ATP ACTIVATES A CATION-PERMEABLE PATHWAY IN RAT PAROTID ACINAR-CELLS

Effects of several purinergic receptor agonists were examined on rat parotid acinar cells. Extracellular ATP stimulated Ca-45(2+) uptake into isolated rat parotid acinar cells in a concentration-dependent fashion (EC50 approximately 125-mu-M ATP) at a maximum rate of approximately 6 nmol.mg protein-1.min-1. In the absence of extracellular Na+, ATP increased the uptake rate by > 100%. Increasing concentrations of extracellular Na+ reduced the ATP-stimulated rate of Ca-45(2+) entry in a graded fashion (IC50 16.6 mM), suggesting that Ca2+ and Na+ compete for entry. Uptake rate was not reduced when intracellular Ca2+ was buffered with 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid, indicating that the effects of ATP were not initiated by an elevation in intracellular free Ca2+ concentration. 3-O-(4'-benzoyl)benzoyl-ATP was much more potent (EC50 approximately 4-mu-M) and stimulated Ca2+ influx at a greater rate (approximately 12 nmol.mg protein-1.min-1) than ATP. Other nucleotide analogues, including adenosine 5'-O-(3-thiotriphosphate), 2-methylthio-ATP, and 5'-adenylylimidodiphosphate, were much less effective than ATP. ATP produced a biphasic effect on membrane potential: an initial hyperpolarization was followed by a rapid depolarization. The depolarization was greatly reduced in the absence of extracellular Na+, but not in the absence of extracellular Ca2+, indicating that the majority of the depolarizing current was due to Na+ entry. Effects of ATP on the membrane potential were distinguishable from those of the Ca2+ ionophore ionomycin and the muscarinic agonist carbachol. Depolarization of the cells by gramicidin or K+ did not produce an increase in Ca-45(2+) uptake. These studies indicate that extracellular ATP activated a Na+- and Ca2+-permeable pathway, presumably an ATP-gated cation channel, and that the effects of the P2Z purinergic receptor are different from the phospholipase C-linked receptors on the parotid cell and the phospholipase C-linked P2Y receptor found on other cells.