| Summary: | Propagating intr~cellular Ca2+ waves in cardiac myocytes occur as a consequence of the overloaded state of the sarcoplasmic reticulum (SR). To examine these events in detail, ventricular cardiomyocytes were isolated from rabbit hearts and permeabilised with p-escin. Cytosolic Ca2+ signals were monitored using Fluo-5F (10J.lM) in combination with laser:scanning confocal microscopy. Through careful calibration ofthe intracellular Ca2+ signals and construction of analysis programs, the fluxes which underlie the Ca2+ wave were derived and subsequently incorporated into a mathematical model. The decline in cytosolic Ca2+ subsequent to rapid application ofcaffeine was used to quantify cellular Ca2+ diffusional loss (diffusional constant = 31.2+/-0.9 S-I). Ca2+ binding to cellular proteins was then calculated and the sum ofthe free Ca2+, bound Ca2+ and Ca2+ lost by diffusion was used as the integral ofthe Ca2+ flux across the SR. The first derivative ofthis was taken as the trans-SR flux rate. From the analysis ofthese signals it was apparent that the Ca2+ released from the SR during a wave was not significantly different from that released on application of lOmM caffeine (0.149 +/-0.10 roM vs. 0.154+/-0.10 mM)). This information, coupled with values ofintra-SR buffering allowed calculation ofintra-SR [Ca2l. This in tum allowed the trans SR [Ca2l gradient to be estimated and the subsequent calculation ofRyR and SERCA mediated Ca2+ flux. These measurements were used to derive parameters for construction of a 3- compartment model of Ca2+ flux using existing models ofCa2+ buffering, SERCA activity and leak. Three experimental interventions were used to study changes in Ca2+ wave properties and assess the effectiveness ofthe model in predicting wave frequency, minimum and maximum [Ca2l. These were: (i) changing extracellular Ca2+, (ii) inhibiting the RyR using tetracaine and (iii) inhibiting SERCA using 2',5'-di(tertbutyl)- I,4-benzohydroquinone (TBQ). As cytosolic Ca2+ was increased from 300 to 900nM, so frequency and systolic Ca2+were shown to 'increase nonlinearly, whilst diastolic [Ca2+] increased linearly. Calculated SR release threshold was found not to change. SERCA Vmax and KD both increased, with Vmax rising from 160 to 380 J.lMs·1 and KD rising from 239±48 to 354±18nM as extracellular [Ca2i was increased from 300nM to ~OOnM. The calculated peak permeability ofRyR mediated flux also increased from 41.1±6.5 to 61.2±3.6 S·l over this range. These changes, when included in the model, subsequently provided acceptable predictions of experimental results. Tetracaine caused frequency ofthe Ca2+ waves to decrease from 0.59±0.03 Hz to 0.35±0.02 Hz, systolic [Ca2l to increase from 2.06±0.11 J.lM to 3..16±0.24 J.lM and diastolic [Ca2l to decrease from 185±9 nM to157±10 nM. Flux analysis indicated that these changes were associated with an increase in the SR release threshold from 1.Ip±0.04 mM to 1.58±0.08 mM (n=6). Implementation ofthis threshold change in the computational model predicted a decrease in Ca2+ wave frequency to a simil~ value to that observed experimentally. The increased systolic [Ca2i was comparable to but greater than that observed experimentally. In contrast, the model predicted diastolic [Ca2l to increase while a decrease diastolic was observed experimentally. Application of the SERCA inhibitor TBQ (lJ.lM) decreased SR Ca2 + content, the amplitude and frequency of Ca2+ waves. Analysis ofthe underlying fluxes suggested that TBQ caused a 43% reduction in SERCA Vmax, with no significant ~hange in KD. Analysis also suggested that this reduction in Vmax was accompanied by a 25% reduction in SR release threshold. While the reduced SERCA Vmax is consistent with TBQ's known action on SERCA, the effect on Ca2+ wave amplitude was unexpected and cannot be easily explained with the current ci+ wave model.
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