Effect of circadian rhythm on the neuronal uptake and sensitivity to the inotropic action of norepinephrine by rat atria.

The circadian rhythm for plasma corticosterone was determined. Animals were then killed at times corresponding to high and low periods of the circadian rhythm in plasma corticosterone. Myocardial sensitivity to norepinephrine was measured at these time periods as the ED50 of the catecholamine, obtained using electrically driven rat atria. The uptake of 3H-norepinephrine by spontaneously beating atria was also measured at both time periods. A circadian variation in the uptake of 3H-norepinephrine by the rat atria was observed. This variation in uptake was associated with a variation in plasma corticosterone, but was not associated with any change in myocardial sensitivity to norepinephrine.