Patch‐clamp studies of slow potential‐sensitive potassium channels in longitudinal smooth muscle cells of rabbit jejunum

Abstract

The patch-clamp technique was used to study single channel currents in membrane patches of longitudinal smooth muscle cells of rabbit jejunum dispersed by collagenase treatment. Recordings were made from both cell-attached and isolated patches. The predominant unit currents observed were outward at membrane potentials positive to the K equilibrium potential (EK), and they were rapidly and reversibly blocked by tetraethylammonium (TEA). Their size varied as EK was changed but was not noticeably affected by changing ENa, ECl or ECa; it was little altered in Ca-free EGTA [ethylene glycol bis[.beta.-aminoethylether]-N,N''-tetraacetic acid] solution. Thus, these currents apparently result mainly, if not exclusively, from the movements of K through channels insensitive to the Ca concentration. The properties of these K channels are described. The unit conductance varied slightly with potential in most experiments; around zero potential it was .apprx. 50 pS [picosiemens]. The conductance was dependent upon the K, but not the Ca gradient. Sublevels of conductance of .apprx. 2/3 and, less commonly, 1/3 of the fully conducting channel state were sometimes seen. Membrane patches were studied which showed 1 to .apprx. 12 levels of outward current which were presumed to result from the opening of up to 12 channels having the same characteristics. The probability of channel open state varied with membrane potential, increasing in the potential range -40 to +40 mV. Channel openings were rare negative to -40 mV. No inward currents through these K channels were observed as openings were not seen at membrane potentials negative to EK. When the probability of channel opening was low, channel openings occurred in bursts which could be separated by several seconds. Analysis of the openings of a single channel revealed that open times and short closed times were exponentially distributed with mean durations of 15-45 ms, and .apprx. 6 ms at zero potential. In some patches regular cyclical openings of several channels occurred. In other patches openings of individual channels appeared to be independent events as they were reasonably fitted by a binomial distribution. Following a step change from negative potentials, where channels were closed, to more positive potentials, channel openings increased during a period of 10 s to reach a steady state. No evidence of inactivation was observed. The existence of a population of potential-sensitive K-selective ion channels is suggested in the smooth muscle cell membrane which was closed at the resting membrane potential and which opened upon depolarization with slow (seconds) kinetics; these may be involved in the slow potential (wave) activity of this muscle.