[Effect of purinergic agents on ion movement in submaxillary glands]

Rat submandibular glands have been digested with collagenase P and a crude cellular suspension has been prepared. The [Ca2+]i and the pHi of these cells have been measured using fluorescent probes extracellular ATP increases the [Ca2+]i. At low concentrations ATP binds to a metabotropic receptor coupled to the mobilization of intracellular calcium stores. At high concentrations ATP activates a ionotropic receptor coupled to a non-specific cation channel permeant to calcium, sodium or zinc. The increase of the [Ca2+]i in response to ATP opens an anion permeability leading to a drop of the pHi. The acidification in response to ATP is potentiated by the removal of extracellular chloride or by the addition of an inhibitor of chloride channels in the incubation medium. Acetazolamide, an inhibitor of carbonic anhydrase blocks by 30% the intracellular acidosis in response to ATP. It is concluded that anions (among which bicarbonate is only a fraction) leave the cells by a chloride channel. The pHi does not decrease below 6.8 for two reasons: 1) the uptake of sodium by the non-specific cation channel and the ensuing depolarization on one hand and the decrease of the pHi on the other hand reverse the proton-moving force; 2) at low pHi the non-specific cation channel becomes permeable to protons. These results suggest that purinergic agonists are secretagogues. But at the opposite to classical secretagogues which increase the efflux of chloride, purinergic agonists rather increase the permeability to organic anions.