Over-expression of FK506-binding protein FKBP12.6 alters excitation-contraction coupling in adult rabbit cardiomyocytes

Abstract

This study investigated the function of FK506-binding protein (FKBP12.6) using adenoviral-mediated gene transfer to over-express FKBP12.6 (Ad-FKBP12.6) in adult rabbit ventricular cardiomyocytes. Infection with a beta-galactosidase-expressing adenovirus (Ad-LacZ) was used as a control. Peak-systolic intracellular [Ca2+] (measured with Fura-2) was higher in the Ad-FKBP12.6 group compared to Ad-LacZ (1 Hz field stimulation at 37degreesC). The amplitude of caffeine-induced Ca2+ release was also greater, indicating a higher SR Ca2+ content in the Ad-FKBP12.6 group. Voltage clamp experiments indicated that FKBP 12.6 over-expression did not change L-type Ca2+ current amplitude or Ca2+ efflux rates via the Na+-Ca2+ exchanger. Ca2+ transients comparable to those after Ad-FKBP12.6 transfection could be obtained by enhancing SR Ca2+ content of Ad-LacZ infected cells with periods of high frequency stimulation. Line-scan confocal microscopy (Fluo-3 fluorescence) of intact cardiomyocytes stimulated at 0.5 Hz (20-21degreesC) revealed a higher degree of synchronicity of SR Ca2+ release and fewer non-responsive Ca2+ release sites in the Ad-FKBP12.6 group compared to control. Ca2+ spark morphology was measured in beta-escin-permeabilized cardiomyocytes at a free [Ca2+](i) of 150 nm. The average values of the spark parameters (amplitude, duration, width and frequency) were reduced in the Ad-FKBP12.6 group. Increasing [Ca2+](i) to 400 nm caused coherent propagating Ca2+ waves in the Ad-FKBP12.6 group but only limited Ca2+ release events were recorded in the control group. These data indicate that FKBP12.6 over-expression enhances Ca2+ transient amplitude predominately by increasing SR Ca2+ content. Moreover, there is also evidence that FKBP 12.6 can enhance the coupling between SR Ca2+ release sites independently of SR content.