Abstract
The development of a refractory period for Ca2+ spark initiation after Ca2+ release in cardiac myocytes should inhibit further Ca2+ release during the action potential plateau. However, Ca2+ release sites that did not initially activate or which have prematurely recovered from refractoriness might release Ca2+ later during the action potential and alter the cell-wide Ca2+ transient. To investigate the possibility of late Ca2+ spark (LCS) activity in intact isolated cardiac myocytes using fast confocal line scanning with improved confocality and signal to noise. We recorded Ca2+ transients from cardiac ventricular myocytes isolated from rabbit hearts. Action potentials were produced by electric stimulation, and rapid solution changes were used to modify the L-type Ca2+ current. After the upstroke of the Ca2+ transient, LCSs were detected which had increased amplitude compared with diastolic Ca2+ sparks. LCS are triggered by both L-type Ca2+ channel activity during the action potential plateau, as well as by the increase of cytosolic Ca2+ associated with the Ca2+ transient itself. Importantly, a mismatch between sarcoplasmic reticulum load and L-type Ca2+ trigger can increase the number of LCS. The likelihood of triggering an LCS also depends on recovery from refractoriness that appears after prior activation. Consequences of LCS include a reduced rate of decline of the Ca2+ transient and, if frequent, formation of microscopic propagating Ca2+ release events (Ca2+ ripples). Ca2+ ripples resemble Ca2+ waves in terms of local propagation velocity but spread for only a short distance because of limited regeneration. These new types of Ca2+ signaling behavior extend our understanding of Ca2+-mediated signaling. LCS may provide an arrhythmogenic substrate by slowing the Ca2+ transient decline, as well as by amplifying maintained Ca2+ current effects on intracellular Ca2+ and consequently Na+/Ca2+ exchange current.
Highlights
Cardiac excitation-contraction coupling (ECC) is mediated at the cellular level by the nearsynchronous activation of ~104 microscopic Ca2+ release events called Ca2+ sparks [1, 2]. This occurs because the cardiac action potential (AP) opens L-type Ca2+ channels (LTCC) in the surface membrane to produce a local increase in Ca2+ which in turn opens Ca2+-sensitive channels in the adjacent junctional sarcoplasmic reticulum membrane [3]
Individual late Ca2+ sparks (LCS) (Fig. 1B) had an increased amplitude compared to diastolic Ca2+ sparks (Fig. 1C), but did not exhibit any changes in duration (~40 ms) or spatial full width at half maximum (~1.8 μm)
A simple, obvious, explanation for the genesis of the late Ca2+ sparks would be that they arise from junctional sarcoplasmic reticulum membrane (jSR) that was either not activated during the upstroke of the Ca2+ transient or was uncoupled or ‘orphaned’ [17] from t-tubules
Summary
Cardiac excitation-contraction coupling (ECC) is mediated at the cellular level by the nearsynchronous activation of ~104 microscopic Ca2+ release events called Ca2+ sparks [1, 2]. This occurs because the cardiac action potential (AP) opens L-type Ca2+ channels (LTCC) in the surface membrane to produce a local increase in Ca2+ which in turn opens Ca2+-sensitive channels (ryanodine receptors, RyR) in the adjacent junctional sarcoplasmic reticulum membrane (jSR) [3]. Continued SR release (or leak) should oppose SR re-uptake and slow the time course of the Ca2+ transient, as seen in a phospholamban knockout mice with CamKII c overexpression [15]
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