Browsing by Author "Zeitz, O."

FK506 (tacrolimus) is a new immunosuppressant being used in cardiac allograft transplantation. While cyclosporine A has been shown to exert an acute negative inotropic effect on isolated heart muscle preparations, little is known of the inotropic, influence of FK506. The Ca2+ release channel of human skeletal muscle and cardiac muscle is associated with FK506 binding proteins (FKBP), FKBP12 and FKBP12.6, respectively. FKBPs can be dissociated by treatment with FK506. As a consequence of FK506 exposure, isolated skeletal muscle and cardiac muscle ryanodine receptors show altered gating characteristics. Therefore, we analyzed the direct inotropic effect of FK506 exposure to isolated, intact heart muscle preparations from the human and rabbits. Experiments were performed on isolated, electrically stimulated right atrial auricular muscle strips obtained from human myocardium during elective open heart surgery and on intact right ventricular trabeculae from rabbit hearts. The human preparations were exposed to concentrations of 8 X 10(-9), 8 X 10(-8) and 8 X 10(-6) M FK506 followed by a cumulative dose-response curve with isoprenaline as a non-selective beta -adrenoceptor agonist. Our data suggest that FK506 does not exert any positive or negative inotropic effect in either human or rabbit myocardium. (C) 2001 Elsevier Science B.V. All rights reserved.

The FK506-binding protein FKBP12.6 is tightly associated with the cardiac sarcoplasmic reticulum (SR) Ca2+-release channel (ryanodine receptor type 2 [RyR2]), but the physiological function of FKBP12.6 is unclear. We used adenovirus (Ad)-mediated gene transfer to overexpress FKBP12.6 in adult rabbit cardiomyocytes. Western immunoblot and reverse transcriptase-polymerase chain reaction analysis revealed specific overexpression of FKBP12.6, with unchanged expression of endogenous FKBP12. FKBP12.6-transfected myocytes displayed a significantly higher (21%) fractional shortening (FS) at 48 hours after transfection compared with Ad-GFP-infected control cells (4.8+/-0.2% FS versus 4+/-0.2% FS, respectively; n=79 each; P=0.001). SR-Ca2+ uptake rates were monitored in beta -escin-permeabilized myocytes using Fura-2. Ad-FKBP12.6-infected cells showed a statistically significant higher rate of Ca2+ uptake of 0.8+/-0.09 nmol/s(-1)/10(6) cells (n = 8, P<0.05) compared with 0.52+/-0.1 nmol/s(-1)/10(6) cells in sham-infected cells (n = 8) at a [Ca2+] of 1 mol/L. In the presence of 5 mu mol/L ruthenium red to block Ca2+ efflux via RyR2, SR-Ca2+ uptake rates were not significantly different between groups. From these measurements, we calculate that SR-Ca2+ leak through RyR2 is reduced by 53% in FKBP12.6-overexpressing cells. Caffeine-induced contractures were significantly larger in Ad-FKBP12.6-infected myocytes compared with Ad-GFP-infected control cells, indicating a higher SR-Ca2+ load. Taken together, these data suggest that FKBP12.6 stabilizes the closed conformation state of RyR2. This may reduce diastolic SR-Ca2+ leak and consequently increase SR-Ca2+ release and myocyte shortening.