Loading of calcium and strontium into the sarcoplasmic reticulum in rat ventricular muscle.
Previous work suggests that strontium ions (Sr(2+)) are less effective than calcium ions (Ca(2+)) at supporting excitation-contraction (EC) coupling in cardiac muscle. We therefore tested whether this was due to differences in the uptake and release of Ca(2+)and Sr(2+)by the sarcoplasmic reticulum (SR) of rat ventricular trabeculae and myocytes at 22-24 degrees C. In permeabilized trabeculae, isometric contractions activated by exposure to Ca(2+)- and Sr(2+)-containing solutions produced similar maximal force, but were four times more sensitive to Ca(2+)than to Sr(2+). The rate of loading and maximal SR capacity for caffeine-releasable Ca(2+)and Sr(2+)were similar. In isolated, voltage-clamped ventricular myocytes, the SR content was measured as Na(+)-Ca(2+)exchange current during caffeine-induced SR cation releases. The SR Ca(2+)load reached a steady maximum during a train of voltage clamp depolarizations. A similar maximal Sr(2+)load was not observed, suggesting that the SR capacity for Sr(2+)exceeds that for Ca(2+). Therefore, the relative inability of Sr(2+)to support cardiac EC coupling appears not to be due to failure of the SR to sequester Sr(2+). Instead, increases in cytosolic [Sr(2+)] seem to poorly activate Sr(2+)release from the SR.
Spencer
C I
CI
Bockus Research Institute, Allegheny University Hospitals-Graduate, 415 S. 19th St, Philadelphia, PA 19146, USA.
Barsotti
R J
RJ
Berlin
J R
JR
D000818:Animals; D002110:Caffeine / Q000494:pharmacology; D002118:Calcium / Q000378:metabolism* / Q000493:pharmacokinetics; D002412:Cations; D000697:Central Nervous System Stimulants / Q000494:pharmacology; D004594:Electrophysiology; D005260:Female; D006352:Heart Ventricles / Q000187:drug effects / Q000378:metabolism*; D066298:In Vitro Techniques; D007700:Kinetics; D008297:Male; D009200:Myocardial Contraction / Q000187:drug effects; D051381:Rats; D017207:Rats, Sprague-Dawley; D012503:Saponins / Q000494:pharmacology; D012508:Sarcolemma / Q000378:metabolism; D012519:Sarcoplasmic Reticulum / Q000187:drug effects / Q000378:metabolism*; D013324:Strontium / Q000378:metabolism* / Q000493:pharmacokinetics; D013997:Time Factors
D002412:Cations; D000697:Central Nervous System Stimulants; D012503:Saponins; D002110:Caffeine; D002118:Calcium; D013324:Strontium