Increased Ca Sparks and Waves Frequency and Unchanged [Na]i in Hearts from Ankyrin-B Heterozygous Mice

Abstract

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 the T-tubules, larger cellular and SR Ca load and increased frequency of delayed afterdepolarizations (DADs). How the coordinated loss of NCX and NKA affects SR Ca release and [Na]i is poorly understood. We measured Ca sparks and waves in myocytes from AnkB+/- and wild-type (WT) mice with and without 1 μM isoproterenol. Under control conditions, Ca transients, SR Ca load and Ca sparks frequency (CaSpF), even normalized to SR Ca content, were higher in AnkB+/- mice at all frequencies. With isoproterenol, Ca transient amplitude and SR Ca content were similar in myocytes from WT and AnkB+/- mice, but CaSpF was still elevated in AnkB+/- mice. Ca waves occurred preponderantly in AnkB+/- mice. In some cases, Ca waves and sparks evolved into synchronous after-Ca transients that are indicative of DADs. This data suggest that SR Ca release is enhanced in AnkB+/- mice. We measured [Na]i to determine whether the increased CaSpF in AnkB+/- mice is due to higher [Na]i, which may elevate diastolic Ca. [Na]i was slightly, but not significantly, lower in AnkB+/- vs. WT mice (11.4±1.1 vs. 13.2±1.7 mM in WT). Isoproterenol lowered [Na]i by a comparable amount in both AnkB+/- and WT mice (to 8.1±0.9 mM and 11.1±1.5 mM). Thus, altered [Na]i does not explain the increased Ca sparks frequency in AnkB+/- mice.