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 hemidiaphragm 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-Ca(2+), high-Mg(2+) 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 [(125)I]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 microm [1.43-2.20] vs. 0.26 microm [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 [(125)I]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).


CONCLUSIONS
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.
Nguyen-Huu
Tu
T
Laboratoire de Neurobiologie Cellulaire et Moléculaire, Unité Propre de Recherche 9040, Centre National de la Recherche Scientifique, Gif-sur-Yvette, France.
Molgó
Jordi
J
Servent
Denis
D
Duvaldestin
Philippe
P
D000818:Animals; D003964:Diaphragm / Q000187:drug effects / Q000378:metabolism; D004305:Dose-Response Relationship, Drug; D004351:Drug Resistance / Q000187:drug effects / Q000502:physiology*; D005121:Extremities / Q000294:innervation / Q000502:physiology*; D005260:Female; D018482:Muscle, Skeletal / Q000187:drug effects / Q000378:metabolism*; D009469:Neuromuscular Junction / Q000187:drug effects / Q000378:metabolism; D018377:Neurotransmitter Agents / Q000378:metabolism*; D051381:Rats; D017207:Rats, Sprague-Dawley; D011978:Receptors, Nicotinic / Q000502:physiology*; D014403:Tubocurarine / Q000494:pharmacology*
D018377:Neurotransmitter Agents; D011978:Receptors, Nicotinic; D014403:Tubocurarine