Decrease in Maximal Force Generation in the Neonatal Mouse Bladder Corresponds to Shift in Myosin Heavy Chain Isoform Composition

Abstract

A change in calcium handling has been proposed as the cause of decreased maximal force generation by neonatal bladders with growth. Recent studies suggest that increased myosin heavy chain isoform SM1 increases force generation. We studied force generation in neonatal mouse bladders to determine if decreases in SM1 corresponded with decreased force. C57Bl/6 mice were studied from birth to 12 weeks of life (adulthood). The bladder strip contractile response to KCl and bethanechol was followed by the inhibition of rho-kinase activity by Y-27632. The mRNA levels for SM1/SM2 were determined using reverse transcriptase-polymerase chain reaction and protein levels were determined using sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Muscle fraction per cross-sectional area was determined by trichrome staining. Newborn bladders generated significantly more tension in response to KCl (43.3 vs 17.4 mN/mm2, p = 0.02) and bethanechol (40.6 vs 11.9 mN/mm2, p = 0.05) than adult bladders. Inhibition of rho-kinase resulted in similar decreases in tension in all bladders. SM1 mRNA decreased slightly from 60% at birth to 50% at 12 weeks. SM1 protein decreased from 72.5% at birth to 50% by 3 weeks and it remained stable at 12 weeks. Total myosin per gm protein remained stable. Muscle fraction decreased from 63.8% at birth to 58.6% at 12 weeks (p = 0.4). We noted a decrease in SM1 that corresponded to a decrease in bladder force generation. The concept that SM1 contributes to the optimal assembly of myosin filaments suggests that changes in myosin isoforms may have a role in the decrease in voiding pressures seen in normal children.