Angiotensin ii activation of ca++-permeant nonselective cation
channels in rat adrenal glomerulosa cells.
Lotshaw, David P., and Fang Li.
Dept. of Biological Sciences, Northern Illinois Univ., DeKalb, IL
60115
APStracts 3:0197C, 1996.
A Ca++ permeant, nonselective cation channel was observed in cell
-attached and inside-out membrane patches from rat adrenal glomerulosa
cells maintained in primary cell culture. In cell-attached patches
under near physiological ionic conditions, single channel currents
exhibited a reversal potential near -10 mV, inward rectification, a
nearly linear slope conductance between 0 mV and -80 mV of 17.4 pS,
and voltage-dependent block at potentials more negative than -80 mV.
Channels exhibiting identical conductance and gating properties were
observed in inside-out patches however channel gating ran down within
minutes in this configuation. In the inside-out configuration,
channel gating did not require cytosolic Ca++ (Ca++ < 10-9) and
inward rectification was relieved by removal of intracellular Mg++.
Relative ionic permeability was calculated using reversal potential
measurements from inside-out patches under biionic conditions. The
channel discriminated poorly among monovalent cations
(PLi>PK>PCs>PNa) and was not significantly permeable
to anions. The channel was permeable to Ca++, exhibiting a relative
permeability ratio of 0.29 (PCa:PNa) when measured with 110 mM Ca++
on the intracellular face and a permeability ratio of 4.38 (PCa:PNa)
with 110 mM Ca++ on the extracellular face. Channel gating behavior
was episodic with open times ranging from milliseconds to tens of
seconds and closed times lasting up to several minutes or longer.
Channel gating appeared to be relatively voltage-independent except
that mean channel open time and open probability were reduced by
membrane hyperpolarization. In cell-attached patches bath application
of 1 nM angiotensin II increased the channel open probability,
primarily affecting channels exhibiting a low open probability prior
to stimulation. Using nystatin perforated-patch current clamp to
measure membrane potential, AngII was observed to induce large
transient membrane depolarizations, consistent with activation of an
inward current. We hypothesize that this channel is an important
component of angiotensin II-induced membrane depolarization and Ca++
influx during stimulation of aldosterone secretion.
Received 20 November 1995; accepted in final form 13 June 1996.
APS Manuscript Number C697-5.
Article publication pending Am. J. Physiol. (Cell Physiology).
ISSN 1080-4757 Copyright 1996 The American Physiological Society.
Published in APStracts on 4 July 96