AJP - Cell Physiology, Vol 260, Issue 5 C900-C909, Copyright © 1991 by American Physiological Society
Citrate decreases contraction and Ca current in cardiac muscle independent of its buffering action
D. M. Bers, L. V. Hryshko, S. M. Harrison and D. D. Dawson
Division of Biomedical Sciences, University of California, Riverside 92521-0121.
Extracellular Ca (Cao) depletions that occur during cardiac muscle
contractions are indicative of net Ca entry. Buffering Cao concentration
([Ca]o) with citrate can limit the magnitude of these Cao depletions [e.g.,
Shattock and Bers. Am. J. Physiol. 256 (Cell Physiol. 25): C813-C822, 1989]
which theoretically would allow more Ca entry and consequently greater
force at the same free [Ca]o. However, Shimoni and Ginsburg [Am. J.
Physiol. 252 (Cell Physiol. 21): C248-C252, 1987] have shown that citrate
inhibits cardiac contractions and suggested that this was due to its
Ca-buffering action (i.e., dissipating a local elevation of [Ca] at the
outer sarcolemmal surface and thereby decreasing Ca influx). To examine the
effects of Ca buffering per se, we compared the effects of four
low-affinity Ca buffers [citrate, nitrilotriacetic acid (NTA), dipicolinic
acid (DPA), and acetamidoiminodiacetic acid (ADA)] on several cardiac
preparations. In Mg-free medium with 2 mM free Ca (measured using
murexide), citrate, DPA, and ADA (10 mM) decreased the force of twitch
contractions in rabbit ventricle to 76 +/- 2, 60 +/- 2, and 85 +/- 2%,
respectively, but 10 mM NTA increased force slightly to 105 +/- 2%. No
simple correlation was observed between the Ca affinity of the buffer and
its effect on tension. These effects were not due to changes in
sarcoplasmic reticulum (SR) Ca loading because rapid cooling contractures
were not affected and similar results were observed in the presence of
caffeine or ryanodine. The depressant effects of citrate and ADA on tension
were greater at pH 5.5-6 and ADA had no effect at pH 8.5. Thus the
depressant effect is stronger with more protonated forms of citrate and
ADA, which are also poorer Ca buffers. Citrate (but not NTA) decreased Ca
current in whole cell voltage clamp and shifted the current-voltage
relationship and reversal potential to more negative potentials. Citrate
decreased Ca current more effectively at higher citrate and lower Ca
concentrations. We conclude that citrate (and some other weak Ca buffers)
may directly decrease Ca current and contraction in a manner independent of
Ca buffering ability.
