Ca2+ release-induced inactivation of Ca2+ current in rat ventricular myocytes: evidence for local Ca2+ signalling.
1. Inactivation of Ca2+ current (ICa) induced by Ca2+ release from sarcoplasmic reticulum (SR) was studied in single rat ventricular myocytes using whole-cell patch-clamp and indo-1 fluorescence measurement techniques. 2. Depolarizing pulses to 0 mV elicited large Ca2+ transients and ICa with biexponential inactivation kinetics. Varying SR Ca2+ loading by a 20 s pulse of caffeine showed that the fast component of ICa inactivation was dependent on the magnitude of Ca2+ release. 3. Inactivation of ICa induced by Ca2+ release was quantified, independently of voltage and Ca2+ entry, using a function termed fractional inhibition of ICa (FICa). The voltage relation of FICa had a negative slope, resembling that of single-channel Ca2+ current (iCa) rather than the bell-shaped current-voltage (I-V) relation of macroscopic ICa and Ca2+ transients. 4. Intracellular dialysis of myocytes with 10 mM EGTA (150 nM free [Ca2+]) had no effect on ICa inactivation induced by Ca2+ release, despite abolition of Ca2+ transients and cell contraction. Dialysis with 3 or 10 mM BAPTA (180 nM free [Ca2+]) attenuated FICa in a concentration-dependent manner, with greater inhibition at positive than at negative potentials, consistent with more effective buffering of Ca2+ microdomains of smaller iCa. 5. Spatial profiles of [Ca2+] near an opened Ca2+ channel were simulated. [Ca2+] reached submillimolar levels at the mouth of the channel, and dropped steeply as radial distance increased. At any given distance from the channel, [Ca2+] was higher at negative than at positive potentials. The radii of Ca2+ microdomains were significantly reduced by 3 or 10 mM BAPTA, but not by 10 mM EGTA. 6. In conclusion, the distinctive voltage dependence and susceptibility of Ca2+ release-induced ICa inactivation to fast and slow Ca2+ buffers suggests that the process is mediated through local changes of [Ca2+] in the vicinity of closely associated Ca2+ channels and ryanodine receptors.
Sham
J S
JS
Division of Pulmonary and Critical Care Medicine, Johns Hopkins Medical Institutions, Baltimore, MD 21224, USA. jsks@welchlink.welch.jhu.edu
D000818:Animals; D002021:Buffers; D002118:Calcium / Q000378:metabolism* / Q000494:pharmacology; D015220:Calcium Channels / Q000502:physiology; D002148:Calmodulin-Binding Proteins / Q000502:physiology; D004558:Electric Stimulation; D004594:Electrophysiology; D006352:Heart Ventricles / Q000737:chemistry / Q000166:cytology / Q000378:metabolism; D015640:Ion Channel Gating / Q000502:physiology; D008297:Male; D009124:Muscle Proteins / Q000502:physiology; D009206:Myocardium / Q000737:chemistry / Q000166:cytology* / Q000378:metabolism*; D051381:Rats; D017208:Rats, Wistar; D019837:Ryanodine Receptor Calcium Release Channel; D015398:Signal Transduction / Q000187:drug effects / Q000502:physiology*
D002021:Buffers; D015220:Calcium Channels; D002148:Calmodulin-Binding Proteins; D009124:Muscle Proteins; D019837:Ryanodine Receptor Calcium Release Channel; D002118:Calcium