MYOSIN GENE-EXPRESSION IN THE RESPIRATORY MUSCLES

Myosin is one of the basic structural components of skeletal muscles, Its interaction with actin results in muscle contraction, The myosin m olecule is composed of two heavy (MyHC) and two light chains (My LC) t hat, together with the adenosine triphosphatase (ATPase) activity, det ermine the functional characteristics of the fibre, Both MyHC and MyLC present different isoforms, The main MyHC isoforms in adult mammals a re the slow MyHC (My HC-I) and fast MyHCs (MyHC-IIa, MyHC-IIb and MyHC -IIx). Muscle fibres can express only one isoform or coexpress differe nt forms. The muscle phenotype is the product of genome plus environme ntal stimuli. The family of genes that codifies the MyHC isoforms is l ocated in two different clusters, each isoform being encoded by a sepa rate gene, The gene corresponding to slow MyHC is located in chromosom e 14, both in humans and in mice, The other genes are positioned in ch romosome 17 in humans, and in chromosome 11 in mice, The transcription al and translational mechanisms that control the expression of MyHC is oforms are not well known, although it is believed that the main regul ation is dependent on mechanical signals, These signals are probably m ediated by a biochemical messenger, As a general rule, fast MyHC genes seem to be expressed ''by default'', whereas the slow MyHC gene would be expressed as a response to changes in load. So far, few studies ha ve analysed the in vivo regulation of MyHC gene expression in respirat ory muscles, It has recently been reported that breathing against mode rate levels of inspiratory resistance quickly induces an increase in t he genetic expression of slow MyHC in the diaphragm. This suggests the possibility of eliciting a phenotypic adaptation of respiratory muscl es using specific training protocols.