Use of ion channel blockers in studying the regulation of skeletal muscle contractions

Summary Effects of K⁺- and Cl^–-channel blockers on the muscle contraction of mouse diaphragm in response to direct electrical muscle stimulation were studied. K⁺-channel blockers (0.1–1 mmol/1 4-aminopyridine, 0.4–1.2 mmol/l uranyl nitrate and 2-30 mmol/l tetraethylammonium chloride) and a Cl^–-channel blocker (0.01–0.03 mmol/1 9-anthracene carboxylic acid) increased the contractile amplitudes in a limited extent not to exceed over 50% of control. However, the sequential applications of two different channel blockers at a rather low concentration markedly increased the contractile responses mostly over 300% of control except the combination of 4-aminopyridine and uranyl nitrate. It appears that two K⁺-channel blockers synergistically exerted their effects rather than additionally in the regulation of muscle contractions. Investigation on the possible mechanism of the synergistic action of K⁺-channel blockers suggested that prolongation of action potential durations was in a linear correlation with the increased contractions. On the other hand, the contractile potentiation induced by combination of K⁺- and Cl^–-channel blockers was attributed to the production of repetitive action potential firings (150±12 Hz) upon a single electrical stimulation. Similar to Cl^–-channel blocker, low Cl^– as well as low Ca²⁺ enchanced K⁺-channel blockers in producing contractile potentiation accompanied with stimulus-bound repetitive discharges. Tetrodotoxin at a concentration of 0.03 mol/l which did not affect the twitches evoked by electrical stimulations completely inhibited the contractile potentiation induced by the combined application of K⁺- and Cl^–-channel blockers. It was believed that these studies on the contractions of mouse diaphragm carried out in the physiological salt solution provided a better approach in exploring the possible functions of K⁺- and Cl^–-channels in the regulation of skeletal muscle contractions. Moreover, because of the different K⁺-channels inhibited by these blockers (4-aminopyridine and uranyl nitrate majorly on delayed rectifier K⁺-channel and tetraethylammonium ion on Ca²⁺-activated and ATP-sensitive K⁺-channel), it was concluded that the different types of K⁺-channels as well as Cl^–-channels exert their effects in a synergistic manner on the regulation of the skeletal muscle contractions.