TAURINE MODULATION OF CALCIUM-BINDING TO CARDIAC SARCOLEMMA

  • 1 January 1985
    • journal article
    • research article
    • Vol. 232  (2) , 445-451
Abstract
A sarcolemma-enriched membrane fraction (SL) was prepared from the hearts of Sprague-Dawley rats and its ability to bind Ca2+ was measured by equilibrium dialysis. The effect of taurine on SL Ca2+ binding varied with the buffer and with Na+ concentration. In Tris, in the presence of Na+ (140 mM), taurine (10 mM) increased the affinity but decreased the maximal binding of Ca2+ (0.5-7 mM). In the absence of Na+, taurine decreased the affinity without altering the maximal binding. These effects of Ca2+ binding were absent in bicarbonate or Krebs-Henseleit buffers. Incubations with A23187, a Ca2+ ionophore, and lanthanum, a Ca2+ antagonist, indicated that SL membranes incubated in Tris, but not in buffers containing bicarbonate, were sealed vesicles with internal environments low in Ca2+. High-affinity binding of Ca2+ (10-6-10-4 M) was measured in modified Krebs-Henseleit buffers. Taurine decreased Ca2+ binding in a high-Na+ (145 mM), low-K+ (4.7 mM) buffer. Taurine increased Ca2+ binding in both 4.7 mM Na+-145 mM K+ and 25 mM Na+-4.7 mM K+ buffers. Taurine also increased Ca2+ binding in the presence of ATP. Taurine increased high-affinity Ca2+ binding in intracellular buffers, but it did not affect low-affinity Ca2+ binding in extracellular buffers. Taurine may exert its cardiotonic actions through modulation of the high-affinity Ca2+ binding sites on the internal aspect of the SL.