Patch-clamp studies of slow potential-sensitive potassium channels in longitudinal smooth muscle cells of rabbit...

1. 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.2. The predominant unit currents observed were outward at membrane potentials positive to the potassium equilibrium potential (E(K)) and they were rapidly and reversibly blocked by tetraethylammonium (TEA). Their size varied as E(K) was changed but was not noticeably affected by changing E(Na), E(Cl) or E(Ca); it was little altered in calcium-free EGTA solution. Thus, these currents apparently result mainly, if not exclusively, from the movements of potassium ions through channels insensitive to the calcium ion concentration. The present study describes the properties of these potassium channels.3. The unit conductance varied slightly with potential in most experiments; around zero potential it was about 50 pS. The conductance was dependent upon the potassium, but not the calcium, gradient. Sub levels of conductance of about two-thirds and, less commonly, one-third of the fully conducting channel state were sometimes seen.4. Membrane patches were studied which showed one to about twelve levels of outward current which were presumed to result from the opening of up to twelve 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 potassium channels were observed as openings were not seen at membrane potentials negative to E(K).5. 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 about 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.6. 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.7. These results suggest the existence of a population of potential-sensitive potassium-selective ion channels in the smooth muscle cell membrane which are closed at the resting membrane potential and which open upon depolarization with slow (seconds) kinetics; these may be involved in the slow potential (wave) activity of this muscle.