β-Arrestin mediates the Frank–Starling mechanism of cardiac contractility

Significance The Frank–Starling law of the heart describes the heart’s ability to enhance contractility in response to increased cardiac filling. This property is fundamental to how humans maintain cardiovascular function in response to changes in circulating blood volume, and is regulated by enhanced calcium sensitivity of myofilaments with biomechanical stretch. The mechanism of how biomechanical stretch leads to changes in the myofilament calcium sensitivity remains poorly understood. Using genetic and pharmacologic approaches, we show that β-arrestin and the angiotensin II type I receptor act as crucial molecular regulators of the Frank–Starling law of the heart. This work identifies β-arrestins as important regulators of this fundamental principle of cardiac contractility. The Frank–Starling law of the heart is a physiological phenomenon that describes an intrinsic property of heart muscle in which increased cardiac filling leads to enhanced cardiac contractility. Identified more than a century ago, the Frank–Starling relationship is currently known to involve length-dependent enhancement of cardiac myofilament Ca2+ sensitivity. However, the upstream molecular events that link cellular stretch to the length-dependent myofilament Ca2+ sensitivity are poorly understood. Because the angiotensin II type 1 receptor (AT1R) and the multifunctional transducer protein β-arrestin have been shown to mediate mechanosensitive cellular signaling, we tested the hypothesis that these two proteins are involved in the Frank–Starling mechanism of the heart. Using invasive hemodynamics, we found that mice lacking β-arrestin 1, β-arrestin 2, or AT1R were unable to generate a Frank–Starling force in response to changes in cardiac volume. Although wild-type mice pretreated with the conventional AT1R blocker losartan were unable to enhance cardiac contractility with volume loading, treatment with a β-arrestin–biased AT1R ligand to selectively activate β-arrestin signaling preserved the Frank–Starling relationship. Importantly, in skinned muscle fiber preparations, we found markedly impaired length-dependent myofilament Ca2+ sensitivity in β-arrestin 1, β-arrestin 2, and AT1R knockout mice. Our data reveal β-arrestin 1, β-arrestin 2, and AT1R as key regulatory molecules in the Frank–Starling mechanism, which potentially can be targeted therapeutically with β-arrestin–biased AT1R ligands.