Smooth muscle myosin light chain kinase is transiently expressed in skeletal muscle during embryogenesis and muscle regeneration both in vivo and in vitro.

By using a polyclonal antibody raised against smooth muscle Myosin Light Chain Kinase of adult chicken we show that the 135 kDa smooth muscle Myosin Light Chain Kinase isoform is present in neonatal and regenerating rat skeletal muscle, as well as in adult atrial myocardium. No reaction was evident in adult skeletal muscle fibres. In neonatal and in early regenerating muscle smooth muscle Myosin Light Chain Kinase is associated with embryonic myosin as revealed by their co-presence in muscle fibres. Experiments in vitro show the same results in myotubes. In atrial myocardium there is a patchy positivity in certain group of myocytes. Immunoblotting experiments show in muscle cell cultures, in neonatal and in regenerating skeletal muscle a protein band with electrophoretic mobility corresponding to that of smooth muscle Myosin Light Chain Kinase. These results suggest that the expression of smooth muscle Myosin Light Chain Kinase is not fully tissue-specific and that regulation of the contractile machinery could be different during myogenesis and in adulthood, in relation to the peculiar dynamic characteristics of developing muscles.