AJP - Cell Physiology, Vol 263, Issue 5 C948-C952, Copyright © 1992 by American Physiological Society
Single calcium channel currents of arterial smooth muscle at physiological calcium concentrations
M. Gollasch, J. Hescheler, J. M. Quayle, J. B. Patlak and M. T. Nelson
Department of Pharmacology, University of Vermont, Colchester, Vermont 05446.
Entry of Ca through voltage-dependent Ca channels is an important regulator
of the function of smooth muscle, cardiac muscle, and neurons. Although Ca
channels have been extensively studied since the first descriptions of Ca
action potentials (P. Fatt and B. Katz. J. Physiol. Lond. 120: 171-204,
1953), the permeation rate of Ca through single Ca channels has not been
measured directly under physiological conditions. Instead, single Ca
channels have typically been examined using high concentrations (80-110 mM)
of another divalent charge carrier, Ba, so as to maximize the amplitude of
the single-channel currents. Calculations of unitary currents at 2 mM Ca
indicated that the single-channel currents would be immeasurably small
(i.e., < 0.1 pA). We provide here the first direct measurements of
single Ca channel currents at a physiological Ca concentration. Contrary to
earlier estimates, we have found that currents through single Ca channels
in arterial smooth muscle are 0.1-0.3 pA at 2 mM Ca and physiological
membrane potentials. These relatively large unitary currents permit direct
measurement of Ca channel properties under conditions that do not distort
their function. Our data also indicate that Ca permeates these channels at
relatively high rates in physiological Ca concentrations and membrane
potentials.
