Influence of caffeine, Ca2+, and Mg2+ on ryanodine depression of the tension transient in skinned myocardial fibers of the rabbit.
Ryanodine, a blocker for Ca(2+)-release channels of the sarcoplasmic reticulum (SR Ca(2+)-release channels), induces depression of myocardial contraction in isolated intact muscle, which is consistent with depression of the caffeine-induced tension transient in skinned muscle fibers. In isolated SR, ryanodine binds to a specific receptor with high affinity, and this binding is enhanced by caffeine and increasing Ca2+ and decreased by increasing Mg2+. The aim of this study was to test the hypothesis that depression of myocardial contraction is mediated by changes in ryanodine-receptor binding properties. Accordingly, factors (caffeine, Ca2+, and Mg2+) affecting ryanodine-receptor binding properties in the isolated SR membrane were studied in skinned myocardial fibers from adult rabbits. The depression of the caffeine-induced tension transient by ryanodine (ryanodine depression) influenced by these three factors was measured. In a dose-dependent manner, increasing caffeine or Ca2+ concentrations enhanced the ryanodine depression. The concentrations for 50% ryanodine depression (IC50) approximated 7 mM for caffeine, and pCa 5.25 for Ca2+. When 1 microM ryanodine and 25 mM caffeine were combined, ryanodine depression was independent of Ca2+ at low Ca2+ concentrations (20%-30% at pCa greater than 8 and 7.5) and was a direct function of Ca2+ at higher concentrations (pCa 7.5-6.0 with IC50 approx. pCa 6.75). In contrast, increasing Mg2+ reduced the ryanodine depression with IC50 approximately equal to pMg 3.3. In conclusion, the caffeine- or Ca(2+)-enhanced, and Mg(2+)-reduced ryanodine depression observed in this study is consistent with known ryanodine-receptor binding properties.
Su
J Y
JY
Department of Anesthesiology, University of Washington, Seattle 98195.
D000818:Animals; D002110:Caffeine / Q000494:pharmacology*; D002118:Calcium / Q000494:pharmacology*; D004347:Drug Interactions; D066298:In Vitro Techniques; D008274:Magnesium / Q000494:pharmacology*; D008297:Male; D009200:Myocardial Contraction / Q000187:drug effects*; D011817:Rabbits; D011950:Receptors, Cholinergic / Q000378:metabolism; D012433:Ryanodine / Q000378:metabolism / Q000494:pharmacology*; D019837:Ryanodine Receptor Calcium Release Channel
D011950:Receptors, Cholinergic; D019837:Ryanodine Receptor Calcium Release Channel; D012433:Ryanodine; D002110:Caffeine; D008274:Magnesium; D002118:Calcium