The Effects of Fructose 1,6-Diphosphate, Caffeine and Dantrolene Sodium on Suxamethonium-induced Contractures in Denervated Rat Skeletal Muscle
Authors: Andrews Margaret M , Webb Sandra N , Whittaker R 
Published: April 2011

Abstract
Previously unidentified forms of suxamethonium-induced contractures have been investigated in chronically denervated rat extensor digitorum longus (EDL) muscle at 20°C. Contractures were assigned to groups 1–6 on the basis of the peak tension (Tp1) during 0–10 min exposure to the drug (3.0 × 10 −5M), (7.0 × 10−6M), and (3.5 × 10−6M) and the subsequent retention, increase, or decrease in tension (Tp2), during the further 10 min. It is proposed that four stages exist in the development of contractile changes at 1–7, 8–35, 36–70 and 70–130 days after denervation (DPD) and that contractility is lost at 147 days after denervation. Initial changes, although present in EDL muscles in group 1 at 2.0 DPD s.d. ± 1 (n = 7) in response to the drug (3.0 × 10−5M), were more apparent in EDL muscles in group 2 which were identified at 5.5 DPD s.d. ± 1.6 (n = 7) by an excessive contracture response (Tp2) to the drug (3.0 × 10−5M), 18.3 mN s.d. ±10.6. At 5.0 DPD s.d. ±2.7 (n = 5) contracture tension (Tp2) was commensurate with membrane depolarization, 13.1 mN/33.1 mV, but residual tension increased to 23.3 mN during the Krebs wash (80 min) whilst membrane depolarization decreased to 9.2 mV. Also, at 4.3 DPD s.d. ± 2.3 (n = 5) tension (Tp2) increased significantly (P≤0.05) in the presence of caffeine (4.1 × 10−3M). Residual tension, which increased during a Krebs wash (30 min) at 0.42 mN min−1 after suxamethonium (3.0 × 10−5M), was reduced by dantrolene sodium (3.8 × 10−4M) at 0.2 mN min−1 after a latency of 36 min in muscles at 3.8 DPD s.d. ±1.1 (n = 4). However, at 5 and 7 DPD (n= 12) the latency of action of dantrolene sodium shortened from 30 to 6 min, and residual tension was reduced faster, 0.42 mN min−1 (cf. 0.2 mN min−1). The second stage in the development of contractile changes was observed in EDL muscles at 8.4 DPD s.d. ±4.3 and 43.6 DPD s.d. ± 25.3 (groups 3 and 4) which were characterized by a prominent peak in tension (Tp1) in response to the drug (3.0 × 10−5M), 16.5 mN s.d. ± 6.6, c.f. 32.9 mN s.d. ± 13.2, and a smaller second peak in tension (Tp2), 8.4 mN s.d. ±4.3 c.f. 11.7 mN s.d. ± 4.1, respectively. Further measurements, at 6.3 DPD s.d. ± 1 (n = 9) and 20 DPD s.d. ±9.7 (n = 12), showed that although Tp1 was commensurate with membrane depolarization, 21.2 mN/28.4 mV, and 30.5 mN/27.2 mV, in response to suxamethonium (3.0 × 10−5M), Tp2 was low with respect to membrane depolarization, 3.7 mN/38.2 mV and 9.7 mN/38.6 mV, respectively. The third stage in the development of contractile changes was observed in EDL muscles at 58.1 DPD s.d. ± 22 (n = 14) in group 5 which was typified by a sharp first peak in tension (Tp1), 23.8 mN s.d. ± 13.2 and by an absence of tension during 10–20 min exposure to the drug (3.0 × 10−5M). In response to the drug (3.0 × 10−5M) in the presence of fructose 1,6-diphosphate (2.3 × 10−3M) or caffeine (4.1 × 10−3M), the second peak in contracture tension (Tp2) was recorded at 3 mN and 10.3 mN, respectively. In the fourth stage of contractile changes EDL muscles at 124.7 DPD s.d. ± 2.3 (n = 11), designated group 6, produced a minimal contractile response to suxamethonium (3.0 × 10−5M), 6.4 mN (Tp1) and 1.9 mN (Tp2) in comparison with variable depolarization, 12.8 mV and 30.5 mV. Finally, at 147 DPD the drug-induced contracture response was absent (n = 4) whilst membrane depolarization was less than normal control muscles, 7–11 mV (c.f. 10 13 mV). Contractures identified provide a basis for the study of the nature and sequence of changes in skeletal muscle after denervation.