AJP - Cell Physiology, Vol 239, Issue 5 182-C189, Copyright © 1980 by American Physiological Society
Penetration-induced hyperpolarization as evidence for Ca2+ activation of K+ conductance in isolated smooth muscle cells
J. V. Walsh Jr and J. J. Singer
Single smooth muscle cells, freshly isolated by enzymatic digestion of the
stomach muscularis of the toad Bufo marinus were studied under direct
microscopic observation using standard electrophysiological techniques.
Following penetration with a microelectrode, a hyperpolarization lasting
many seconds occurred before the membrane depolarized to a steady-state
level. The following lines of evidence indicate that the
penetration-induced hyperpolarization results from an increase in K+
conductance caused by Ca2+ that enters the cell at the time of penetration:
1) The cell contracted at the time of penetration indicating that [Ca2+]i
was elevated even though no action potential had occurred; the cell
subsequently relaxed. 2) The input resistance was much lower during the
hyperpolarization than during the steady-state resting potential. In the
steady state all cells displayed outward-going rectification. 3) At
constant [Ca2+]0, the amplitude of the hyperpolarization varied with
log[K+]0 (1.3-56 mM) to a much greater degree than did the steady-state
potential. Tetraethylammonium chloride (TEA) (18.2 mM) reduced the
hyperpolarization. 4) At constant [K+]0, the amplitude of the
hyperpolarization increased as the [Ca2+]0 was raised (1.8-52.1 mM). 5)
With [Ca2+]0 low (less than or equal to 0.16 mM), the hyperpolarization was
almost completely abolished in the presence of a high concentration of Ba2+
(80 mM) or Mn2+ (79.2 mM); this was not the case with Sr2+.
