Steady-state twitch ca fluxes and cytosolic ca buffering in rabbit
ventricular myocytes.
Delbridge, Leanne M. D., Jose W. M. Bassani, and Donald M. Bers.
Department of Physiology, Loyola University School of Medicine,
Maywood, IL 60153, USA
APStracts 2:0287C, 1995.
Intracellular Ca2+ ([Ca2+]i) transients and trans-sarcolemmal Ca2+
currents were measured in indo-1 loaded isolated rabbit ventricular
myocytes during whole cell voltage clamp to quantitate the components
of cytosolic Ca2+ influx and to describe the dynamic aspects of
cytosolic Ca2+ buffering during steady state contraction (0.5 Hz, 22
C). Sarcolemmal Ca2+ influx was directly measured from the integrated
Ca2+ current (ICa) recorded during the clamp (158 +/- 10 amol).
Sarcoplasmic reticulum (SR) Ca2+ content was determined from the
integrated electrogenic Na+-Ca2+ exchange current (IX) induced during
rapid application and sustained exposure of cells to caffeine to
elicit the release of the SR Ca2+ load (1208 +/- 170 amol). The mean
steady state SR Ca2+ load was calculated to be 87 +/- 13 [mu]M
([mu]mol/l non-mitochondrial cytosolic volume). Ca2+ influx via ICa
represented about 14 % of the stored SR Ca2+ and 23 % of the total
cytosolic Ca2+ flux during a twitch (47 +/- 6 [mu]M). Comparison of
electrophysiologically measured Ca2+ fluxes with Ca2+ transients
yield apparent buffering values of 60 for caffeine contractures and
110 for twitches (DCa2+ total / D Ca2+ free). This is consistent with
the occurrence of 'active' buffering of cytosolic Ca2+ by SR Ca2+
uptake during the twitch.
Received 17 May 1995; accepted in final form 25 July 1995.
APS Manuscript Number C281-5.
Article publication pending Am. J. Physiol. (Cell Physiology).
ISSN 1080-4757 Copyright 1995 The American Physiological Society.
Published in APStracts on 10 August 1995.