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

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 . DLA fiber cross-sectional areas (CSA) were determined from digitized musc le sections, and fiber succinate dehydrogenase (SDH) activity was determine d histochemically using a microdensitometric procedure. An acidic ATPase re action was used to visualize capillaries. Myosin heavy chain (MyHC) isoform s were identified by SDS-PAGE, and their proportions were determined by sca nning densitometry. The body weight of IGF-I(-/-) animals was 32% that of W T 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 6 3, 68, and 65%, respectively, those of WT animals (P < 0.001). The DIA thic kness and the number of fibers spanning its entire thickness were reduced b y 36 and 25%, respectively, in IGF-I(-/-) mice (P < 0.001). SDH activity wa s significantly increased in all three types of DIA fibers of IGF-I(-/-) mu tants (P < 0.05). The number of capillaries per fiber was reduced similar t o 30% in IGF-I(-/-) animals, whereas the capillary density was preserved. T he proportions of MyHC isoforms were similar between the groups. Muscle hyp oplasia likely reflects the importance of IGF-I on cell proliferation, diff erentiation, and apoptosis (alone or in combination) during development, al though 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. Impro ved oxidative capacity likely reflects DIA compensatory mechanisms in IGF-I (-/-) mutants.