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

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 β‐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 37°C). 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 FKBP12.6 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 (20−21°C) 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 β‐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 FKBP12.6 can enhance the coupling between SR Ca2+ release sites independently of SR content.