Rationale
Intra-sarcoplasmic reticulum (SR) free [Ca] ([Ca]SR) provides the driving force for SR Ca release and is a key regulator of SR Ca release channel gating during normal SR Ca release or arrhythmogenic spontaneous Ca release events. However, little is known about [Ca]SR spatiotemporal dynamics.
Objective
To directly measure local [Ca]SR with subsarcomeric spatiotemporal resolution during both normal global SR Ca release and spontaneous Ca sparks, and to evaluate the quantitative implications of spatial [Ca]SR gradients.
Methods & Results
Intact and permeabilized rabbit ventricular myocytes were subjected to direct simultaneous measurement of cytosolic [Ca] and [Ca]SR and fluorescence recovery after photobleach (FRAP). We found no detectable [Ca]SR gradients between SR release sites (junctional SR) and Ca uptake sites (free SR) during normal global Ca release, clear spatio-temporal [Ca]SR gradients during isolated Ca blinks, faster intra-SR diffusion in the longitudinal vs. transverse direction, 3-4 fold slower diffusion of fluorophores in the SR than in cytosol, and that intra-SR Ca diffusion varies locally, dependent on local SR connectivity. A computational model clarified why spatiotemporal gradients are more detectable in isolated local releases vs. global releases and provides a quantitative framework for understanding intra-SR Ca diffusion.
Conclusions
Intra-SR Ca diffusion is rapid, limiting spatial [Ca]SR gradients during excitation-contraction coupling. Spatiotemporal [Ca]SR gradients are apparent during Ca sparks, and these observations constrain models of dynamic Ca movement inside the SR. This has important implications for myocyte SR Ca handling, synchrony and potentially arrhythmogenic spontaneous contraction.
Journal of Physiologyventricle in rabbit heart 8-9 weeks after an experimentally induced apical infarct, and compares these values with equivalent measurements from sham-operated (sham) hearts.
The aim of the study was to correlate intracellular Ca(2+) transients with Ca(2+) uptake and efflux characteristics of the sarcoplasmic reticulum (SR) in ventricular myocytes isolated from rabbits with left-ventricular dysfunction (LVD). Chronic (8 weeks) ligation of a coronary artery caused marked LVD in rabbits. Measurements of intracellular [Ca(2+)] were made using Fura-2 on intact, single, left-ventricular myocytes. SR Ca(2+) flux rates associated with sarco-endoplasmic reticulum Ca(2+) ATPase type 2 (SERCA2)-mediated uptake, ryanodine receptor type 2 (RyR2)-mediated Ca(2+) efflux and background SR Ca(2+) leak were measured in suspensions of permeabilised myocytes. Measurements on single, permeabilised myocytes were used to assess the steady-state Ca(2+) content of the SR and the characteristics of spontaneous SR Ca(2+) release. Peak systolic [Ca(2+)] was significantly lower; time-to-peak and Ca(2+) transient duration were significantly longer in LVD myocytes. SERCA2-mediated Ca(2+) uptake was reduced to approximately 50% in myocytes from the LVD group. Ruthenium red (RuR)-sensitive Ca(2+) efflux (mediated by the RyR2) was also reduced in the LVD group by approximately 50%, as was the remaining (RuR-insensitive) background Ca(2+) leak. Measurements from single, permeabilised myocytes showed a lower steady-state SR Ca(2+) content. The frequency and amplitude of spontaneous SR Ca(2+) release from LVD hearts was also reduced. Partial inhibition of SERCA2 by thapsigargin depressed both the amplitude and the frequency of spontaneous release. Partial inhibition of RyR2-mediated-Ca(2+) efflux with tetracaine enhanced spontaneous Ca(2+) release amplitude and decreased frequency. Increased background Ca(2+) leak with ionomycin decreased the frequency of spontaneous release. It is concluded that partial inhibition of SERCA2 mimics some aspects of altered SR function in LVD, but reduced RyR2 function cannot explain the other functional alterations observed. Reduced background Ca(2+) leak from the SR may compensate partly for the reduced Ca(2+) uptake capacity of the SR in the LVD group.
Measurements of sarcoplasmic reticulum (SR) Ca(2+) uptake were made from aliquots of dissociated permeabilized ventricular myocytes using fura 2. Equilibration with 10 mM oxalate ensured a reproducible exponential decline of [Ca(2+)] from 600 nM to a steady state of 100-200 nM after addition of Ca(2+). In the presence of 5 microM ruthenium red, which blocks the ryanodine receptor, the time course of the decline of [Ca(2+)] can be modeled by a Ca(2+)-dependent uptake process and a fixed Ca(2+) leak. Partial inhibition of the Ca(2+) pump with 1 microM cyclopiazonic acid or 50 nM thapsigargin reduced the time constant for Ca(2+) uptake but did not affect the SR Ca(2+) leak. Addition of 10 mM inorganic phosphate (P(i)) decreased the rate of Ca(2+) accumulation by the SR and increased the Ca(2+) leak rate. This effect was reversed on addition of 10 mM phosphocreatine. 10 mM P(i) had no effect on Ca(2+) leak from the SR after complete inhibition of the Ca(2+) pump. In conclusion, P(i) decreases the Ca(2+) uptake capacity of cardiac SR via a decrease in pump rate and an increase in Ca(2+) pump-dependent Ca(2+) leak.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.