Expression and trans-synaptic regulation of P(2x4) and P(2z) receptors for extracellular ATP in parotid acinar cells: Effects of parasympathetic denervation

Abstract

Trans-synaptic regulation of muscarinic, peptidergic, and purinergic responses after denervation has been reported previously in rat parotid acinar cells (McMillian, M. K., Soltoff, S. P., Cantley, L. C., Rudel, R., and Talamo, B. R. (1993) Br. J. Pharmacol. 108, 453-461). Characteristics of the ATP-mediated responses and the effects of parasympathetic denervation were further analyzed through assay of Ca2+ influx, using fluorescence ratio imaging methods, and by analysis of P(2x) receptor expression. ATP activates both a high affinity and a low affinity response with properties corresponding to the recently described P(2x4) and the P(2z) (P(2x7))-type purinoceptors, respectively. Reverse transcription-polymerase chain reaction analysis reveals mRNA for P(2x4) as well as P(2x7) subtypes but not P(2x1), P(2x2), P(2x3), P(2x5), or P(2x6). P(2x4) protein also is detected by Western blotting. Distribution of the two types of ATP receptor responses on individual cells was stochastic, with both high and low affinity responses on some cells, and only a single type of response on others. Sensitivity to P(2x4)-type activation also varied even among cells responsive to low concentrations of ATP. Parasympathetic denervation greatly enhanced responses, tripling the proportion of acinar cells with a P(2x4)-type response and increasing the fraction of highly sensitive cells by 7-fold. Moreover, P(2x4) mRNA is significantly increased following parasympathetic denervation. These data indicate that sensitivity to ATP is modulated by neurotransmission at parasympathetic synapses, at least in part through increased expression of P(2x4) mRNA, and suggest that similar regulation may occur at other sites in the nervous system where P(2x4) receptors are widely expressed.