AJP - Cell Physiology, Vol 262, Issue 5 C1149-C1153, Copyright © 1992 by American Physiological Society
A discrete Na-Ca exchange-dependent Ca compartment in rat ventricular cells: exchange and localization
G. A. Langer and T. L. Rich
Department of Medicine, University of California, School of Medicine, Los Angeles 90024-1760.
Application of a new rapid perfusion (up to 4.8 ml/s) technique to
45Ca-labeled ventricular cells from adult rat heart has defined a discrete
intracellular calcium (Ca) compartment with the following characteristics:
1) its exchange is absolutely dependent on operation of the Na-Ca
exchanger, i.e., its isotopic content remains constant during washout in
the absence of Na and Ca and is released only upon addition of Na and Ca to
the perfusate. 2) At an extracellular Ca concentration of 1.0 mM it
contains 350 mumol/kg dry wt cells and exchanges with half time of 650 ms.
Ca flux from the compartment is 385 mumol.kg dry wt-1.s-1 or 20% of the
total nonperfusion limited flux from the cells. 3) Its content is decreased
19% by 10 mM caffeine but not diminished by exposure of the cells to 10(-6)
M ryanodine and not accessible to lanthanum (La) displacement. 4) Only
limited exchange occurs when only Na or Ca is present and exchange is
virtually eliminated by substitution of extracellular Li for extracellular
Na. 5) Replacement of Na and Ca to the perfusate after various periods of
removal produces no contraction (despite immediate Ca release from the
cell). The results define a discrete intracellular Ca compartment which
exchanges only via the Na-Ca exchanger. It is not La accessible, not in the
ryanodine- or caffeine-sensitive portions of the sarcoplasmic reticulum,
not in the mitochondria nor at the myofilaments, but may reside at inner
sarcolemmal leaflet binding sites.
