Adenosine 5'‐triphosphate activates acetylcholine receptor channels in cultured Xenopus myotomal muscle cells.

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Abstract
1. Single-channel currents activated by extracellular adenosine 5''-triphosphate (ATP-induced currents) were recorded in cultured muscle cells of Xenopus laevis using the cell-attached patch clamp technique. 2. The amplitude histogram of the ATP-induced currents had two distinct peaks, corresponding to 60 pS (high-conductance (.gamma.) channels currents) and 41 pS (low-.gamma. channel currents). The peak values of the currents were unaltered during 1-6 days in culture. 3. The mean open time of the two types of ATP-induced currents was 0.93 ms for high-.gamma. and 0.86 ms for low-.gamma. channel currents at 50 mV hyperpolarization. The reversal potential of the ATP-induced current estimated from the I-V relationship, ranged between -5 and -15 mV. 4. The open-state probability of currents induced by 10 .mu.M-ATP decreased in the presence of 20 .mu.M-d-tubocurarine. 5. The frequency of ATP-induced current events depended upon the ATP concentration. The current events were first detected at 0.1 .mu.M-ATP and occurred with increasing frequency up to 10 .mu.M-ATP. At concentrations higher than 10 .mu.M the frequency of current events decreased. 6. When acteylcholine (ACH, 0.1 nM) was applied together without ATP, current events were rarely observed. 7. Two types of ATP-induced currents were also observed with adenylylimdo 5''-diphosphate (AMP-PNP) at one-hundred micromolar concentrations. Neither AMP (adenosine 5''-monophosphate) nor ADP (adenosine 5''-diphosphate) (1-500 .mu.M) induced channel events. 8. It is concluded that the nicotinic ACh receptor channels in cultured Xenopus skeletal muscle cells are opened by micromolar concentrations of exogenous ATP. The possible physiological significance is discussed.