Summary
To shed some light on the homeostatic regulation of plasma fibrinogen, metabolic studies
were made in healthy females, and in normal, thyroidectomized, and thyroxine-treated
rabbits. In females, compared with normal males, plasma fibrinogen concentration,
plasma and interstitial fibrinogen decreased consequent to an increased fractional
catabolic rate and a normal fibrinogen synthesis rate. The interstitial/plasma fibrinogen
ratio remained unchanged. In normal rabbits, with increasing body weight fractional
catabolic rate and catabolic rate decreased, while fibrinogen concentration and plasma
fibrinogen remained constant owing to a simultaneous decrease in fibrinogen synthesis.
In addition, fractional transcapillary transfer rate and transcapillary flux also
decreased resulting in a shrinkage of interstitial fibrinogen. Thyroidectomy and thyroxine-injection
markedly altered fibrinogen metabolism: thyroid hormone accelerated fibrinogen catabolism
but also stimulated synthesis. The net result was an increase in plasma fibrinogen
and fibrinogen concentration. The interstitial/plasma fibrinogen ratio decreased in
thyroxine-treated, and increased in thyroidectomized animals. This study defines the
variations of the fibrinogen system parameters in these physiologic and pathologic
conditions, and illustrates some patterns of alterations in fibrinogen metabolism.
