C57bl 6j male mice

Hearts were isolated from male C57BL/6J male mice (~20–25 g) (Charles River JAX^TM stock number 000664) or from WT and PLM^3SA knock-in mice^{18 (link)}. This investigation complied with all the relevant ethical regulations for animal testing and research: UK Home Office Guidance on the Operation of the Animals (Scientific Procedures) Act, 1986.
PLM^3SA knock-in mice were backcrossed with C57BL/6J mice (Charles River, UK) for >5 generations and were generated by heterozygous pair mating. The PLM^3SA mouse expresses a non-phosphorylatable form of PLM in which Ser 63, 68, and 69 have been mutated to alanine and exhibits a Na/K ATPase that is unresponsive to kinase regulation and hence shows chronically elevated Na_i^{15 (link)}. Unless otherwise stated, littermates were used as the appropriate wild-type controls (PLM^WT). Animals were kept under pathogen-free conditions, 12-h light–dark cycle, controlled humidity (~40%), temperature (20–22 °C), and fed chow and water ad libitum. All animals used in studies were male. For pharmacologically induced acute Na_i elevation studies, 6-week-old C57BL/6J male mice (~25 g body weight) were purchased from Charles River (UK). Myocardial hypertrophy was induced in 6-week-old C57BL/6J mice (20–22 g) (Charles River, UK). For cellular compartmentation of the ²³Na TQF NMR signal experiment, Male Wistar rats (250 g) were purchased from Charles River, UK.

Animal studies were performed using 8-weeks-old C57BL/6J male mice purchased from Vital River Laboratory Animal Technology Co (Beijing, China). All mice were housed in Tsinghua animal house, an adequately ventilated temperature-controlled rodent-housing system with free access to food and water.
Mice were weighed and grouped into three clusters of 5 mice each: Control (ground control group), HU (hindlimb unloading mouse group), and HU + 3HB (hindlimb unloading mice fed with 3HB group). Before the mice were suspended with their hindlimbs, R-3-hydroxybutyric acid sodium salt (3HB, Sigma-Aldrich, USA) or an equal volume of deionized water were administrated in the pretreatment period for 7 days, then continued for another 14 days during the hindlimb unloading model construction. 3HB and deionized water were administrated for 21 days by oral gavage once a day at 8:00 a.m.