Disparate dysfunction of skeletal muscles located near and distant from burn site in the rat

This study tested the hypotheses that burn-induced change in muscle function varies at sites local and distant from burn and is related to changes in expression of acetylcholine receptors (AChRs) and muscle mass. In anesthetized rats, approximately 4% burn was inflicted over the tibialis anterior muscle of one limb. The contralateral leg served as control. In another study, a approximately 45% body surface area burn was produced on the trunk; controls were body sham-burned rats. The evoked twitch tensions of tibialis anterior muscles in both legs were measured together with AChR proteins and their transcripts. Compared with the contralateral leg, absolute tensions in the burned leg declined at days 1, 4, and 7 without loss of muscle mass so that tension per unit wet muscle mass (specific tension) decreased; at day 14, the tension decreased with muscle atrophy so that specific tension was unchanged. Membrane AChRs and/or the immature subunit transcript, AChRgamma messenger ribonucleic acid (mRNA) increased at days 4, 7, and 14, and both were inversely related to evoked tension (r =.43, P <.01 and r =.61, P <.0001, respectively). There was a direct correlation between AChR and AChRgamma mRNA (r =.82, P <.001), suggesting that the upregulated AChRs may contain the immature gamma-subunit isoform. After approximately 45% body burn, AChRs and mRNA did not change and the evoked tensions did not decline, but there was relative loss of muscle mass at days 7 and 14 so that specific tension increased. Burn trauma initially causes weakness of muscles directly under the burn, and this weakness may be partially related to increased expression of immature AChRs and later to muscle atrophy.