CHARACTERISTICS OF NOREPINEPHRINE RETENTION BY A SUBCELLULAR FRACTION OF RABBIT HEART

¹ Department of Pharmacology, Yale University School of Medicine, New Haven, Connecticut

Retention of norepinephrine-H³ in vitro by microsomal fractions of rabbit heart was related linearly to its concentration in the incubation medium (0.3 M sucrose), over the range, 10 ng to 400 ng per ml. At each of the concentrations used, the simultaneous presence in the medium of 40 times as much nonisotopic norepinephrine failed to decrease the retention of labeled amine. When concentrations of norepinephrine-H³ up to 50 ng/ml were used, retention was not altered significantly by increasing the duration of incubation from 10 minutes to 60 minutes; it was reduced, however, with concentrations of 100 ng of norepinephrine-H³ per ml, or higher. At least part of the process responsible for retention was temperaturedependent, since the concentration of the amine in particles was significantly lower after incubation at 4° than at 23°. Pretreatment of animals with reserpine failed to influence the retention of norepinephrine in vitro, as did inclusion of T.E.C. (p-tolyl ether of choline), 60 µg/ml, in the incubation medium. On the other hand, tyramine 20 µg/ml, in the medium decreased retention and also liberated previously bound norepinephrine-H³. It is concluded that the ability of subcellular particles of rabbit heart (which sediment in the microsomal fraction) to limit the concentration of circulating norepinephrine that they can retain (reported earlier; Gillis, 1964) is a phenomenon requiring the integrity of the myocardial cell.

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