Inhibition of potassium uptake by low concentrations of norepinephrine and dibutyryl cyclic AMP.

(1) The inhibition of potassium uptake by low concentration of norepinephrine (3 X 10-8 M) and of dibutyryl cyclic AMP (DBcAMP, 10 minus5 M) was studied in cardiac Purkynĕ fibres. (2) The inhibitory action of DBcAMP on K uptake was abolished by the alpha blocker phentolamine. (3) Norepinephrine alone decreased K uptake and such inhibition was somewhat larger when DBcAMP was added. DBcAMP alone caused the usual decrease in K uptake but addition of norepinephrine abolished it. (4) The inhibition caused by norepinephrine reduced the increase in uptake caused by a high concentration (10 minus 3 M) of DBcAMP. (5) The inhibitory effect of norepinephrine was reversed in the presence of high concentration of magnesium (5.25 mM). (6) The inhibitory effect of norepinephrine was reversed by aminophylline and abolished by caffeine. (7) The inhibitory action of norepinephrine and BCcAMP was reversed or abolished, respectively, by imidazole. (8) It is concluded that the inhibition of potassium uptake by low concentration of DBcAMP is mediated by an alpha receptor mechanism and that possibly the "receptors" for this effect of norepinephrine and DBcAMP are located at different sites. Also it appears that DBcAMP may be acting at the membrane and that the action of methylxanthines and imidazole is not necessarily mediated only by a modification of phosphodiesterase activity.