Dilazep: an inhibitor of adenosine uptake with intrinsic calcium antagonistic properties

Abstract

Abstract Concentrations of dilazep which were ineffective in altering the muscular tone of the guinea-pig taenia caeci (0·03, 0·3 μM) or the phasic mechanical activity of the rabbit proximal ileum (0·03 μM) markedly potentiated the inhibitory action of adenosine on both these parameters. Dilazep, 0·3 μM or greater, dose-dependently inhibited the mechanical activity of the proximal ileum. This inhibitory action was probably mediated by more than one mechanism, as shown by the fact that theophylline (50,100 μM) antagonized the effect at lower dilazep concentrations (up to 3 μM) leaving essentially unchanged the response to higher concentrations (6, 10 μM). Similarly, the responses to low doses of dilazep were reduced after desensitization of the organ to adenosine, whilst the responses to higher doses were unaffected by this procedure. In a Ca2+-free, high-K+ medium, dilazep (1-10 μM) caused a parallel shift to the right of the Ca2+-induced contractions of the guinea-pig taenia caeci. Adenosine showed only slight Ca2+-antagonistic properties within the mM range of concentrations. These findings suggest that, at the higher concentrations tested, dilazep exhibits Ca2+-antagonistic properties unrelated to its adenosine-mediated mode of action.