AJP - Cell Physiology, Vol 269, Issue 1 C48-C54, Copyright © 1995 by American Physiological Society
Possible contribution of long open state to noninactivating Ca2+ current in detrusor cells
S. Nakayama and A. F. Brading
University Department of Pharmacology, Oxford, United Kingdom.
The whole cell patch-clamp technique was used to measure Ca2+ current in
isolated smooth muscle cells from guinea pig urinary bladder.
Noniactivating Ca2+ channel current was modeled incorporating the long open
state of the Ca2+ channel. When inactivation was examined over a wide
voltage range, a completely U-shaped curve was obtained. Lack of
inactivation at +80 mV could be attributed to the long open state induced
by large depolarization as well as to minimal Ca2+ influx and
Ca(2+)-dependent inactivation. Activation parameters were obtained by
comparing the amplitudes of conditioned (by +80 mV, 5 s) and unconditioned
test potentials. With the use of the activation curve and the U-shaped
inactivation curve, a noninactivating current that peaks around +20 mV was
obtained. This current is composed of a so-called "window" current and a
persistent current brought about by the long open state. Differences in the
voltage dependence of the development of the long open state in various
smooth muscles, as well as differences in the equilibrium constant between
open and inactivated states, could underlie the different patterns of
contractile behavior that characterize smooth muscles.
