Neonatal mouse ileum: functional properties and protein composition of the contractile machinery

Immature motility of the ileum may contribute to life-threatening diseases. Little is known about the normal biomechanics of the neonatal ileum in relation to the protein composition of its contractile machinery.

We analyzed the tissue architecture, the biomechanics in intact and β-escin-permeabilized preparations, and the protein composition in neonatal (P0) and adult murine ileum.

Muscle thickness of the P0 ileum was -50% of the adult ileum and passive compliance was higher. Carbachol- and KCl-elicited contractions were tonic rather than phasic as in the adult. Ca(2+) sensitivity was higher and relaxation rate was slower in β-escin-permeabilized P0 compared with adult ileum. The expression level of β-actin relative to α-actin was higher, and those of total actin, myosin, myosin light chain kinase, the catalytic subunit of myosin phosphatase and telokin were lower compared with the adult. The expression level of MYPT1 was similar, but P0 ileum expressed only the M133; the adult ileum also expressed the M130 isoform.

The mechanical features and protein composition of the P0 ileum are similar to those of adult tonic smooth muscles. We propose that this is highly adaptive during fetal life allowing the small intestine to act predominantly as a container.