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Disruption of Ca²⁺ homeostasis is a key pathomechanism in heart failure. CaMKII‐dependent hyperphosphorylation of ryanodine receptors in the sarcoplasmic reticulum (SR) increases the arrhythmogenic SR Ca²⁺ leak and depletes SR Ca²⁺ stores. The contribution of conversely acting serine/threonine phosphatases [protein phosphatase 1 (PP1) and 2A (PP2A)] is largely unknown.

Human myocardium from three groups of patients was investigated: (i) healthy controls (non‐failing, NF, n = 8), (ii) compensated hypertrophy (Hy, n = 16), and (iii) end‐stage heart failure (HF, n = 52). Expression of PP1 was unchanged in Hy but greater in HF compared to NF while its endogenous inhibitor‐1 (I‐1) was markedly lower expressed in both compared to NF, suggesting increased total PP1 activity. In contrast, PP2A expression was lower in Hy and HF compared to NF. Ca²⁺ homeostasis …