Expression of a Gi-coupled receptor in the heart causes impaired Ca2+ handling myofilament injury and dilated cardiomyopathy

摘要

Increased signaling by Gi-coupled receptors has been implicated in dilated cardiomyopathy. To investigate the mechanisms, we used transgenic mice that develop dilated cardiomyopathy after conditional expression of a cardiac-targeted Gi-coupled receptor (Ro1). Activation of Gi signaling by the Ro1 agonist spiradoline caused decreased cellular cAMP levels and bradycardia in Langendorff-perfused hearts. However, acute termination of Ro1 signaling with the antagonist nor-binaltorphimine did not reverse the Ro1-induced contractile dysfunction, indicating that Ro1 cardiomyopathy was not due to acute effects of receptor signaling. Early after initiation of Ro1 expression, there was a 40% reduction in the abundance of the sarcoplasmic reticulum Ca²⁺-ATPase (P < 0.05); thereafter, there was progressive impairment of both Ca²⁺ handling and force development assessed with ventricular trabeculae. Six weeks after initiation of Ro1 expression, systolic Ca²⁺ concentration was reduced to 0.61 ± 0.08 vs. 0.91 ± 0.07 μM for control (n = 6–8; P < 0.05), diastolic Ca²⁺ concentration was elevated to 0.41 ± 0.07 vs. 0.23 ± 0.06 μM for control (n = 6–8; P < 0.01), and the decline phase of the Ca²⁺ transient (time from peak to 50% decline) was slowed to 0.25 ± 0.02 s vs. 0.13 ± 0.02 s for control (n = 6–8; P < 0.01). Early after initiation of Ro1 expression, there was a ninefold elevation of matrix metalloprotein-ase-2 (P < 0.01), which is known to cause myofilament injury. Consistent with this, 6 wk after initiation of Ro1 expression, Ca²⁺-saturated myofilament force in skinned trabeculae was reduced to 21 ± 2 vs. 38 ± 0.1 mN/mm² for controls (n = 3; P < 0.01). Furthermore, electron micrographs revealed extensive myofilament damage. These findings may have implications for some forms of human heart failure in which increased activity of Gi-coupled receptors leads to impaired Ca²⁺ handling and myofilament injury, contributing to impaired ventricular pump function and heart failure.