Ca2+ activation and tension cost in myofilaments from mouse hearts ectopically expressing enteric γ-actin

Abstract

To determine the significance of actin isoforms in chemomechanical coupling, we compared tension and ATPase rate in heart myofilaments from nontransgenic (NTG) and transgenic (TG) mice in which enteric γ-actin replaced >95% of the cardiac α-actin. Enteric γ-actin was expressed against three backgrounds: mice expressing cardiac α-actin, heterozygous null cardiac α-actin mice, and homozygous null cardiac α-actin mice. There were no differences in maximum Ca2+activated tension or maximum rate of tension redevelopment after a quick release and rapid restretch protocol between TG and NTG skinned fiber bundles. However, compared with NTG controls, Ca2+sensitivity of tension was significantly decreased and economy of tension development was significantly increased in myofilaments from all TG hearts. Shifts in myosin isoform population could not fully account for this increase in the economy of force production of TG myofilaments. Our results indicate that an exchange of cardiac α-actin with an actin isoform differing in only five amino acids has a significant impact on both Ca2+ regulation of cardiac myofilaments and the cross-bridge cycling rate.