Disappearance of Iodine-131-Iabelled Human Growth Hormone from the Plasma of Diabetic, non-Diabetic and Acromegalic Subjects

Abstract

THE disappearance of iodine-131-labelled human growth hormone (HGH) from plasma has been investigated as part of a study designed to measure the secretion rate of the hormone. This was undertaken because plasma levels alone may not be indicative of the secretion rate of the hormone. Twenty-six adult patients consented to the procedure. Seventeen were diabetics (six required insulin and seven had never been given insulin). Eight were acromegalic (four with diabetes), and five were normal. Thyroidal uptake of radio-iodine was blocked with Lugol's iodine. HGH prepared by the Raben¹ technique and purified² was iodinated by the method of Hunter and Greenwood³. Material of specific activity 30–150 µc./µg was used. It was diluted at once and stored in a diluent containing 0.3 per cent human serum albumin. Each batch was used within 14 days of preparation. The subjects were studied resting, fasting and after 12 h without food or drugs. Care was taken to ensure that where possible each member of the various groups of patients was studied with a different batch of ¹³¹I-HGH, and this meant that each batch was used in several types of patients. A weighed dose of 2–5 µc. was given intravenously at about 9 a.m. Blood samples were collected at intervals during the next 2–4 h, cooled, and the plasma radioactivity counted in a well-type scintillation counter to give > 95 per cent accuracy. The plasma proteins were then precipitated with cold 20 per cent trichloroacetic acid (TCA) and the supernatant counted. The protein-bound ¹³¹I could then be calculated and was expressed as a percentage of the dose injected and plotted semi-logarithmically against time. In each of the five normals a disappearance curve similar to Fig. 1A was found. Equilibrium was reached in 10 min and a straight-line exponential curve thereafter gave half-times of from 23 to 45 min (see Fig. 2). All seven non-insulin-requiring diabetics gave curves similar to Fig. 1B, reaching equilibrium by 20–60 min and giving final straight-line curves with half-times of from 60 to 185 min (see Fig. 2).