Abstract 20651: Ankyrin-B (+/−) Mice show Enhanced Sarcoplasmic Reticulum Ca Release in Cardiac Myocytes due to Altered Local Ca Concentration Regulation
The interplay between Na/Ca exchanger (NCX) and Na/K-ATPase (NKA) is essential in regulating cardiac [Na]i, [Ca]i and contractility. The membrane targeting and stability of both NCX and NKA require direct interaction with cytoskeletal protein ankyrin B (AnkB). Humans with AnkB loss-of-function mutations and AnkB heterozygous mice (AnkB+/−) display a complex cardiac phenotype that includes ventricular arrhythmias and sudden death. Cardiac myocytes from AnkB+/− mice show reduced NCX and NKA expression at T-tubules, larger cellular and SR Ca load and increased frequency of delayed afterdepolarizations (DADs). We examined how the loss of NCX and NKA affects SR Ca release, by measuring Ca sparks and waves in myocytes from AnkB+/− and wild-type (WT) mice (±1 µM isoproterenol). In intact myocytes, Ca spark frequency (CaSpF) was significantly higher in AnkB+/− vs. WT mice at all stimulation frequencies (0.5–2 Hz), both with and without isoproterenol. This resulted in significantly larger SR Ca leak in myoyctes from AnkB+/− mice (4.54±1.9 vs. 0.61±0.144 mF/F0.sec-1 under control conditions (p<0.05) and 7.97±2.57 vs 3.72±0.7 mF/F0.sec-1 with isoproterenol in AnkB+/− and WT myocytes, respectively). Moreover, spontaneous Ca waves occurred preponderantly in AnkB+/− myocytes (5/22 cells vs. 0/27 cells under control conditions (p<0.05), 12/21 cells vs. 2/24 cells with isoproterenol, p<0.01). This enhanced SR Ca release in AnkB+/− mice was due in part to a larger SR Ca content in AnkB+/− mice, but was still present when normalized to SR Ca load. This suggests that ryanodine receptor (RyR) function is altered in AnkB+/− mice. CaSpF was similar in saponin-permeabilized myocytes from AnkB+/− and WT mice (3.76±0.18 vs. 3.46±0.45 in AnkB+/− and WT myocytes, respectively). Global diastolic [Ca]i in intact myocytes was not different in AnkB+/− vs. WT mice (96±7 vs. 89±5 nM). This suggests that the enhanced SR Ca release in intact AnkB+/− myocytes is due to different RyR regulation rather than differences in intrinsic RyR function or expression. We propose that the loss of NCX and NKA at the T-tubules leads to a higher local [Ca] in the dyadic cleft and thus a higher activation level of RyRs.
- © 2010 by American Heart Association, Inc.