Evidence for Enhanced Cellular Uptake and Binding of Thyroxine In Vivo during Acute Infection with Diplococcus pneumoniae

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Abstract

Previous work has demonstrated that acute pneumococcal infections in man and in the rhesus monkey are accompanied by accelerated metabolic disposal of L-thyroxine (T4). In order to study the influence of acute pneumococcal infection on the kinetics of hormone distribution, the early cellular uptake of T4 (CT4), reflecting the net effect of plasma and cellular binding factors, was assessed in rhesus monkeys from the differences in instantaneous distribution volumes of T4-131I and albumin-125I during the first 60 min after their simultaneous injection. Hepatic and renal uptakes of 131I were also determined. Plasma binding of T4 was assessed by measuring the per cent of free T4 (% FT4) in serum. Six monkeys were studied 12 hr (INF-12) and seven 24 hr (INF-24) after intravenous inoculation with Diplococcus pneumoniae; seven controls were inoculated with a heat-killed culture. CT4 at 60 min as per cent administered dose was 31.5 ±2.0 (mean ±SE) in INF-12 and 33.0±0.8 in INF-24, values significantly greater than control (22.4±1.3). By contrast, mean% FT4 was identical in control and INF-12 (0.028 ±0.002 and 0.028 ±0.001) and variably increased in INF-24 (0.034 ±0.003). Thus, in the infected monkeys CT4 and% FT4 were not significantly correlated. The increased CT4 in the infected monkeys could not be ascribed to an increase in vascular permeability and did not correlate with the magnitude of fever. Although the increased CT4 could not be accounted for by increased hepatic or renal uptake of hormone, hepatic and renal T4 spaces were increased, results consistent with increased binding by these tissues. Our data indicate that the cellular uptake of T4 is increased early in acute pneumococcal infection and suggest that this results from a primary enhancement of cell-associated binding factors for T4.

Authors

Frederick R. DeRubertis, Kenneth A. Woeber