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) w ere recorded from electrically paced rat atria before, during and afte r treatment with phenylephrine in vitro. Phenylephrine caused a prolon gation of APD and an increase in SDT, the magnitude of which depended upon the frequency of pacing of the atria. Effects very similar to tho se elicited by phenylephrine were produced by exposing the atria to 4- aminopyridine, 3,4-diaminopyridine, CsCl, amantadine or sparteine. Atr ia that had been pre-treated with any of the latter five compounds sho wed a diminished responsiveness when subsequently treated with maximal ly effective concentrations of phenylephrine, and vice versa. These fi ndings are consistent with blockade of potassium ion channels being re sponsible for the observed responses to all of the above agents.