Effects of Adenovirus-Mediated Sorcin Overexpression on Excitation-Contraction Coupling in Isolated Rabbit Cardiomyocytes

Abstract

To evaluate the effect of sorcin on cardiac excitation-contraction coupling, adult rabbit ventricular myocytes were transfected with a recombinant adenovirus coding for human sorcin (Ad-sorcin). A β-galactosidase adenovirus (Ad-LacZ) was used as a control. Fractional shortening in response to 1-Hz field stimulation (at 37°C) was significantly reduced in Ad-sorcin–transfected myocytes compared with control myocytes (2.10±0.05% [n=311] versus 2.42±0.06% [n=312], respectively; P <0.001). Action potential duration (at 20°C) was significantly less in the Ad-sorcin group (458±22 ms, n=11) compared with the control group (520±19 ms, n=10; P <0.05). In voltage-clamped, fura 2–loaded myocytes (20°C), a reduced peak-systolic and end-diastolic [Ca 2+ ] i was observed after Ad-sorcin transfection. L-type Ca 2+ current amplitude and time course were unaffected. Caffeine-induced Ca 2+ release from the sarcoplasmic reticulum (SR) and the accompanying inward Na + -Ca 2+ exchanger (NCX) current revealed a significantly lower SR Ca 2+ content and faster Ca 2+ -extrusion kinetics in Ad-sorcin–transfected cells. Higher NCX activity after Ad-sorcin transfection was confirmed by measuring the NCX current-voltage relationship. β-Escin–permeabilized rabbit cardiomyocytes were used to study the effects of sorcin overexpression on Ca 2+ sparks imaged with fluo 3 at 145 to 160 nmol/L [Ca 2+ ] using a confocal microscope. Under these conditions, caffeine-mediated SR Ca 2+ release was not different between the two groups. Spontaneous spark frequency, duration, width, and amplitude were lower in sorcin-overexpressing myocytes. In summary, sorcin overexpression in rabbit cardiomyocytes decreased Ca 2+ -transient amplitude predominately by lowering SR Ca 2+ content via increased NCX activity. The effect of sorcin overexpression on Ca 2+ sparks indicates an effect on the ryanodine receptor that may also influence excitation-contraction coupling.