Control of ion distribution in isolated smooth muscle cells. I. Potassium

We describe a technique for examining unidirectional ion movements in suspensions of enzymatically disaggregated smooth muscle cells derived from stomach muscle of the toad. This technique has been used to analyze the movement of 42K across these cells. This analysis was greatly simplified by the finding that the cells were in a steady state with respect to K+ distribution after isolation. The potassium contents of the isolated cells were identical to those of intact smooth muscle (131 mM/liter intracellular fluid) and stable for over 4 h; moreover, the unidirectional influx and efflux rates were equal. An additional simplification was provided by the finding that virtually all the K+ exchanges in a manner predicted for a simple two-compartment system consisting of an extracellular and an intracellular space. Transmembrane K+ flux in these cells averaged 1.2 pmol.cm-2.s-1 at room temperature. A large portion (approximately 80%) of 42K influx appeared to be mediated by a saturable transport system with an apparent Km of 0.6 mM and an apparent Vmax of 1.3 pmol.cm-2.s-1. The calculated resting membrane permeability to K+ in these isolated smooth muscle cells, assuming a membrane potential of -50 mV, was 2.9 X 10(-8) cm/s. The calculated gK+ was 2.7 mumho/cm2 constituting only a small fraction of the total membrane conductance as measured electrophysiologically. The latter finding suggests that the resting membrane potential in the isolated cells must be determined by ions in addition to K+. We propose that these methods for studying ion movements in smooth muscle should aid in unraveling the mechanisms responsible for controlling the distribution of ions both at rest, as in the present study, as well as in response to neurotransmitters.