Lack of run-down of smooth muscle P2X receptor currents recorded with the amphotericin permeabilized patch technique, physiological and pharmacological characterization of the properties of mesenteric artery P2X receptor ion channels

1 Immunoreactivity for P2X(1), P2X(4) and P2X(5) receptor subtypes was detected in the smooth muscle cell layer of second and third order rat mesenteric arteries immunoreactivity, for P2X(2), P2X(3), P2X(6) and P2X(7) receptors was below the level of detection in the smooth muscle layer. 2 P2X receptor-mediated currents were recorded in patch clamp studies on acutely dissociated mesenteric artery smooth muscle cells. Purinergic agonists evoked transient inward currents that decayed rapidly in the continued presence of agonist (iota similar to 200 ms). Standard whole cell responses to repeated applications of agonist at 5 min intervals ran down. Run-down was unaffected by changes in extracellular calcium concentration, intracellular calcium buffering or the inclusion of ATP and GTP in the pipette solution. 3 Run-down was overcome and reproducible responses to purinergic agonists were recorded using the amphotericin permeabilized patch recording configuration. 4 The rank order of potency at the P2X receptor was ATP=2 methylthio ATP>alpha,beta -methylene ATP>CTP = 1-beta,gamma -methylene ATP. Only ATP and 2meSATP were full agonists. The P2 receptor antagonists suramin and PPADS inhibited P2X receptor-mediated currents with IC(50)s of 4 muM and 70 nM respectively. 5 These results provide further characterization of artery P2X receptors and demonstrate that the properties are dominated by a P2X(1)-like receptor phenotype. No evidence could be found for a phenotype corresponding to homomeric P2X(4) or P2X(5) receptors or to heteromeric P2X(1/5) receptors and the functional role of these receptors in arteries remains unclear.