l -Erythro-methoxamine is more potent than phenylephrine in effecting contraction of internal anal sphincter in vitro

BACKGROUND: Topical phenylephrine has been shown to increase resting anal canal pressure in normal and incontinent individuals. However, high concentrations of gel (10-40 per cent) are required that may cause local side-effects. The aim of this study was to determine whether methoxamine, another alpha-1-adrenoceptor agonist, might be a more potent alternative to phenylephrine. METHODS: Porcine internal anal sphincter (IAS) tissue was cut into strips, suspended in a superfusion organ bath and allowed to equilibrate. Strips were subjected to each drug under test for 20 s, sufficient to obtain stable tone. Phenylephrine, methoxamine (1 : 1 : 1 : 1 ratio of its four isomers) and each of the individual isomers of methoxamine were evaluated in turn. RESULTS: In vitro, methoxamine racemate and phenylephrine were similarly potent in causing contraction of IAS strips (mean(s.e.m.) dose giving half maximal effect (EC(50)) at 74.7(16.5) versus 58.3(13.4) micro M respectively; P = 0.443). However, one of the methoxamine isomers, L-erythro-methoxamine (EC(50) 17.6(3.7) micro M), was significantly more potent than the other three isomers, methoxamine racemate and phenylephrine (P = 0.002). CONCLUSION: L-Erythro-methoxamine is four times more potent than phenylephrine and is a possible treatment for incontinence. Trials are under way to examine the efficacy of L-erythro-methoxamine in vivo.