AJP - Cell Physiology, Vol 265, Issue 2 C556-C561, Copyright © 1993 by American Physiological Society
Further characterization of the Na-Ca exchange-dependent Ca compartment in rat ventricular cells
G. A. Langer and T. L. Rich
Department of Medicine, University of California, School of Medicine, Los Angeles 90024-1760.
A discrete subcellular Ca compartment, absolutely dependent upon Na-Ca
exchange for its exchange, has been previously identified in ventricular
cells from both adult rat and from neonatal rat cultures. The present study
further characterizes this compartment in the adult ventricular myocyte.
The compartment's exchange is significantly decreased by 20 microM
3,4-dichlorobenzamil. Its content responds significantly to variation of
the extracellular Ca concentration ([Ca]o) and to reversal of the Na-Ca
exchanger by reduction of extracellular Na concentration to zero. Its
content decreases to 155 +/- 24 mumol/kg dry cell wt in 0.2 mM [Ca]o and
increases to 623 +/- 48 mumol/kg at 6.5 mM [Ca]o, where it appears to be
near saturation. The compartment is significantly reduced by 45Ca labeling
in the presence of 10 mM caffeine (23%) and 1 mM ryanodine (22%). This
compartment is not significantly affected by 1 microM thapsigargin. 45Ca
activity is retained in the compartment when 45Ca washout is conducted
without Na or Ca in the washout solution, i.e., Na-Ca exchange is not
operative. Under these conditions, addition of 1 mM Cd to the washout
produces a significant release of 45Ca from the compartment. The results
are interpreted to indicate that the compartment in the adult cell has two
components that are in equilibrium: a relatively small sarcoplasmic
reticulum (SR) component and a larger "non-SR" component. This is in
contrast to the compartment in the cultured cell in which the compartment
is 2.6 times larger and has a larger SR component. The possibility that the
non-SR component represents Ca bound to the cytoplasmic leaflet of the
sarcolemma at SR junction regions is discussed.
