Cytosolic Calcium Wave Propagation Depends on Local Calcium Movement Inside Cardiac Sarcoplasmic Reticulum

Abstract

The propagation of cytosolic Ca waves in cardiac myocytes is thought to occur through Ca-induced Ca release (CICR) with Ca being released from the sarcoplasmic reticulum (SR) via ryanodine receptors (RyR) activating neighboring RyRs and propagating throughout the cell. This widely accepted paradigm has recently been challenged by the hypothesis that an intra-SR ‘sensitization’ Ca wave precedes the cytosolic Ca wave and primes RyRs for activation by CICR (Keller et al., Cardiovasc. Res., 2007). Here we tested the intra-SR sensitization wave hypothesis by direct simultaneous measurements of cytosolic ([Ca]i) and intra-SR ([Ca]SR) calcium signals during wave propagation in intact and permeabilized rabbit ventricular myocytes. [Ca]i and [Ca]SR were measured with the fluorescent probes rhod-2 and fluo-5N, respectively, using high-speed confocal imaging. Our data show that the cytosolic Ca wave front and the corresponding intra-SR Ca depletion wave is preceded by a transient elevation of [Ca]SR. This transient elevation of [Ca]SR preceded the cytosolic wave front along the path of wave propagation and could be identified at individual release junctions with high-resolution imaging. Colliding waves that originated at separate subcellular locations annihilated as expected, however the preceding elevation of [Ca]SR of each wave resulted in a further increase of [Ca]SR at the site of collision. Increasing SR Ca load (beta-adrenergic stimulation) and SR Ca buffer capacity (with the exogenous Ca buffer ADA) enhanced Ca wave amplitude and [Ca]SR elevation preceding the wave. Furthermore, the cytosolic Ca wave front was preceded by a small transient decrease of [Ca]i that coincided with the transient elevation of [Ca]SR and possibly indicates the involvement of SERCA activity. These data suggest that local Ca movement inside the SR is a prerequisite for the propagation of spontaneous Ca waves in cardiac myocytes.