TISSUE TO PLASMA RATIOS OF RADIOACTIVITY IN THE RAT HYPOTHALAMO-HYPOPHYSEAL SYSTEM AFTER INTRAVENOUS INJECTION OF 3H-LYSINE8-VASOPRESSIN AND 3H-MANNITOL

Abstract

ABSTRACT After intravenous injection of synthetic 3H-Lys8-vasopressin to intact rats and rats in which the liver and kidneys had been excluded from the circulation, the tissue water to plasma water ratios of radioactivity were determined, usually 12 min after injection, for the following tissues: The adenohypophysis, the neurohypophysis, the median eminence, parts of the hypothalamus containing the supraoptic nuclei, and pieces of the cerebellar cortex. The results were compared with those obtained in similar experiments using 3H-mannitol. Radioactivity could not be shown to accumulate in any part of the hypothalamo-hypophyseal system. Radioactivity disappeared from the plasma in the rats with excluded liver and kidneys at a rate not strikingly different from that in intact rats. In separate experiments, identical in procedure, radiochromatography was performed on plasma samples. The ratio of 3H-tyrosine to 3H-Lys8-vasopressin was about 1 already two min after the injection, both in intact and operated rats. These results seem to indicate a considerable extra-abdominal breakdown of 3H-Lys8-vasopressin, and demonstrate that without special precautions, measurements of tissue and plasma radioactivities cannot be taken as indicators of labelled hormone, even when the organs which have been considered to be the major site of hormone breakdown have been excluded from the circulation.