One calcium ion may suffice to open the tetrameric cardiac ryanodine receptor in rat ventricular myocytes.
1. The release of Ca2+ from sarcoplasmic reticulum in response to Ca2+ entering through L-type Ca2+ channels was studied in isolated voltage clamped rat ventricular myocytes at room temperature using the fluorescent Ca2+ indicators fluo-3 and Oregon Green 488 Bapta 5N. 2. Depolarizations to positive potentials elicited fluo-3 Ca2+ transients with rates of rise that were linearly related to the magnitude of the peak measured Ca2+ current in the presence of Cs+-containing pipette solutions. 3. Further experiments utilizing prepulses to preactivate a constant number of channels also revealed a linear relationship between the Ca2+ transient rate of rise and the magnitude of entering Ca2+ current at positive potentials. Under these conditions as well, the maximal rates of rise of global myoplasmic Ca2+ transients were due primarily to Ca2+ release from the sarcoplasmic reticulum as revealed by effects of ryanodine and caffeine on the Ca2+ transients. Using such prepulses, linearity between the Ca2+ transient rate of rise and the magnitude of the peak Ca2+ current was found under a variety of pulse protocols. 4. Using one such pulse protocol, linearity between the Ca2+ transient rate of rise and the magnitude of the peak Ca2+ current was also found when Ca2+ currents assessed at one potential were reduced in magnitude during the onset of block by application of Co2+. Using the same pulse protocol, linearity between the Ca2+ transient rate of rise and the magnitude of the peak Ca2+ current was also found when use of Cs+ was avoided by blocking K+ currents with extracellular TEA and 4-aminopyridine. Linearity in the relationship between the Ca2+ transient rate of rise and the magnitude of the peak Ca2+ current was also found when Ca2+ transients were measured using the low affinity Ca2+ indicator Oregon Green 488 Bapta 5N in place of fluo-3. 5. These results appear to indicate that the cardiac ryanodine receptor is capable of being activated by only one calcium ion. Alternative interpretations of the data are discussed.
Fan
J S
JS
Department of Physiology & Biophysics, University of Texas Medical Branch, Galveston, TX 77555-0641, USA.
Palade
P
P
D000818:Animals; D002110:Caffeine / Q000494:pharmacology; D002118:Calcium / Q000494:pharmacology*; D015220:Calcium Channels / Q000187:drug effects / Q000378:metabolism; D020746:Calcium Channels, L-Type; D002586:Cesium / Q000378:metabolism; D003035:Cobalt / Q000494:pharmacology; D004594:Electrophysiology; D006321:Heart / Q000187:drug effects*; D006352:Heart Ventricles / Q000166:cytology / Q000187:drug effects / Q000378:metabolism; D066298:In Vitro Techniques; D008297:Male; D008564:Membrane Potentials / Q000502:physiology; D009206:Myocardium / Q000166:cytology / Q000378:metabolism*; D018408:Patch-Clamp Techniques; D010726:Phosphodiesterase Inhibitors / Q000494:pharmacology; D051381:Rats; D017207:Rats, Sprague-Dawley; D019837:Ryanodine Receptor Calcium Release Channel / Q000187:drug effects*; D012519:Sarcoplasmic Reticulum / Q000187:drug effects / Q000378:metabolism
D015220:Calcium Channels; D020746:Calcium Channels, L-Type; D010726:Phosphodiesterase Inhibitors; D019837:Ryanodine Receptor Calcium Release Channel; D002586:Cesium; D003035:Cobalt; D002110:Caffeine; D002118:Calcium