Abstract 3221: A Human Inhibitor-1 Polymorphism, G109E, Impairs Cardiomyocyte Function and Promotes Susceptibility to Arrhythmia
Depressed sarcoplasmic reticulum (SR) Ca handling and increased SR protein phosphatase-1 (PP1) activity are characteristic of human failing hearts. One of the causes of this elevated PP1 activity is inhibitor-1 (I-1), whose levels and activity are decreased in failing hearts. I-1 is a negative regulator of PP1 activity and considered a promising therapeutic target. We recently screened DNA samples from 191 heart failure patients and identified an I-1 polymorphism with a glutamic acid substitution for glycine at position 109 (G109E). This point mutation occurs at a frequency of 5%. To elucidate the functional consequences of the G109E I-1 variant, adult rat cardiomyocytes were isolated and infected with adenoviruses expressing either green fluorescent protein, wild-type I-1, or G109E mutated I-1 (Ad.GFP, Ad.WT, and Ad.G109E), respectively. Ad.G109E and Ad.WT infected cells had similar I-1 expression, and did not alter the levels of sarcoplasmic reticulum Ca ATPase or phospholamban (n=4). Interestingly, cardiomyocytes infected with Ad.G109E had depressed fractional shortening by ~30% compared to cells infected with Ad.GFP or Ad.WT (P=0.07, n=6). Cardiomyocytes infected with Ad.G109E also exhibited decreased rates of contraction and relaxation (by ~25% and 20% respectively). However, treatment with 100 nM isoproterenol (Iso) restored the depressed contractility in G109E I-1 expressing cells. The maximally stimulated parameters by Iso were similar among Ad.GFP, Ad.WT, and Ad.G109E cells. To assess the response of G109E I-1 infected cells to stress, high frequency stimulation (2 Hz) alone or in combination with 100 nM Iso was applied. Cells expressing G109E I-1 tended to exhibit more aftercontractions, than cells expressing WT I-1 or GFP. Thus, G109E I-1 appears to compromise cardiomyocyte function, which can be restored upon β-adrenergic stimulation. However, G109E I-1 appears to destabilize the cells under stress conditions and promote aftercontractions. These data support the importance of I-1 in cardiac function and its relevance in heart failure.