MEMBRANE CURRENT RESPONSES TO EXTERNALLY-APPLIED ATP IN THE LONGITUDINAL MUSCLE OF THE CHICKEN RECTUM

1 Membrane current responses to ATP in enzymically-dispersed single smooth muscle cells from the chicken rectum were investigated by the whole-cell voltage clamp technique. 2 In cells dialysed with a KCl-rich solution under voltage clamp at a holding potential of -40 mV, ATP (10 mum) produced an inward current followed by an outward current. When the holding potential was changed to 0 mV and -80 mV, the biphasic current response to ATP was converted to an outward current alone and an inward current alone, respectively. 3 External application of tetraethylammonium (TEA, 5 mm), intracellular dialysis with a CsCl-rich solution, or inclusion of EGTA (10 mm) in the pipette abolished the outward current response to ATP. 4 Neither depletion of Ca2+ store with caffeine (10 mm) nor block of voltage-gated Ca2+ channels with nifedipine (10 mum) affected the biphasic current response to ATP. After removal of the extracellular Ca2+ the outward current response to ATP was abolished. 5 Alpha,beta-methylene ATP (100 mum) elicited a current similar to the ATP-induced current. In the presence of alpha,beta-methylene ATP (100 muM), application of ATP (100 mum) was without effect. 6 In CsCl-filled cells, ATP analogues elicited an inward current and the order of potency was ATP is-approximately-equal-to alpha,beta-methylene ATP>ADP>>AMP. 7 Inclusion of GTP gamma S (0.2 mm) or GDP beta S (2 mm) in the pipette did not affect the ATP-induced inward current in CsCl-filled cells. The reversal potential of the ATP-induced inward current was about 0 mV and was completely inhibited after replacement of the cations in the bath solution by Tris. The reversal potential remained almost unchanged after replacement of Na+ in the bath solution with 110 mm Ca2+, but shifted in the negative direction after replacement of Na+ or both Na+ and Ca2+ with glucosamine. 8 The results suggest that ATP acts on P2 purinoceptors to cause activation of cation channels with selectivity for Ca2+ over Na+. Moreover, it appears that no G-protein-mediated mechanism is involved and increased Ca2+ entry through the cation channels causes activation of Ca2+-activated K+ channels.