Differences between temporal patterns of plasma cortisol and corticosteroid-binding globulin binding capacity throughout the twenty-four hour day and the menstrual cycle

Abstract

Plasma cortisol and corticosteroid-binding globulin (transcortin, CBG) binding capacity were determined by a gel-exchange method employing Sephadex G-25 on: i) plasma samples drawn at 4-h intervals during a 28-h span in ten hospitalized healthy subjects (six men, four women); ii) plasma samples drawn daily for one month at 08:00–09:00 in four normally ovulating women. Clear fluctuations of CBG binding capacity along the 24-h cycle were noted. Lowest values were in the nocturnal hours and mean minimum was at 04:00; mean values recorded at midnight and at 04:00 were significantly reduced in comparison with mean values recorded at any other time (paired t-test). The least squares fit of a 24-h cosine curve allowed the demonstration of the highly significant rhythmicity in plasma transcortin binding capacity. Acrophase was calculated to occur at about 13:30 and to lag behind the acrophase of the plasma cortisol rhythm of approximately three hours without overlap of 95% confidence arcs. A cyclic pattern throughout the menstrual cycle was found in three out of four women, who presented significantly higher values of cortisol-binding capacity during the luteal phase than during the early follicular phase (p<0.01). No significant variation was found in circulating cortisol throughout cycles examined. It is suggested that circadian and circatrigintidian variations in CBG binding capacity may periodically interfere with the effects of glucocorticoids at the cellular level, including the feedback control of the magnitude of ACTH release. Rhythms of ACTH-cortisol secretion and of cortisol-CBG interaction may be influenced by different control mechanisms. Their interplay in ensuring sensitive modulation of the hypothalamo-hypophyseal-adrenal function has to be considered.