Presence of a metabotropic and an ionotropic purinergic receptor on rat submandibular ductal cells

ATP (1 μM-1 mM) increased the intracellular Ca²⁺ concentration ([Ca²⁺](i)) in rat submandibular ductal cells. The dose-response curve was biphasic with a first plateau ~10 μM and a second increase at concentrations higher than 100 μM. This second increase was abolished in the absence of extracellular calcium or in the presence of Coomassie blue and could be mimicked by benzoyl-ATP. The activation of this response increased the uptake of extracellular manganese and the release of kallikrein, a ductal cell marker. The response at low concentrations of ATP was mimicked by ADP, 2-methylthioadenosine 5'-triphosphate (2-MeS-ATP; which was the most potent agonist), adenosine 5'-O-(2-thiodiphosphate), and UTP. Removal of extracellular calcium did not abolish this response, but ADP or 2-MeS-ATP had no effect after a preincubation with thapsigargin. A preincubation with 2- MeS-ATP desensitized the receptor involved in the response to both ADP and UTP. Similarly, a pretreatment with UTP prevented a further response to ADP. ADP increased the intracellular concentration of inositol 1,4,5-triphosphate but did not affect the secretion of kallikrein. In conclusion, rat submandibular ductal cells possess two types of purinergic receptors: a metabotropic (P(2y1)) receptor and a P(2x) ionotropic purinergic receptor coupled to a manganese-permeant calcium channel and to kallikrein secretion.