Implications of uptake and storage of norepinephrine by sympathetic nerve endings

Abstract

The rate of synthesis of a neurohormone is usually considered to be equivalent to the turnover rate as measured from the disappearance of the radioactive substance formed from an isotopically labeled precursor. The disappearance of radioactivity is determined at a time when the decline is exponential, the tacit assumption being made that the endogenous substance is present in only a single pool. However, if the neurohormone is present in two pools, one diffusible and the other in the form of a reversible complex with tissue components, the two forms will have a precursor-product relationship. In this case, synthesis will be more rapid than is apparent from the turnover rate because some of the endogenous amine will be synthesized and metabolized (or released) without forming a complex in the second pool (1). However, the interrelationship between the two pools may be studied by measurement of the decline in labeled neurohormone after the intravenous administration of the hormone itself rather than a precursor (2). Previous studies from this laboratory (2) on the dynamic equilibrium between the norepinephrine (NE) pools at sympathetic nerve endings have shown that tritiated norepinephrine (H 3NE) is rapidly taken up from the circulation into various tissues of a number of animal species. The labeled amine is specifically localized in peripheral sympathetic nerve endings (3). For the first few hours the concentration of H 3NE declines rapidly and then much more slowly in an exponential fashion. Since the amount of H 3NE declines as it is displaced by newly formed amine, it was concluded that the labeled amine is first taken up into a readily miscible pool which is in direct contact with processes that synthesize and utilize endogenous amine, and then slowly enters a second or reserve pool in dynamic equilibrium with the first (2). By mathematical treatment of the data, the rate of NE synthesis and the relative size of the two pools may be estimated (4). In these studies, d, 1-H 3NE was used on the assumption that the active transport system at nerve endings was stereospecific for the 1-isomer. In a recent paper, Kopin and Bridgers (5) questioned, and correctly so, the validity of this assumption, pointing out that if both isomers were taken up into nerve endings, the initial rapid decline in H 3NE would result mainly from the selective disappearance of the d-form; in this case, our calculations of the, rate of NE synthesis and size of the pools, which depend on the initial rate, would be incorrect. Based on results of a double labeling technique, they concluded that the isomers are taken up by heart and spleen in equal amounts, but that the d-isomer disappeared more rapidly than the 1-form. Since this result would logically lead to the conclusion that the specialized mechanism which takes up circulating NE is not directly related to the mechanism that maintains NE at a constant level in the nerve endings, we have determined the specificity of the NE uptake in rat heart by the classical procedure of isotopic dilution. In contrast to the reported results of Kopin and Bridgers, our results show that NE is taken up by a process that is essentially stereospecific for the 1-isomer.