Influences of IGF-I gene disruption on the cellular profile of the diaphragm

Abstract

The impact of a targeted disruption of the Igf1 gene, encoding the insulin-like growth factor I (IGF-I), on diaphragm (DIA) cellularity was studied in 2-mo-old homozygous mutant [IGF-I(−/−)] mice and their wild-type [WT; i.e., IGF-I(+/+)] littermates. DIA fiber types were classified histochemically. DIA fiber cross-sectional areas (CSA) were determined from digitized muscle sections, and fiber succinate dehydrogenase (SDH) activity was determined histochemically using a microdensitometric procedure. An acidic ATPase reaction was used to visualize capillaries. Myosin heavy chain (MyHC) isoforms were identified by SDS-PAGE, and their proportions were determined by scanning densitometry. The body weight of IGF-I(−/−) animals was 32% that of WT littermates. DIA fiber type proportions were unchanged between the groups. The CSAs of types I, IIa, and IIx DIA fibers of IGF-I(−/−) mutants were 63, 68, and 65%, respectively, those of WT animals ( P < 0.001). The DIA thickness and the number of fibers spanning its entire thickness were reduced by 36 and 25%, respectively, in IGF-I(−/−) mice ( P < 0.001). SDH activity was significantly increased in all three types of DIA fibers of IGF-I(−/−) mutants ( P< 0.05). The number of capillaries per fiber was reduced ∼30% in IGF-I(−/−) animals, whereas the capillary density was preserved. The proportions of MyHC isoforms were similar between the groups. Muscle hypoplasia likely reflects the importance of IGF-I on cell proliferation, differentiation, and apoptosis (alone or in combination) during development, although reduced cell size highlights the importance of IGF-I on rate and/or maintenance of DIA fiber growth in the postnatal state. Reduced capillarity may result from both direct and indirect influences on angiogenesis. Improved oxidative capacity likely reflects DIA compensatory mechanisms in IGF-I(−/−) mutants.