Thermal modulation of epicardial Ca(2+) dynamics uncovers molecular mechanisms of Ca(2+) alternans

Ca(2+) alternans (Ca-Alts) are alternating beat-to-beat changes in the amplitude of Ca(2+) transients that frequently occur during tachycardia, ischemia, or hypothermia that can lead to sudden cardiac death. Ca-Alts appear to result from a variation in the amount of Ca(2+) released from the sarcoplasmic reticulum (SR) between two consecutive heartbeats. This variable Ca(2+) release has been attributed to the alternation of the action potential duration, delay in the recovery from inactivation of RYR Ca(2+) release channel (RYR2), or an incomplete Ca(2+) refilling of the SR. In all three cases, the RYR2 mobilizes less Ca(2+) from the SR in an alternating manner, thereby generating an alternating profile of the Ca(2+) transients. We used a new experimental approach, fluorescence local field optical mapping (FLOM), to record at the epicardial layer of an intact heart with subcellular resolution. In conjunction with a local cold finger, a series of images were recorded within an area where the local cooling induced a temperature gradient. Ca-Alts were larger in colder regions and occurred without changes in action potential duration. Analysis of the change in the enthalpy and Q(10) of several kinetic processes defining intracellular Ca(2+) dynamics indicated that the effects of temperature change on the relaxation of intracellular Ca(2+) transients involved both passive and active mechanisms. The steep temperature dependency of Ca-Alts during tachycardia suggests Ca-Alts are generated by insufficient SERCA-mediated Ca(2+) uptake into the SR. We found that Ca-Alts are heavily dependent on intra-SR Ca(2+) and can be promoted through partial pharmacologic inhibition of SERCA2a. Finally, the FLOM experimental approach has the potential to help us understand how arrhythmogenesis correlates with the spatial distribution of metabolically impaired myocytes along the myocardium.