Effect of pre-treating rat atria with potassium channel blocking drugs on the electrical and mechanical responses to phenylephrine.

Action potential duration (APD) and systolic developed tension (SDT) were recorded from electrically paced rat atria before, during and after treatment with phenylephrine in vitro. Phenylephrine caused a prolongation of APD and an increase in SDT, the magnitude of which depended upon the frequency of pacing of the atria. Effects very similar to those elicited by phenylephrine were produced by exposing the atria to 4-aminopyridine, 3,4-diaminopyridine, CsCl, amantadine or sparteine. Atria that had been pre-treated with any of the latter five compounds showed a diminished responsiveness when subsequently treated with maximally effective concentrations of phenylephrine, and vice versa. These findings are consistent with blockade of potassium ion channels being responsible for the observed responses to all of the above agents.