EFFECTS OF EXTRACELLULAR ATP ON ION-TRANSPORT SYSTEMS AND [CA-2+]I IN RAT PAROTID ACINAR-CELLS - COMPARISON WITH THE MUSCARINIC AGONIST CARBACHOL

The effects of extracellular ATP on ion fluxes and the intracellular free Ca2+ concentration ([Ca2+]i) were examined using a suspension of rat parotid acinar cells and were contrasted with the effects of the muscarinic agonist carbachol. Although ATP and carbachol both rapidly increased [Ca2+]i about threefold above the resting level (200-250 nM), the effect of ATP was due primarily to an influx of Ca2+ across the plasma membrane, while the initial response to carbachol was due to a release of Ca2+ from intracellular stores. Within 10 s, ATP (1 mM) and carbachol (20 µM) reduced the cellular Cl− content by 39-50% and cell volume by 15-25%. Both stimuli reduced the cytosolic K+ content by 57-65%, but there were marked differences in the rate and pattern of net K+ movement as well as the effects of K+ channel inhibitors on the effluxes initiated by the two stimuli. The maximum rate of the ATP-stimulated K+ efflux (~2,200 nmol K+/mg protein per min) was about two-thirds that of the carbachol-initiated efflux rate, and was reduced by ~30% (vs. 60% for the carbachol-stimulated K+ efflux) by TEA (tetraethylammonium), an inhibitor of the large conductance (BK) K+ channel. Charybdotoxin, another K+ channel blocker, was markedly more effective than TEA on the effects of both agonists, and reduced the rate of K+ effiux initiated by both ATP and carbachol by ~80%. The removal of extracellular Ca2+ reduced the ATPand the carbachol-stimulated rates of K+ efflux by 55 and 17%, respectively. The rate of K+ effiux initiated by either agonist was reduced by 78-95% in cells that were loaded with BAPTA to slow the elevation of [Ca2+]i. These results indicated that ATP and carbachol stimulated the effiux of K+ through multiple types of K+ permeable channels, and demonstrated that the relative proportion of efflux through the different pathways was different for the two stimuli. ATP and carbachol also stimulated the rapid entry of Na+ into the parotid cell, and elevated the intracellular Na+ content to 4.4 and 2.6 times the normal level, respectively. The rate of Na+ entry through Na+-K+-2Cl− cotransport and Na+-H+ exchange was similar whether stimulated by ATP, carbachol, or ionomycin, and uptake through these two carrier-mediated transporters accounted for 50% of the ATP-promoted Na+ influx. The remainder may be due to a nonselective cation channel and an ATP-gated cation channel that is also permeable to Ca2+ The consumption of extracellular ATP by an ecto-ATPase (apparent K0.5 for ATP is 0.93 mM) on the plasma membrane limited the duration of the response to ATP when large concentrations of cells were used. The effects of ATP on [Ca2+]i and ion fluxes were blocked specifically by DIDS (4,4'-diisothiocyanostilbene-2,2'-disulfonic acid), and were more potent in the absence of Mg2+, suggesting that the active nucleotide moiety was ATP4−. These studies suggest that ATP may function as a neurotransmitter and modulate fluid secretion by stimulating Ca2+-sensitive Cl− and K+ channels and multiple Na+ uptake pathways in the rat parotid acinar cell. Although some of these pathways were similar to those activated by carbachol, others were unique to ATP, which suggests that purinergic receptors may play a regulatory role in salivary secretion. © 1990, Rockefeller University Press., All rights reserved.