Carbachol-Activated Monovalent Cation-Selective Channels in the Murine Small Intestine

1 Bogomolets Institute of Physiology, National Academy of Sciences of Ukraine, Kyiv, Ukraine. 2 St. George’s Hospital Medical School, London, Great Britain. Correspondence should be addressed to A. V. Zholos (e-mail: zholos@serv.biph.kiev.ua). A cation current (I cat ), which is selective with respect to monovalent cations, has been observed in several types of smooth muscles [1, 2]. In visceral smooth muscles, carbachol (CCh) produces depolarization by activation of the monovalent cation-selective channels and subsequent opening of the voltagegated Ca channels. So far, there is no information on the action of CCh on single smooth muscle cells of the murine ileum. Using symmetrical Cs solutions, both the external application of 50 μM CCh and the internal action of 200 μM GTPγS evoked an inward current with a peak amplitude exceeding 0.5 nA (n = 50). The current-voltage (I-V) relationship for this current was Ushaped at negative potentials, and the reversal potential, E rev , was close to 0 mV. At least two types of monovalent cation-selective channels with unitary conductances of 17 and 70 pS (n = 12) were identified in outside-out patches. The open probability (P o ) for the 70 pS channel increased with depolarization, and the U shape of the I-V relationship of the whole-cell current at negative potentials was mostly determined by the voltage-dependent P o typical of this channel. External application of 20 μM quinine completely blocked single channel currents evoked by CCh or GTPγS in the outside-out patch configuration. Single channel openings in outside-out patches were rather stable and could be observed for more than 10 min after patch excision. The number of active channels in a patch was usually from one to four, and outside-out patches generally had either no channels or more than one active channel. The effects of external Ca and Mg ions on the behavior of single channels were also tested. Addition of Mg and Ca at 2.0, 2.5, or 10 mM concentrations to the bath solution reduced the unitary cationic channel conductance from 70 pS down to 42 pS and 32 pS, respectively, and somewhat increased the open channel noise. Our data suggest that the CCh-evoked I cat in smooth muscle cells of the murine ileum is mediated by at least two types of cationic channels with conductances of 17 or 70 pS. The 70 pS channel plays a major role in the generation of the whole-cell CCh-evoked I cat , and the U-shaped I-V relationship at negative potentials is mostly determined by the voltage dependence of P o typical of this channel.