Circadian and circaseptan rhythmicities in corticosteroid-binding globulin (CBG) binding activity of human milk

It has been demonstrated that human milk contains a corticosteroid-binding globulin (CBG, transcortin) which is very similar to that isolated from plasma. It is agreed, moreover, that the cortisol environment of the breast is important for the synthesis of major milk proteins. It was of interest, therefore, to investigate whether milk transcortin exhibits circadian variations in the binding activity as does the plasma counterpart. Twelve lactating women, aged 23-35 years, collected specimens of milk just before feeding at different times of the 24-h cycle for 3 consecutive days in the first week of lactation. Six women extended milk collection for at least a 28-days span in the first 2 months of lactation. To obtain whey, milk was centrifuged in cellulose nitrate tubes at 105,000 g for 30 min at 4 degrees C. Specific cortisol binding activity of CBG was measured by an adsorption technique and data were expressed as a function of protein content. Cortisol levels in milk were measured by radioimmunoassay. Rhythmometric analyses (single- and mean-cosinor procedures) yielded detection of a significant circadian rhythm in 'apparent specific binding activity' for cortisol (ASBA-F) whereas no significant oscillation was found as far as milk cortisol concentrations were concerned. Mean acrophase of the CBG-binding activity in milk was located in the evening, lagging behind the corresponding activity in serum by approximately 8 h. A circaseptan rhythmicity in milk binding activity was also present in our longitudinal series at a high level of significance. These results provide evidence that CBG binding activity in human milk has a clear circadian rhythm in the face of ecchronic milk cortisol concentrations. Phase-shift with regard to the rhythm of the plasma counterpart may be accounted for by processes of milk accumulation before nursing The significance of circadian and circaseptan variations in specific cortisol (and other steroids) binding in milk awaits further investigation.