The retention of exogenous norepinephrine by rabbit tissues

Abstract

The retention of 3H-norepinephrine by rabbit heart and by spleen was linearly related to the dose administered, over the range 0.75 to 10.0 μg per kg. The percentage distribution of the labeled amine among the nuclear, mitochondrial, and microsomal fractions of sucrose homogenates of heart, prepared by differential centrifugation, remained constant, regardless of the dose of amine administered. In the microsomal fraction, which contained a maximum of 6 per cent of the amine in the total homogenate, the concentration of 3H-norepinephrine, in terms of protein (i.e. the relative specific activity), was up to fourteen times that found in the total homogenate. The ability of microsomal elements to concentrate exogenous amine reached a saturation point when approximately 3.0 μg per kg of 3H-norepinephrine was administered; the simultaneous administration of nonradioactive and tritium-labeled norepinephrine greatly depressed the relative specific activity of the amine in this fraction. Microsomal fractions, prepared by centrifugation of heart homogenates over a discrete sucrose density gradient, retained up to 28 per cent of the total 3H-norepinephrine in the homogenates; the relative specific activity of the amine in this fraction, however, was always lower than that of the corresponding fraction isolated by differential centrifugation. It is suggested that concentration of exogenous norepinephrine by microsomal elements can account for almost all of the labeled amine retained by heart. Further, it appears that the technique of differential centrifugation yields a sample of these elements that is less contaminated by other structures with the same sedimentation characteristics (but which do not contribute to the retention of exogenous norepinephrine) than are those obtained by sucrose density gradient centrifugation. If microsomal structures are responsible for the retention of 3H-norepinephrine, and this property is saturated at 3.0 μg/kg, then the linear relationship between dose administered and total heart retention may be explained by postulating the existence of discrete locations of these structures, which vary in tlie ease with which exogenous amine can reach them.