Chronic β-adrenergic stimulation reverses depressed Ca handling in mice overexpressing inhibitor-2 of protein phosphatase 1

Abstract

RationaleA higher expression/activity of type 1 serine/threonine protein phosphatase 1 (PP1) may contribute to dephosphorylation of cardiac regulatory proteins triggering the development of heart failure. ObjectiveHere, we tested the putatively protective effects of PP1 inhibitor-2 (I2) overexpression using a heart failure model induced by chronic β-adrenergic stimulation. Methods and resultsTransgenic (TG) and wild-type (WT) mice were subjected to isoprenaline (ISO) or isotonic NaCl solution supplied via osmotic minipumps for 7 days. I2 overexpression was associated with a depressed PP1 activity. Basal contractility was unchanged in catheterized mice and isolated cardiomyocytes between TGNaCl and WTNaCl. TGISO mice exhibited more fibrosis and a higher expression of hypertrophy marker proteins as compared to WTISO. After acute administration of ISO, the contractile response was accompanied by a higher sensitivity in TGISO as compared to WTISO. In contrast to basal contractility, the peak amplitude of [Ca]i and SR Ca load were reduced in TGNaCl as compared to WTNaCl. These effects were normalized to WT levels after chronic ISO stimulation. Cardiomyocyte relaxation and [Ca]i decay kinetics were hastened in TGISO as compared to WTISO, which can be explained by a higher phospholamban phosphorylation at Ser16. Chronic catecholamine stimulation was followed by an enhanced expression of GSK3β, whereas the phosphorylation at Ser9 was lower in TG as compared to the corresponding WT group. This resulted in a higher I2 phosphorylation that may reactivate PP1. ConclusionOur findings suggest that the basal desensitization of β-adrenergic signaling and the depressed Ca handling in TG by inhibition of PP1 is restored by a GSK3β-dependent phosphorylation of I2.