Abstract
Cardiac muscle expresses three neuronal nitric oxide synthase (nNOS) splice variants: nNOSα, nNOSμ and nNOSβ. The functions of these nNOS splice variants in cardiac muscle, particularly myofilament-associated nNOSβ are unclear. To decipher cardiac nNOS splice variant function we investigated myofilament function and intracellular calcium and force transients in demembranated and intact papillary muscles from two lines of nNOS knockout mice. The first line (KN1) lacks nNOSα and nNOSμ. The second line (KN2) lacks active nNOSα, nNOSμ and nNOSβ. Demembranated KN1 papillary muscles exhibited reduced myofilament ATPase activity (-35%) and specific force (-10%) relative to controls. Demembranated KN2 muscles exhibited a smaller decrease in myofilament ATPase activity (-21%), but a greater reduction in specific force (-26%) relative to controls. Myofilament calcium sensitivity in demembranated KN1 and KN2 papillary muscles was similar to controls. Thus, papillary muscle-expressed nNOS splice variants are necessary for control levels of myofilament ATPase activity and force generation, but dispensable for myofilament calcium sensitivity. The greater reduction in myofilament ATPase relative to specific force in KN1, but not KN2 muscle, reduced the energy cost of muscle contraction, suggesting that nNOSβ increased the energetic efficiency of contraction in the absence of nNOSμ and nNOSα. Analyses of intact KN1 and KN2 papillary muscles showed that both intracellular calcium transients and their evoked force transients were similar to controls at stimulation frequencies between 1 and 3 Hz. Therefore, nNOS was dispensable for baseline excitation-contraction coupling. In summary, these data suggest that nNOS splice variants differentially regulate myofilament function, but not baseline calcium handling in papillary muscles. More importantly, they suggest that nNOSβ is a novel modulator of myofilament function, and ultimately the energetic efficiency of cardiac papillary muscle contraction.
Highlights
Neuronal nitric oxide synthase splice variants encoded by the NOS1 gene are an important source of cardioprotective nitric oxide (NO) in the heart [1,2,3,4]
Myofilament ATPase activity was reduced to a lesser degree (-21%) in knockout 2 (KN2) mice, suggesting that nNOSβ may suppress maximal myofilament ATPase activity in muscles lacking nNOSα and nNOSμ
Specific force was decreased to a greater degree in KN2 than KN1 papillary muscles compared to controls indicating a role for nNOSβ in modulating cardiac myofilament force output
Summary
Neuronal nitric oxide synthase (nNOS) splice variants encoded by the NOS1 gene are an important source of cardioprotective nitric oxide (NO) in the heart [1,2,3,4]. Cardiac muscle cells express three nNOS splice variants (nNOSα, nNOSμ and nNOSβ) at distinct subcellular locations. NNOSα and/or nNOSμ associate with the sarcoplasmic reticulum, while nNOSβ associates with myofilaments [5, 6] These distinct localizations suggest isoform-specific functions for nNOS splice variants that remain to be deciphered. Cardiomyocyte-specific nNOSα overexpression or viral delivery of nNOSα to the KN1 myocardium impaired cardiac contractility [12, 13]. Another example of the lack of consensus comes from divergent findings regarding the role of nNOS in modulating myocardial contractile reserve. KN1 mice can exhibit impaired or enhanced β-adrenergic inotropic responses [10, 14, 15]
Sign-in/Register to view highlights & summary Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
