The Journal of General Physiology, Vol 75, 163-182, Copyright © 1980 by The Rockefeller University Press
Control of ion distribution in isolated smooth muscle cells. I. Potassium
CR Scheid and FS Fay
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.
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