The role of luminal Ca2+ in the generation of Ca2+ waves in rat ventricular myocytes.
1. We used confocal Ca2+ imaging and fluo-3 to investigate the transition of localized Ca2+ releases induced by focal caffeine stimulation into propagating Ca2+ waves in isolated rat ventricular myocytes. 2. Self-sustaining Ca2+ waves could be initiated when the cellular Ca2+ load was increased by elevating the extracellular [Ca2+] ([Ca2+]o) and they could also be initiated at normal Ca2+ loads when the sensitivity of the release sites to cytosolic Ca2+ was enhanced by low doses of caffeine. When we prevented the accumulation of extra Ca2+ in the luminal compartment of the sarcoplasmic reticulum (SR) with thapsigargin, focal caffeine pulses failed to trigger self-sustaining Ca2+ waves on elevation of [Ca2+]o. Inhibition of SR Ca2+ uptake by thapsigargin in cells already preloaded with Ca2+ above normal levels did not prevent local Ca2+ elevations from triggering propagating waves. Moreover, wave velocity increased by 20 %. Tetracaine (0.75 mM) caused transient complete inhibition of both local and propagating Ca2+ signals, followed by full recovery of the responses due to increased SR Ca2+ accumulation. 3. Computer simulations using a numerical model with spatially distinct Ca2+ release sites suggested that increased amounts of releasable Ca2+ might not be sufficient to generate self-sustaining Ca2+ waves under conditions of Ca2+ overload unless the threshold of release site Ca2+ activation was set at relatively low levels (< 1.5 microM). 4. We conclude that the potentiation of SR Ca2+ release channels by luminal Ca2+ is an important factor in Ca2+ wave generation. Wave propagation does not require the translocation of Ca2+ from the spreading wave front into the SR. Instead, it relies on luminal Ca2+ sensitizing Ca2+ release channels to cytosolic Ca2+.
Lukyanenko
V
V
Department of Physiology, University Health Sciences Center, Texas Tech University, Lubbock, TX, USA.
Subramanian
S
S
Gyorke
I
I
Wiesner
T F
TF
Gyorke
S
S
D000465:Algorithms; D000779:Anesthetics, Local / Q000494:pharmacology; D000814:Aniline Compounds; D000818:Animals; D002110:Caffeine / Q000494:pharmacology; D002118:Calcium / Q000378:metabolism / Q000502:physiology*; D000252:Calcium-Transporting ATPases / Q000037:antagonists & inhibitors; D000697:Central Nervous System Stimulants / Q000494:pharmacology; D003198:Computer Simulation; D003600:Cytosol / Q000187:drug effects / Q000378:metabolism; D004594:Electrophysiology; D004791:Enzyme Inhibitors / Q000494:pharmacology; D005456:Fluorescent Dyes; D006321:Heart / Q000187:drug effects / Q000502:physiology*; D008051:Lipid Bilayers; D018613:Microscopy, Confocal; D008954:Models, Biological; D009206:Myocardium / Q000166:cytology / Q000378:metabolism; D051381:Rats; D017207:Rats, Sprague-Dawley; D012519:Sarcoplasmic Reticulum / Q000187:drug effects / Q000378:metabolism; D019284:Thapsigargin / Q000494:pharmacology; D014966:Xanthenes
D000779:Anesthetics, Local; D000814:Aniline Compounds; D000697:Central Nervous System Stimulants; D004791:Enzyme Inhibitors; D005456:Fluorescent Dyes; D008051:Lipid Bilayers; D014966:Xanthenes; C059715:Fluo-3; D002110:Caffeine; D019284:Thapsigargin; D000252:Calcium-Transporting ATPases; D002118:Calcium