Control of dual isoforms of Ca2+ release channels in muscle.
Here we compare excitation-contraction coupling in single muscle cells of frogs and rats. Because amphibians have isoform 3 (or 3) of the ryanodine receptor/Ca2+ release channel, in addition to 1 (alpha), which is also present in the mammal, any extra feature present in the frog may in principle be attributed to isoform 3. Ca2+ release under voltage clamp depolarization has a peak and a steady phase in both taxonomic classes, but the peak is more marked in the frog, where the ratio of amplitudes of the two phases is voltage-dependent. This dependence is a hallmark of CICR. Confocal imaging identified Ca2+ sparks in the frog, but not in the voltage-clamped rat cells. Because Ca2+ sparks involve CICR both observations indicate that the contribution of CICR is minor or null in the mammal. The "couplon" model well accounts for observations in the frog, but assumes a structure that we now know to be valid only for the rat. A revised model is proposed, whereby the isoform 3 channels, located parajunctionally, are activated by CICR and contribute its characteristic global and local features. Several issues regarding the roles of different channels remain open to further study.
Ríos
Eduardo
E
Section of Cellular Signaling, Department of Molecular Biophysics and Physiology, Rush University, Chicago, IL 60612, USA. erios@rush.edu
Zhou
Jingsong
J
D000818:Animals; D001001:Anura; D002118:Calcium / Q000378:metabolism*; D020013:Calcium Signaling / Q000502:physiology; D004558:Electric Stimulation; D008564:Membrane Potentials / Q000502:physiology; D008954:Models, Biological; D009119:Muscle Contraction / Q000502:physiology*; D020033:Protein Isoforms / Q000502:physiology; D051381:Rats; D019837:Ryanodine Receptor Calcium Release Channel / Q000378:metabolism / Q000502:physiology*; D012519:Sarcoplasmic Reticulum / Q000378:metabolism / Q000502:physiology*; D013045:Species Specificity
D020033:Protein Isoforms; D019837:Ryanodine Receptor Calcium Release Channel; D002118:Calcium