION OF REGULATORY VOLUME DECREASE BY P-2U PURINOCEPTORS IN HSG-PA CELLS

HSG-PA human salivary gland duct cells exhibit progressively increased regulatory volume decrease (RVD) in response to decreased medium osmo larity. The P-2U purinoceptor agonist UTP causes a potentiation of RVD , the extent of which is most pronounced in 220 mosM medium and is lea st apparent in 180 mosM medium. We examined the underlying mechanisms for this effect. Exposure of HSG-PA cells to UTP promotes Ca2+ mobiliz ation, hyperpolarization, and net K+ efflux, suggesting the participat ion of Ca2+-activated K+ channels in RVD. To delineate the anion count erpart of K+ movement during RVD, cell swelling in the presence of gra micidin, which abolishes the membrane potential, was measured. In resp onse to a sudden dilution in hypotonic media, gramicidin-treated cells swelled immediately, followed by a ''secondary swelling'' in 180 but not in 220 mosM medium. The results suggest that in 180 mosM cells per form spontaneous RVD mediated by increased anion conductance. In 220 m osM medium in which RVD is minimal, the increase in anion conductance is marginal. In our model of RVD in which cells were challenged by UTP , the ensuing hyperpolar ization provides the driving force for net Cl - efflux, which is confirmed by tracer flux studies during purinocepto r-activated RVD. Thus RVD, which has long been regarded as a self-suff icient cellular program, appears to be subject to extracellular contro l in HSG-PA cells through receptor-mediated processes.