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 [Ca 2+ ] (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 Ca 2+ release was also greater, indicating a higher SR Ca 2+ content in the Ad‐FKBP12.6 group. Voltage clamp experiments indicated that FKBP12.6 over‐expression did not change L‐type Ca 2+ current amplitude or Ca 2+ efflux rates via the Na + –Ca 2+ exchanger. Ca 2+ transients comparable to those after Ad‐FKBP12.6 transfection could be obtained by enhancing SR Ca 2+ 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 Ca 2+ release and fewer non‐responsive Ca 2+ release sites in the Ad‐FKBP12.6 group compared to control. Ca 2+ spark morphology was measured in β‐escin‐permeabilized cardiomyocytes at a free [Ca 2+ ] i of 150 n m . The average values of the spark parameters (amplitude, duration, width and frequency) were reduced in the Ad‐FKBP12.6 group. Increasing [Ca 2+ ] i to 400 n m caused coherent propagating Ca 2+ waves in the Ad‐FKBP12.6 group but only limited Ca 2+ release events were recorded in the control group. These data indicate that FKBP12.6 over‐expression enhances Ca 2+ transient amplitude predominately by increasing SR Ca 2+ content. Moreover, there is also evidence that FKBP12.6 can enhance the coupling between SR Ca 2+ release sites independently of SR content.