Hypertonic Saline‐Induced Cramp‐Like Muscle Contractions Occur Through Acetylcholine Receptors at the Neuromuscular Junction in Rats

Introduction/Aims Muscle cramps are characterized by sudden, involuntary, painful contractions of skeletal muscle. However, adequate treatment and prevention methods and experimental models are poorly established. In this study (Experiment I), we developed a putative model that mimics clinical muscle cramps using intramuscular hypertonic saline (HS) infusion. In Experiment II, we tested the hypothesis that HS‐induced cramp‐like contractions occur via nicotinic acetylcholine receptors (nAChRs) at the neuromuscular junction (NMJ). Methods Under isoflurane anesthesia, electromyographic (EMG) activity was recorded using a needle‐type electrode inserted into the rat tibialis anterior (TA) muscle. Through a 30‐gauge needle, 5% HS was continuously administered to the TA approximately 1.5 mm distal from the recording electrode. Results In Experiment I, the number, amplitude, and root mean square values of EMG events were significantly higher in the group treated with HS than in the group treated with isotonic saline (IS). The HS‐induced EMG activity increased as the volume of HS increased (10–100 μL for 1 min). In Experiment II, HS‐induced EMG activity was nearly abolished following the intravenous administration of pancuronium bromide, a blocker of nAChRs, at the NMJ, but not μ‐conotoxin GIIIB, a selective blocker of skeletal muscle Na v 1.4 channels. HS‐treated muscle showed greater histological alterations with enlarged interstitial spaces in the cross‐sectional area. Discussion HS infusion elicited robust, reproducible EMG activity, which occurred largely via nAChRs at the NMJ but not via Na v 1.4 channels on the muscle cell membrane. Thus, this model is useful for studying muscle cramps, and the NMJ could be a therapeutic target.