Ca2+ diffusion and sarcoplasmic reticulum transport both contribute to [Ca2+]i decline during Ca2+ sparks in rat ventricular myocytes.
1. We sought to evaluate the contribution of the sarcoplasmic reticulum (SR) Ca2+ pump (vs. diffusion) to the kinetics of [Ca24]i decline during Ca2+ sparks, which are due to spontaneous local SR Ca2+ release, in isolated rat ventricular myocytes measured using fluo-3 and laser scanning confocal microscopy. 2. Resting Ca2+ sparks were compared before (control) and after the SR Ca2(+)-ATPase was either completely blocked by 5 microM thapsigargin (TG) or stimulated by isoprenaline. Na(+)-Ca2+ exchange was blocked using Na(+)-free, Ca(2+)-free solution (0 Na+, O Ca2+) and conditions were arranged so that the SR Ca2+ content was the same under all conditions when Ca2+ sparks were measured. 3. The control Ca2+ spark amplitude (281 +/- 13 nM) was not changed by TG (270 +/- 21 nM) or isoprenaline (302 +/- 10 nM). However, the time constant of [Ca2+]i decline was significantly slower in the presence of TG (29.3 +/- 4.3 ms) compared with control (21.6 +/- 1.5 ms) and faster with isoprenaline (14.5 +/- 0.9 ms), but in all cases was much faster than the global [Ca2+]i decline during a control twitch (177 +/- 10 ms). 4. The spatial spread of Ca2+ during the Ca2+ spark was also influenced by the SR Ca2+ pump. The apparent 'space constant' of the Ca2+ sparks was longest when the SR Ca2+ pump was blocked, intermediate in control and shortest with isoprenaline. 5. We conclude that while Ca2+ diffusion from the source of Ca2+ release is the dominant process in local [Ca2+]i decline during the Ca2+ spark, Ca2+ transport by the SR contributes significantly to both the kinetics and spatial distribution of [Ca2+]i during the Ca2+ spark.
Department of Physiology, University of Maryland, Baltimore 21201, USA.
A M
AM
Gómez
H
H
Cheng
W J
WJ
Lederer
D M
DM
Bers
D000318:Adrenergic beta-Agonists / Q000494:pharmacology; D000818:Animals; D001693:Biological Transport, Active / Q000502:physiology; D002118:Calcium / Q000378:metabolism*; D000252:Calcium-Transporting ATPases / Q000037:antagonists & inhibitors / Q000378:metabolism*; D004058:Diffusion; D004791:Enzyme Inhibitors / Q000494:pharmacology; D006352:Heart Ventricles / Q000166:cytology / Q000201:enzymology / Q000378:metabolism; D007091:Image Processing, Computer-Assisted; D066298:In Vitro Techniques; D007545:Isoproterenol / Q000494:pharmacology; D008297:Male; D018613:Microscopy, Confocal; D009206:Myocardium / Q000201:enzymology / Q000378:metabolism* / Q000648:ultrastructure; D051381:Rats; D017207:Rats, Sprague-Dawley; D012519:Sarcoplasmic Reticulum / Q000201:enzymology / Q000378:metabolism*; D019284:Thapsigargin / Q000494:pharmacology
D000318:Adrenergic beta-Agonists; D004791:Enzyme Inhibitors; D019284:Thapsigargin; D000252:Calcium-Transporting ATPases; D007545:Isoproterenol; D002118:Calcium