AN ATP-GATED CATION CHANNEL WITH SOME P2Z-LIKE CHARACTERISTICS IN GASTRIC SMOOTH-MUSCLE CELLS OF TOAD

1. Whole-cell and single-channel currents elicited by extracellular AT P were studied in freshly dissociated smooth muscle cells from the sto mach of the toad Bufo marinus using standard patch clamp and microfluo rimetric techniques. 2. This ATP-gated cation channel shares a number of pharmacological and functional properties with native rat myometriu m receptors, certain native P2Z purinoceptors and the recently cloned P2X(7) purinoceptor. But, unlike the last two, the ATP-gated channel d oes not mediate the formation of large non-specific pores. Thus, it ma y represent a novel member of the P2X or P2Z class. 3. Extracellular a pplication of ATP (greater than or equal to 150 mu M) elicited an inwa rd whole-cell current at negative holding potentials that was inwardly rectifying and showed no sign of desensitization. Na+, Cs+ and, to a lesser degree, the organic cation choline served as charge carriers, b ut Cl- did not. Ratiometric fura-2 measurements indicated that the cur rent is carried in part by Ca2+. The EC(50) for ATP was 700 mu M in so lutions with a low divalent cation concentration. 4. ATP (greater than or equal to 100 mu M) at the extracellular surface of cell-attached o r excised patches elicited inwardly rectifying single-channel currents with a 22 pS conductance. Cl- did not serve as a charge carrier but b oth Na+ and Cs+ did, as did choline to a lesser extent. The mean open time of the channel was quite long, with a range in hundreds of millis econds at a holding potential of -70 mV. 5. Mg2+ and Ca2+ decreased th e magnitude of the ATP-induced whole-cell currents. Mg(2+)decreased bo th the amplitude and the activity of ATP-activated single-channel curr ents. 6. ADP, UTP, P-1,P-5-di-adenosine pentaphosphate (AP(5)A), adeno sine and alpha,beta-methylene ATP (alpha,beta-Me-ATP) did not induce s ignificant whole-cell current. ATP-gamma-S and 2-methylthio ATP (2-Me- S-ATP) were significantly less effective than ATP in inducing whole-ce ll currents, whereas benzoylbenzoyl ATP (BzATP) was more effective. Bz ATP, alpha,beta-Me-ATP, ATP-gamma-S and 2-Me-S-ATP induced single-chan nel currents, but a higher concentration of alpha,beta-Me-ATP was requ ired. 7. BzATP did not induce the formation of large non-specific pore s, as assayed using mag-fura-2 as a high molecular mass probe.