THE EFFECTS OF PHYSOSTIGMINE ON THE MECHANICAL AND ELECTRICAL RESPONSES OF THE CAT NICTATING MEMBRANE

Neuromuscular blocking and ganglion blocking activities of some acetylcholine antagonists in the cat

The potencies of tubocurarine, gallamine, pancuronium, benzoquinonium, hexamethonium and mecamylamine in blocking neuromuscular transmission in the soleus muscle, and in blocking contractions of the nictitating membrane evoked by preganglionic sympathetic stimulation have been compared in cats under chloralose anaesthesia. On a molar basis, pancuronium was about 8 times and benzoquinonium about 2·5 times more potent than tubocurarine in blocking the soleus muscle; gallamine was less than half as potent, mecamylamine about 128 times and hexamethonium about 380 times less potent. In blocking the superior cervical ganglion, mecamylamine was about 17 times more, tubocurarine was about 5 times more and pancuronium about twice as potent as hexamethonium. Benzoquinonium was about half as potent as hexamethonium, and gallamine about 5 times less potent. The results emphasize that the shorter distance between charged centres, as in hexamethonium, reduces affinity for muscle receptors but does not necessarily enhance affinity for ganglion receptors, and from the point of view of deductions concerning the configuration of the ganglionic receptor, the ganglion blocking potencies of some neuromuscular blocking drugs should be taken into account.

The effect of some anticholinesterases on the response of the taenia to sympathetic nerve stimulation

1. A preparation of the taenia from the caecum of the guinea-pig has been used. Acetylcholine caused contraction and noradrenaline caused relaxation. When the perivascular nerves running to the taenia were stimulated, at low frequencies there was sometimes contraction and sometimes relaxation. At high frequencies there was always relaxation. In the presence of hyoscine there was relaxation at all frequencies, the relaxation increasing as the frequency rose up to 20/sec. The relaxation was not affected by the presence of hexamethonium, but was blocked by bretylium.2. When stimulation was applied in the presence of hyoscine, the addition of physostigmine, Mipafox or Dyflos increased the relaxation to stimulation of low frequency, the increase becoming smaller as the frequency rose. At high frequencies Mipafox decreased the relaxation.