Contractile function in vitro of slow-twitch skeletal muscle from weanlingmice subjected to wasting malnutrition

Our hypothesis was that malnutrition sufficient to produce weight loss in w eanling mice would decrease the ability of slow-twitch skeletal muscle to d evelop and maintain force. We isolated muscles from 3 groups (n = 5) of wea nling C57BL/6J mice of both sexes (i) mice at 19 days of age serving as zer o-time or baseline controls (CONT) (ii) mice fed for the next 14 days with a low-protein diet that produces features of incipient kwashiorkor (LPD) an d (iii) mice fed for the next 14 days with a complete diet (NORM). Muscles were also obtained from 5 adult mice 7-9 months of age (MAT). We stimulated the soleus at 50 Hz for 500 ms at 0.6 tetanic contractions per min (tet.mi n(-1)), 6 tet.min(-1), and 30 tet.min(-1) in Krebs-Henseleit bicarbonate bu ffer at 27 degreesC gassed with 95% O-2 and 5% CO2. The initial developed f orce (mN.mm(-2)) at 0.6 tet.min(-1) did not differ across groups (CONT 211. 7 +/- 16.0, LPD 274.2 +/- 41.6, NORM 246.8 +/- 38.0, MAT 210.8 +/- 10.6). T he fatigue rate (mN.mm(-2).min(-1)) at 6 tet.min(-1) was significantly slow er in muscles from CONT (0.6 +/- 0.3) and LPD (0.6 +/- 0.4) than in NORM (2 .4 +/- 0.6) and MAT (2.3 +/- 0.2). At 30 tet.min(-1), the fatigue rate (mN. mm(-2).min(-1)) did not differ across groups (CONT 2.4 +/- 0.5, LPD 2.7 +/- 0.5, NORM 2.5 +/- 0.4, MAT 2.0 +/- 0.2). After stimulation at 6 tet. min(- 1) and 30 tet.min(-1), only muscles from CONT and LPD recovered to 100%. Be cause muscles from LPD mice developed equal force, fatigued less, and recov ered from fatigue to a greater extent than muscles from NORM mice, we rejec ted the hypothesis. The function of the tissue remaining in the muscles fro m LPD mice approximated that of muscles from mice at 19 days of age rather than muscles from either mice of the same age fed a complete diet or adult mice.