AJP - Heart and Circulatory Physiology, Vol 250, Issue 3 360-H365, Copyright © 1986 by American Physiological Society
Effects of changing Ca2+-to-H+ ratio on Ca2+ uptake by cardiac sarcoplasmic reticulum
D. O. Levitsky and D. S. Benevolensky
Ca uptake by fragments of cardiac sarcoplasmic reticulum (SR) in the
absence of oxalate depends on the ratios between concentrations of free
Ca2+ and H+ and on the level of Ca loading. Using a rapid quenching
technique, we showed that at [Ca2+]f less than 1 microM or in short time
intervals after ATP addition (less than or equal to 1 s), pH decrease led
to a significant inhibition of Ca accumulation. If Ca transport was
quenched in 1 s, [Ca2+]f for half-maximal stimulation of the process would
be 0.3, 1.0, and 4.0 microM at pH 7.2, 6.8, and 6.2, respectively. At
higher [Ca2+]f and at longer intervals, low pH increases the amount of Ca
taken up by SR. These phenomena could not be explained by an interaction of
ethyleneglycol-bis(beta-aminoethyl ether)N,N,N',N'-tetraacetic acid (EGTA)
with the Ca2+-pump system since they were also demonstrated without
application of Ca-EGTA buffers. Scatchard plot analysis of ATP-independent
Ca binding to SR shows that apparent Kd for Ca increases from 0.3 microM at
pH 7.2 to 7.4 microM at pH 6.2. pH shifts do not change the number of
high-affinity Ca-binding sites. The results suggest a competition between
Ca2+ and H+ for high-affinity binding sites on Ca2+-ATPase molecules. This
may cause a delay of myocardial relaxation during ischemia when a decrease
in intracellular pH takes place.
