Comparison between bioactivity and immunoreactivity of serum prolactin in uraemia

The lactogenic potency of sera from 24 male uraemic patients on regular haemodialysis and 14 control subjects was measured by Nb2 node rat lymphoma cell bioassay, and was compared to serum prolactin levels measured by radioimmunoassay. Sera with immunoreactive growth hormone levels exceeding 5 ng/ml were excluded from comparisons. The uraemic patients had a higher serum immunoreactive prolactin (48.2 +/- 10.5 ng/ml) than controls (13.3 +/- 1.3 ng/ml (P less than 0.01). Similarly the lactogenic potency of uraemic serum was higher than that of the control sera (25.5 +/- 3.9 vs 12.6 +/- 1.4 ng/ml, P less than 0.02). The ratio of immunoreactive serum prolactin to the lactogenic potency of the serum was significantly higher in the uraemic group (1.80 +/- 0.14 vs 1.10 +/- 0.10, P less than 0.01) suggesting decreased bioactivity of prolactin in uraemic serum. To examine whether low molecular weight inhibitory molecules were responsible for this discrepancy, the lactogenic potency of 8 uraemic sera was studied before and after 4 h of dialysis against 1 litre of haemodialysis medium. Two of the eleven sera studied showed a significant increment in lactogenic potency after dialysis (47% and 57%). We conclude that there is a disparity between the bioactive and the immunoreactive serum prolactin concentrations in patients with chronic renal failure and that a dialysable factor may be partly responsible for this discrepancy in some cases.