Ca2+-mobility in the sarcoplasmic reticulum of ventricular myocytes is low
Pawel Swietach 1, Kenneth W Spitzer 2 and Richard D Vaughan-Jones 1*
1 University of Oxford
2 University of Utah
* To whom correspondence should be addressed. E-mail: richard.vaughan-jones{at}dpag.ox.ac.uk.
Submitted on January 28, 2008
Revised on February 26, 2008
Accepted on 21 March 2008
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Abstract |
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The sarcoplasmic reticulum (SR) in ventricular myocytes contains releasable Ca2+, for activating cellular contraction. Recent measurements of intra-SR (luminal) Ca2+ suggest a high diffusive Ca2+-mobility constant (DCaSR). This could help spatially to unify SR Ca2+-content ([Ca2+]SRT), and standardize Ca2+-release throughout the cell. But measurements of localized depletions of luminal Ca2+ (Ca2+-blinks), associated with local Ca2+-release (Ca2+-sparks), suggest DCaSR may actually be low. Here we describe a novel method for measuring DCaSR. Using a cytoplasmic Ca2+-fluorophore, we estimate regional [Ca2+]SRT from localized, caffeine-induced SR Ca2+-release. Caffeine-microperfusion of one end of a guinea-pig or rat myocyte diffusively empties the whole SR, at a rate indicating DCaSR 8-9µm2/s, up to tenfold lower than previous estimates. Ignoring background SR Ca2+-leakage in our measurement protocol produces an artefactually high DCaSR (>40µm2/s), which may also explain the previous high values. Diffusion modeling suggests a low DCaSR would be sufficient to support local SR Ca2+-signaling within sarcomeres during excitation-contraction coupling. Low DCaSR also implies that [Ca2+]SRT may readily become spatially non-uniform, particularly under pathological conditions of spatially non-uniform Ca2+-release. Local control of luminal Ca2+, imposed by low DCaSR, may complement the well-established local control of SR Ca2+-release by Ca2+-channel/ryanodine receptor couplons.
Key Words:
Ca2+, Ca2+-signaling, Diffusion, Fluorescence, Sarcoplasmic reticulum, Ventricular myocyte
