Decrease in the density of t-tubular L-type Ca²⁺ channel currents in failing ventricular myocytes

Abstract

In some forms of cardiac hypertrophy and failure, the gain of Ca(2+)-induced Ca(2+) release [CICR; i.e., the amount of Ca(2+) released from the sarcoplasmic reticulum normalized to Ca(2+) influx through L-type Ca(2+) channels (LTCCs)] decreases despite the normal whole cell LTCC current density, ryanodine receptor number, and sarcoplasmic reticulum Ca(2+) content. This decrease in CICR gain has been proposed to arise from a change in dyad architecture or derangement of the t-tubular (TT) structure. However, the activity of surface sarcolemmal LTCCs has been reported to increase despite the unaltered whole cell LTCC current density in failing human ventricular myocytes, indicating that the "decreased CICR gain" may reflect a decrease in the TT LTCC current density in heart failure. Thus, we analyzed LTCC currents of failing ventricular myocytes of mice chronically treated with isoproterenol (Iso). Although Iso-treated mice exhibited intact t-tubules and normal LTCC subunit expression, acute occlusion of t-tubules of isolated ventricular myocytes with osmotic shock (detubulation) revealed that the TT LTCC current density was halved in Iso-treated versus control myocytes. Pharmacological analysis indicated that kinases other than PKA or Ca(2+)/calmodulin-dependent protein kinase II insufficiently activated, whereas protein phosphatase 1/2A excessively suppressed, TT LTCCs in Iso-treated versus control myocytes. These results indicate that excessive β-adrenergic stimulation causes the decrease in TT LTCC current density by altering the regulation of TT LTCCs by protein kinases and phosphatases in heart failure. This phenomenon might underlie the decreased CICR gain in heart failure.

Myoscape controls cardiac calcium cycling and contractility via regulation of L-type calcium channel surface expression

“…While several groups have shown that LTCC currents are reduced in both experimental48 and human495051 heart failure, others reported only little differences52. In this context it is important to note that LTCC currents not only initiate excitation–contraction coupling in cardiac muscle, but have also been implicated in calcium-dependent signalling in cardiomyocytes.…”

β2-Adrenergic and M2-muscarinic receptors decrease basal t-tubular L-type Ca2+ channel activity and suppress ventricular contractility in heart failure

“…The morphology of the heart was evaluated as previously described (Horiuchi-Hirose et al, 2011). Briefly, the hearts were fixed in 20% phosphate-buffered formalin at room temperature for 24 h and cut transversely to obtain ventricular cross sections.…”

“…Cardiac function was assessed with ultrasound cardiography (GE Yokogawa Medical System K.K., Tokyo, Japan) in anesthetized mice as previously described (Horiuchi-Hirose et al, 2011). Briefly, hearts were viewed at the level of the papillary muscles in the short axis.…”

“…In most cardiac hypertrophy and failure, whole-cell LTCC current density is reported not to change markedly (Bers, 2008;Mukherjee and Spinale, 1998). However, we previously found for the first time that basal t-tubular (TT) LTCC activity was halved by activation of protein phosphatase (PP)2A whereas basal surface sarcolemmal (SS) LTCC activity was doubled by inhibition of PP1 in failing ventricular myocytes of mice chronically treated with isoproterenol (Horiuchi-Hirose et al, 2011;Kashihara et al, 2012). Because Gi/o proteins are known to activate PP2A (Herzig et al, 1995;Ke et al, 2004;Liu and Hofmann, 2003) and suppress PP1 in cardiac myocytes (Zhu et al, 2008), we examined the effect on isoproterenol-treated mice of pertussis toxin, a selective inhibitor of the coupling between G protein-coupled receptors and Gi/o proteins (Kashihara et al, 2012).…”

“…The hearts and lungs were excised from the animals, rinsed in ice-cold modified Tyrode solution containing (in mmol/l): NaCl, 136.5; KCl, 5.4; CaCl2, 1.8; MgCl2, 0.53; HEPES (Dojindo Laboratories, Kumamoto, Japan), 5.5; and glucose, 5.5 (pH = 7.4 with NaOH) and weighed. Because the body weight of all groups of mice treated with drugs was significantly higher than that of saline-treated mice (Horiuchi-Hirose et al, 2011), the heart weight (HW; in mg) and lung weight (LW; in mg) were normalized to the tibial length (TL; in mm), which was not significantly different among the five groups of mice (i.e. HW/TL and LW/TL).…”