Properties of average-conductance cationic channels that mediate cholinergic excitation of guinea-pig ileum myocytes under conditions close to the physiological norm

The properties of cationic channels of an average unitary conductance were studied in guinea-pig ileum smooth muscle cells using a patch-clamp technique in the outside-out configuration. Cationic channels were activated by addition of 200 µM GTPγS to an intrapipette solution, which resulted in stable activation of G proteins. The replacement of external cesium-containing (in the absence of Ca2+ and Mg2+ ions) solution with a “more physiological” sodium solution containing 2.5 mM Ca2+ and 2.0 mM Mg2+ led to smaller values of both the amplitude of currents through the unitary cationic channels under study and the probability of the stay of the channel in the open state (Po). The drop in current amplitude was related mostly to the blocking effect of bivalent cations, while a decrease in the Po resulted from the replacement of Cs+ with Na+. Just a drop in the Po, which was responsible for approximately 85% of the inhibitory effect, played a crucial role in the suppression of the integral transmembrane current.