The interaction between hexamethonium and tubocurarine on the rat neuromuscular junction

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Abstract
The ability of hexamethonium (C6) to reverse the neuromuscular blocking action of tubocurarine (Tc) has been reinvestigated at the voltage clamped endplate of the omohyoid muscle of rat. The possibility that a weak anticholinesterase action of C6 could contribute to the paradoxical potentiation of the peak amplitude of the endplate response has been examined. C6 (50–200 μm) caused an increase in the amplitude of nerve‐evoked endplate currents (e.p.cs) recorded in the presence of 0.6 μm Tc. The effect decreased with hyperpolarization of the muscle fibre. Irreversible inhibition of acetylcholinesterase resulted in a loss of the anti‐curare effect of C6. C6 did not cause an increase in e.p.c. amplitude when acetylcholine (ACh) receptors were blocked irreversibly by α‐bungaratoxin. When transmission was blocked by increased Mg2+ concentration, C6 (50–400 μm) reduced the amplitude of e.p.cs without appreciably affecting their time course. C6 caused a decrease in the amplitude of miniature endplate currents (m.e.p.cs) the effect being slightly increased when the fibre was hyperpolarized. An e‐fold increase in the effectiveness of C6 occurred with approximately 58 mV hyperpolarization. High concentrations (> 400 μm) affected the time course of m.e.p.cs in a manner suggestive of open channel block, but this was not evident at 200 μm, the concentration that was most effective in reversing Tc block. When tested against responses to short ionophoretic pulses of agonists, C6 was less effective against ACh ((EC50 ca. 300 μm) than against carbachol (CCh) (EC50 100 μm). When cholinesterase was irreversibly inhibited, C6 blocked responses to both agonists equally (EC50 ca. 100 μm). The effectiveness of C6 in blocking the action of CCh was reduced 10 fold in the presence of 0.6 μm Tc, implying that the two antagonists compete for the same binding site. C6 (50–200 μm) in the presence of Tc (0.6 μm) increased the response to ionophoretically applied ACh but not that to CCh. C6 was equipotent in blocking m.e.p.cs and responses to ionophoretically applied ACh whereas Tc was more potent against the exogenously applied agonist. C6 was a weak inhibitor of acetylcholinesterase activity in rat muscle homogenates (EC50 1.5 mm). The results are discussed in terms of the kinetic hypothesis advanced by Ginsborg & Stephenson (1974) to account for the Tc reversal phenomenon. It is concluded that this theory can explain most of the effect on e.p.cs, but that the weak anticholinesterase action of C6 is also a factor, particularly in the reversal of Tc block of ionophoretic responses.