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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 microM-ATP decreased in the presence of 20 microM-d-tubocurarine. 5. The frequency of ATP-induced current events depended upon the ATP concentration. The current events were first detected at 0.1 microM-ATP and occurred with increasing frequency up to 10 microM-ATP. At concentrations higher than 10 microM, the frequency of current events decreased. 6. When acetylcholine (ACh, 0.1 nM) was applied together with various concentrations of ATP, the frequency of current events increased 2- to 3-fold at the ATP concentration range between 0.1 and 10 microM. At higher concentrations of ATP the frequency decreased again. When ACh (0.1 nM) was applied without ATP, current events were rarely observed. 7. Two types of ATP-induced currents were also observed with adenylylimido 5'-diphosphate (AMP-PNP) at one-hundred micromolar concentrations. Neither AMP (adenosine 5'-monophosphate) nor ADP (adenosine 5'-diphosphate) (1-500 microM) 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.
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