Characterization of single-channel depolarizations in smooth muscle by analysis of ATP-activated membrane voltage noise

Membrane voltage “noise” produced by adenosine 5´-triphosphate (ATP) in smooth muscle cells has been analyzed to infer the properties of individual ATP-activated membrane ionic channels and compare these with the kinetics of the quantal synaptic potential in smooth muscle, the spontaneous excitatory junction potential. (sEJP). Pressure application of ATP (10 μM in Krebs solution) for 10-30 seconds through a micropipette, but not application of Krebs alone, produced steady, low-amplitude membrane depolarizations accompanied by an increase in membrane voltage noise. Spectral noise analysis yielded a channel mean open time (m.o.t) for ATP-activated channels of 45.7±4.5 ms at 37°C. The decay time constant of SEJPs under similar conditions was 42.2±18.3 ms. This was not significantly different (P>0.05) compared with the m.o.t. of ATP-activated channels. By analogy with other synapses, it is concluded that ATP produces channel activations in smooth muscle consistent with its mediation of sEJPs. The authors´ analysis suggests the amplitude of the elementary depolarization to be ~20-30 μV, thus the number of ATP-activated channels underlying typical sEJPs of amplitude 3-12 mV may be approximately 100-500. These findings are discussed in relation to the microphysiology of operation of the autonomic nerve-smooth muscle synapse