Cellular and subcellular alternans in the canine left ventricle

Previous studies indicate that action potential duration (APD) alternans is initiated in the endocardial (END) and midmyocardial (MID) regions rather than the epicardium (EPI) in the canine left ventricle (LV). This study examines regional differences in the rate dependence of Ca²⁺ transient characteristics under conditions that give rise to APD and associated T wave alternans. The role of the sarcoplasmic reticulum (SR) was further evaluated by studying Ca²⁺ transient characteristics in myocytes isolated from neonates, where an organized SR is poorly developed. All studies were performed in cells and tissues isolated from the canine LV. Isolated canine ENDO, MID, and EPI LV myocytes were either field stimulated or voltage clamped, and Ca ²⁺ transients were measured by confocal microscopy. In LV wedge preparations, increasing the basic cycle length (BCL) from 800 to 250 ms caused alternans to appear mainly in the ENDO and MID region; alternans were not observed in EPI under these conditions. Ca²⁺ transient alternans developed in response to rapid pacing, appearing in EPI cells at shorter BCL compared with MID and ENDO cells (BCL=428 ± 17 vs. 517 ± 29 and 514 ± 21, respectively, P < 0.05). Further increases in pacing rate resulted in the appearance of subcellular alternans of Ca²⁺ transient amplitude, which also appeared in EPI at shorter BCL than in ENDO and MID cells. Ca²⁺ transient alternans was not observed in neonate myocytes. We conclude that 1) there are distinct regional differences in the vulnerability to rate-dependent Ca²⁺ alternans in dog LV that may be related to regional differences in SR function and Ca²⁺ cycling; 2) the development of subcellular Ca²⁺ alternans suggests the presence of intracellular heterogeneities in Ca²⁺ cycling; and 3) the failure of neonatal cells to develop Ca²⁺ alternans provides further support that SR Ca²⁺ cycling is a major component in the development of these phenomena.