The use of rat diaphragm muscle combined with tracer studies together with slicing methods gives the possibility of separating junctional (end-plate) from non-junctional exchange (Creese et al. 1977), and the response of the sodium efflux to the effects of exposure to depolarising drugs can be followed.
Rats were killed humanely by stunning and exsanguination, before rapid removal of the diaphragm muscle. Following equilibration in a solution with labelled sodium (24Na), and wash in inactive solution, the steady-state efflux in untreated diaphragm muscles gave a rapid fraction and a single exponential curve with a half-time of 5.8 min (30 min wash at 38 °C, rate constant: 0.119 min-1, 95 % limits 0.109, 0.126 min-1, regression from 46 muscles).
Brief exposure to depolarising drugs plus labelled sodium produces loading in the junctional region of rat diaphragm, as shown by the additional radioactivity in slices of muscle in the region of the endplates, above the level assigned to the ends of the muscle. Decamethonium (100 µmol l-1, 30 s) plus labelled sodium, gave junctional sodium (pmol mg-1), and the subsequent wash in inactive solution showed curvature in semilog plot and a power curve which could be linearised by a change to reciprocals, with efflux proportional to the square of the junctional sodium: the slope of the reciprocal plot was 0.306 X 10-3 min-1 (limits 0.301, 0.312, regression from 54 muscles), and the slope of the log-log plot was close to 2.0. In frog muscle, efflux of labelled sodium can give power curves which may be interpreted as indication of multiple carrier sites on the membrane transport complex, or in terms of a two-compartment model which would involve both the intra-fibre tubular system and the sarcoplasmic sodium (Keynes & Steinhardt, 1968).
In prolonged treatment with depolarising drugs, the resting potential is spontaneously restored (Creese & Mitchell, 1981). When exposure to decamethonium (100 µmol l-1) was increased to 30 min followed by 30 s with labelled sodium plus decamethonium, the efflux from the junctional region reverted to the exponential form, with a half-time 3.0 min (from medians of the semilog plot, with rate constant -0.220 min-1, limits: 0.21, 0.24, regression from 68 muscles).
With carbachol 100 µmol l-1 plus labelled sodium (30 s) the efflux of junctional sodium appeared to be completely prevented by ouabain (100 µmol l-1, 32 muscles), with partial inhibition of non-junctional sodium.