Cardiac actin capping protein reduction and protein kinase C inhibition maintain myofilament function during cardioplegic arrest.

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 -e 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.