The action and interaction of diphenhydramine (Benadryl) hydrochloride at the neuromuscular junction

Effects of H1-receptor blocking drugs on the frog sartorius neuromuscular junction

The effects of H1-receptor blocking agents, pyrilamine, promethazine, and diphenhydramine, on the amplitude and time course of endplate potentials (e.p.ps) were studied in the sartorius muscles of frogs. H1-receptor blockers (10(-5)-10(-4) M) reduced the amplitude of e.p.ps recorded intracellularly without affecting the resting membrane potential. The acetylcholine potential was decreased by perfusion of H1-receptor blockers. However, when the muscle fibre was stimulated directly, threshold and magnitude of action potential were not affected by H1-receptor blockers. The time constant of decay of the e.p.ps recorded extracellularly was reduced by H1-receptor blockers and the decay remained exponential with a single time constant. The quantal content was not reduced by perfusion of H1-receptor blockers at a concentration of 10(-4) M. It is assumed that the major site of action of these H1-receptor blocking agents is at the postsynaptic membrane.

Drugs that may exacerbate myasthenia gravis

Myasthenia gravis is an uncommon disease. The emergency physician should be cautious when prescribing medications to myasthenics for problems not related to myasthenia gravis. We have discussed some of those agents (Figure 3) that are recognized to cause exacerbation of MG or that may have the potential to exacerbate MG. We recommend that management of any medical or surgical problem of the myasthenic be done in consultation with a managing neurologist, and that either early follow-up or admission is necessary when these agents are used in the patient with myasthenia gravis.