Abstract
An internal sample metal cathode proportional counter for measurements of low tritium activity in biologic samples of microgram size has been designed. Using a guard detector operating in anticoincidence together with a thick steel castle, a background of 0.5 cpm and an efficiency of 54.6% for tritium were realized. Radioactivities down to 2 pC (4.4 dpm) can be measured with an accuracy of ±16%. Tritium activity was measured in single adrenal cortical sections of 7 µm in thickness and 0.4 mm in diameter having a mean dry weight of 1.2 µg. The cortical layers were identified histologically from alternate sections. Two days after the injection of tritiated cholesterol the highest radioactivity was encountered in the outer fascicular zone (39 dpm/µg). The distribution of the radioactivity within the cortex was equal in all animals, although the mean level varied individually. The glomerulosa and reticularis contained usually about one-third of the activity in the outer fasciculata. Ether anesthesia and unilateral adrenalectomy caused significant decrease in the incorporation of the cholesterol, which was not further reduced by an injection of adrenocorticotropin. Ninety-five per cent of the radioactivity in the adrenals was bound to cholesterol or its esters 2 days after the injection.