Ca flux, contractility, and excitation-contraction coupling in hypertrophic rat ventricular myocytes

Left ventricular hypertrophy (∼40%) was induced in rats by banding of the abdominal aorta. After 16 wk, ventricular homogenates were prepared for biochemical measurements and ventricular myocytes were isolated for functional studies. In myocytes, the effects of banding on intracellular Ca handling, contraction, and excitation-contraction (E-C) coupling were determined using indo 1 fluorescence and whole cell voltage clamp. After steady-state field or voltage-clamp stimulation to load the sarcoplasmic reticulum (SR), SR Ca content assessed by caffeine-induced Ca transients was the same in sham and banded groups. Despite this, cell shortening amplitudes were significantly depressed in the banded group, suggesting altered contractile properties. In banded rats, the SR Ca-adenosinetriphosphatase (Ca-ATPase) mRNA level was reduced, as was homogenate thapsigargin-sensitive SR Ca-ATPase, but cytosolic free Ca concentration ([Ca]_i) decline attributed to SR CaATPase activity in intact cells was not slowed. Banding also reduced Na/Ca exchange mRNA level but did not affect either Na-dependent sarcolemmal ⁴⁵Ca transport in homogenate or the rate of [Ca]_i decline in intact cells attributed to Na/Ca exchange (during caffeine-induced contractures). Banding also did not change the rate of [Ca]_i decline mediated by the combined function of the mitochondrial Ca uptake and sarcolemmal Ca-ATPase in intact cells. Ca current (I_Ca) density and voltage dependence were the same in sham and banded groups. Ryanodine receptor mRNA, protein content, and ryanodine affinity were also unchanged in the banded group. At 1 mM extracellular Ca concentration ([Ca]_o), banding did not affect E-C coupling efficacy in intact cells under voltage clamp (i.e., same contraction for given I_Ca and SR Ca load). However, when [Ca]_o was reduced to 0.5 mM, the efficacy of E-C coupling was greatly depressed in the banded group (even though I_Ca and SR Ca content were matched). In summary, unloaded myocyte contraction was depressed in these hypertrophic hearts, but Ca transport was little altered, at 1 mM [Ca]_o. However, reduction of [Ca]_o to 0.5 mM appears to unmask a depressed fractional SR Ca release in response to a given I_Ca trigger and SR Ca load.