Changes of SERCA activity have only modest effects on sarcoplasmic reticulum Ca²⁺ content in rat ventricular myocytes

Abstract

Changes of the activity of the sarco-endoplasmic reticulum Ca(2+)-ATPase (SERCA) affect the amplitude of the systolic Ca(2+) transient and thence cardiac contractility. This is thought to be due to alterations of SR Ca(2+) content. Recent work on mice in which the expression of SERCA is decreased found that a large reduction of SERCA expression resulted in a proportionately much smaller decrease of SR Ca(2+) content. The aim of the current work was to investigate the quantitative nature of the dependence of both the amplitude of the systolic Ca(2+) transient and SR Ca(2+) content on SERCA activity during acute partial inhibition of SERCA. Experiments were performed on rat ventricular myocytes. Brief application of thapsigargin (1 μm) resulted in a decrease of SERCA activity as measured from the rate of decay of the systolic Ca(2+) transient. This was accompanied by a decrease in the amplitude of the systolic Ca(2+) transient which was linearly related to that of SERCA activity. However, the fractional decrease in the SR Ca(2+) content was much less than that of SERCA activity. On average SR Ca(2+) content was proportional to SERCA activity raised to the 0.38 ± 0.07 power. This shallow dependence of SR content on SERCA activity arises because Ca(2+) release is a steep function of SR Ca(2+) content. In contrast SR Ca(2+) content was increased 4.59 ± 0.40 (n = 8)-fold by decreasing ryanodine receptor opening with tetracaine (1 mm). Therefore a modest decrease of SR Ca(2+) content results in a proportionately larger fall of Ca(2+) release from the SR which can balance a larger initiating decrease of SERCA. In conclusion, the shallow dependence of SR Ca(2+) content on SERCA activity is expected for a system in which small changes of SR Ca(2+) content produce larger effects on the amplitude of the systolic Ca(2+) transient.

Balanced changes in Ca buffering by SERCA and troponin contribute to Ca handling during β-adrenergic stimulation in cardiac myocytes

“…Figure 3A shows the effects of the SERCA inhibitor, thapsigargin, on both stimulated Ca transients and caffeine responses. The experiments were performed in ISO (100 nM); thapsigargin (5 µM) was applied for 90 s. In agreement with previous work, 27–29 and as shown by the normalized traces of Figure 3C , thapsigargin slowed the decay of the systolic Ca transient. This was associated with a fall in SR Ca content as shown by the decrease in both the caffeine-evoked increase of Ca and the integral of the accompanying NCX current ( Figure 3B ).…”

There and back again: Iterating between population-based modeling and experiments reveals surprising regulation of calcium transients in rat cardiac myocytes

“…Many published studies have shown that strong inhibition of I to can increase CaTA [3, 39, 47–49] and inhibition of SERCA can decrease it [50, 51]. Some studies have also further shown that I to inhibition can have greater effects in epicardial cells [3, 39].…”

“…Bode et al . [51] demonstrated that when voltage-clamped rat CMs were excited with 100 ms square depolarizing pulses, partial SERCA inhibition has a near-linear relationship with CaTA, a much stronger relationship than predicted by the Pandit model. However, in the absence of the endogenous AP waveform, it is impossible to draw conclusions about the relative importance of membrane currents and Ca 2+ handling proteins.…”

Trypanosoma brucei cathepsin-L increases arrhythmogenic sarcoplasmic reticulum-mediated calcium release in rat cardiomyocytes

“…However, sarcolemmal efflux from the cell also contributes to this rate constant of decay. When the rate constant of decay of the caffeine-induced Ca 2+ transient (which includes sarcolemmal efflux but no SR uptake contribution) is subtracted from that of the electrically stimulated Ca 2+ transient (which includes the combined SR uptake and sarcolemmal efflux) 20 the % change in K SERCA induced by the supernatant was 114.3 ± 2.2% of media control. This compares well with the value (110.4 ± 1.2; Figure 2D iii) when using the rate decay constant of the electrically induced transient alone.…”