Expression of green fluorescent protein impairs the force-generating ability of isolated rat ventricular cardiomyocytes

Green fluorescent protein (GFP) is widely used as a biologically inert expression marker for studying the effects of transgene expression in heart tissue, but its influence on the contractile function of cardiomyocytes has not yet been fully evaluated. We measured the contractile function of isolated rat ventricular myocytes before and after infection with a recombinant adenovirus expressing GFP (Adv-GFP). Myocytes infected with a non-transgene-containing adenovirus (Adv-Null) or uninfected myocytes (UI) served as controls. Using a carbon-fiber-based force-length measurement system for single cardiomyocytes, we evaluated the contractile function over a wide range of loading conditions including the shortening fraction (%FS) and maximal shortening velocity (Vmax) under the unloaded condition, and isometric force. At 24 hours after infection, nearly 80% of the Adv-GFP-infected myocytes expressed GFP. We found that the %FS and Vmax did not differ among the three groups, however, the isometric force showed a mild, but significant, decrease only in Adv-GFP myocytes (Adv-GFP: 29.1 +/- 4.0 mN/mm2; Adv-Null: 42.8 +/- 6.2 mN/mm2; UI: 47.1 +/- 4.8 mN/mm2; p = 0.03). An evaluation of the contractile function of isolated cardiomyocytes under high load conditions revealed impaired isometric contractility by GFP expression. Adv-GFP expression may not be an ideal control for specific gene expression experiments in myocardial tissue.