Differential activation by Ca2+, ATP and caffeine of cardiac and skeletal muscle ryanodine receptors after block by Mg2+.
The block of rabbit skeletal ryanodine receptors (RyR1) and dog heart RyR2 by cytosolic [Mg2+], and its reversal by agonists Ca2+, ATP and caffeine was studied in planar bilayers. Mg2+ effects were tested at submaximal activating [Ca2+] (5 microM). Approximately one third of the RyR1s had low open probability ("LA channels") in the absence of Mg2+. All other RyR1s displayed higher activity ("HA channels"). Cytosolic Mg2+ (1 mM) blocked individual RyR1 channels to varying degrees (32 to 100%). LA channels had residual P(o) <0.005 in 1 mM Mg2+ and reactivated poorly with [Ca2+] (100 microM), caffeine (5 mM), or ATP (4 mM; all at constant 1 mM Mg2+). HA channels had variable activity in Mg2+ and variable degree of recovery from Mg2+ block with Ca2+, caffeine or ATP application. Nearly all cardiac RyR2s displayed high activity in 5 microM [Ca2+]. They also had variable sensitivity to Mg2+. However, the RyR2s consistently recovered from Mg2+ block with 100 microM [Ca2+] or caffeine application, but not when ATP was added. Thus, at physiological [Mg2+], RyR2s behaved as relatively homogeneous Ca2+/caffeine-gated HA channels. In contrast, RyR1s displayed functional heterogeneity that arises from differential modulatory actions of Ca2+ and ATP. These differences between RyR1 and RyR2 function may reflect their respective roles in muscle physiology and excitation-contraction coupling.
Copello
J A
JA
Department of Biological Sciences, Vanderbilt University, Nashville, TN, USA. jcopell@lumc.edu
Barg
S
S
Sonnleitner
A
A
Porta
M
M
Diaz-Sylvester
P
P
Fill
M
M
Schindler
H
H
Fleischer
S
S
D000255:Adenosine Triphosphate / Q000378:metabolism* / Q000494:pharmacology; D000818:Animals; D002110:Caffeine / Q000378:metabolism* / Q000494:pharmacology; D002118:Calcium / Q000378:metabolism* / Q000494:pharmacology; D004285:Dogs; D066298:In Vitro Techniques; D008274:Magnesium / Q000378:metabolism* / Q000494:pharmacology; D018482:Muscle, Skeletal / Q000378:metabolism; D009206:Myocardium / Q000378:metabolism; D018408:Patch-Clamp Techniques / Q000379:methods; D011817:Rabbits; D015203:Reproducibility of Results; D019837:Ryanodine Receptor Calcium Release Channel / Q000187:drug effects / Q000378:metabolism*; D012519:Sarcoplasmic Reticulum / Q000378:metabolism; D012680:Sensitivity and Specificity
D019837:Ryanodine Receptor Calcium Release Channel; D002110:Caffeine; D000255:Adenosine Triphosphate; D008274:Magnesium; D002118:Calcium