Ca2+ Wave Velocity in Cardiomyocytes is Regulated by Ryanodine Receptor Ca2+ Sensitivity and SR Ca2+ Content

Abstract

Arrhythmias can be elicited by sudden release of Ca2+ from the sarcoplasmic reticulum (SR) via the SR Ca2+ release channel (RyR) in cardiomyocytes. Such release may initiate a self-propagating process called a Ca2+ wave, which may trigger a spontaneous action potential. We hypothesized that likelihood of arrhythmia is augmented when waves propagate more rapidly, and investigated the role of RyR Ca2+ sensitivity and SR Ca2+ content. Ca2+ waves were studied in isolated ventricular cardiomyocytes from mice using confocal microscopy and Ca2+ fluorescence. Waves preceding spontaneous action potentials propagated more rapidly than those that did not generate action potentials (p<0.05). Thus, mechanisms controlling wave speed may determine arrythmogenic potential. We investigated effects of increased RyR Ca2+ sensitivity by rapidly exposing cells to 1 mM caffeine. The first wave following the switch to caffeine exhibited increased propagation velocity (p<0.05). However, at steady-state, velocity was not altered from pre-treatment values, and mean wave magnitude and SR Ca2+ content were reduced (p<0.05). The opposite intervention, reducing RyR sensitivity with 100 μM tetracaine, increased wave magnitude and SR Ca2+ content at steady state (p<0.05) but did not alter wave speed. These observations suggest that SR content-induced alterations in RyR sensitivity could account for differences in wave speed between initial and steady-state conditions. To test this hypothesis, we increased SR content by pacing cells at 5 Hz, and then stopped the stimulation to allow SR content to decline. Wave speed was observed to progressively decrease following termination of stimulation (p<0.05). Our data suggest that RyR sensitivity and SR Ca2+ content are important determinants of Ca2+ wave speed. An induced increase in RyR sensitivity, possibly relevant in heart failure, increases wave speed only until counteracted by steady-state reduction in SR content.