Resistance to D-Tubocurarine of the Rat Diaphragm as Compared to a Limb Muscle Influence of Quantal Transmitter Release and Nicotinic Acetylcholine Receptors

Background: The diaphragm is resistant to competitive neuromuscular blocking agents, as compared to peripheral muscles. The basis of this difference may be a higher concentration of acetylcholine released or higher number of postsynaptic nicotinic acetylcholine receptors in diaphragmatic neuromuscular junctions. Methods: Nerve-evoked twitch-tension was measured in rat hemidiapragm as was Extensor digitorum longus (EDL) nerve-muscle preparation to determine the effective D-tubocurarine concentration that decreased twitch responses by 50%. The mean quantal content of endplate potentials was determined in single junctions in a low-Ca2+, high-Mg2+ Krebs-Ringer medium. Strips of hemidiaphragm and EDL muscle, containing the endplate regions, were used to determine the number of nAChR nicotinic acetylcholine receptor binding sites with the aid of radiolabeled [I-125]alpha-bungarotoxin. Results: The effective D-tubocurarine concentration that decreased twitch responses by 50% (median [interquartile range]) was seven-fold higher in the hemidiaphragm than in the EDL (1.82 mu M [1.43-2.20] vs. 0.26 mu M [0.23-0.29], P < 0.01). The median of the mean quantal content was higher in the hemidiaphragm than in the EDL (0.57 [0.44-0.84] vs. (0.14 [0.11-0.19], P < 0.01). The number of specific [125I]alpha-bungarotoxin binding sites to junctional nicotinic acetylcholine receptors was higher in the diaphragm than in the EDL (1-15 fmol/mg [0.48-1.70] vs. 0.55 fmol/mg [0.23-0.70], P < 0.05). Conclusion: The current study indicates that the resistance of the diaphragm to neuromuscular blocking agents can be explained by both a higher mean quantal content of endplate potentials and a higher number of nicotinic acetylcholine receptor binding sites than in the peripheral EDL muscle.