Negative inotropic effect of pinacidil on the rat left atria

Cromakalim at 50 microM and pinacidil at 0.1-10 microM had no effect, but pinacidil at 0.1 mM had a negative inotropic effect on the rat electrically-driven left atria without altering the positive inotropic responses to isoprenaline or phenylephrine alone. Glibenclamide had no effect but 4-aminopyridine, procaine (30 microM) and tetraethylammonium (3 mM) augmented the cardiac stimulation response. The ability of pinacidil to attenuate the cardiac stimulation response was not altered by glibenclamide, 4-aminopyridine or procaine but was prevented by pretreatment with tetraethylammonium. Thus, on the rat left atria, pinacidil has a negative inotropic effect which is unrelated to the opening of ATP-sensitive potassium channels, but may be due to opening the inward rectifying potassium channels.