Cardiac Actin Capping Protein Reduction and Protein Kinase C Inhibition Maintain Myofilament Function During Cardioplegic Arrest

Abstract

Background: Heart transplantation is associated with cold, cardioplegic arrest that impairs myocardial function. Protein Kinase C (PKC) suppression of myofilaments may contribute to this dysfunction. CapZ-deficient cardiac myofilaments are unresponsive to PKC. We hypothesized that myofilaments from CapZ-deficient transgenic hearts are resistant to cardioplegic dysfunction and that PKC inhibition improves function. Methods: Heart function was assessed using a Langendorff apparatus. Myofilaments isolated from murine hearts were assessed with an actomyosin MgATPase assay and protein phosphorylation gels. PKC activation was examined by immunoblotting. Results: Wildtype hearts showed impaired function after cardioplegic arrest. CapZ-deficient transgenic mouse hearts performed significantly better after 1 h cardioplegia than wildtype hearts, but not after 4 h cardioplegic arrest. Wildtype myofilaments had depressed activation at 1 and 4 h cardioplegic arrest, as demonstrated by reduced actomyosin MgATPase activity. CapZ-deficient myofilaments showed no reduced actomyosin MgATPase activity at either time. Troponin I (TnI) phosphorylation increased by approximately 20% at 1 and 4 h in wildtype mice. Myosin binding protein C (MyBP-C),and troponin T (TnT) phosphorylation increased by less than 10% at 1 h, and tended to rise at 4 h. Myofilament protein phosphorylation was largely unchanged in CapZ-deficient hearts at 1 h, but MyBP-C tended to be dephosphorylated at 4 h cardioplegic arrest. Myofilament-associated PKC-α, -βII, -δ, and -ε increased at 1 and 4 h cardioplegia in wildtype hearts, whereas only PKC-α increased in transgenic myofilaments at 1 h. PKC inhibition abolished the cardioplegic-dependent changes in actomyosin MgATPase activity and TnI phosphorylation of wildtype myofilaments. Conclusions: We demonstrate a direct link between PKC activation and myofilament dysfunction associated with cold, cardioplegic arrest. Moreover, we show for the first time a cardioprotective benefit of decreased cardiac CapZ.