Bioassayable and immunoassayable prolactin responses to thyrotropin-releasing hormone: use of the Nb2 lymphoma cell bioassay

Abstract

The peak prolactin response to thyrotropin-releasing hormone (TRH) varies among patients. "Exaggerated" responses have been described and linked to ovulatory dysfunction. Herein we describe our initial observations on bioassayable prolactin (BA-PRL) versus immunoassayable prolactin (RIA-PRL) in women with normal baseline RIA-PRL concentrations but with varying peak RIA-PRL responses to TRH. Twenty-three women of reproductive age with baseline RIA-PRL of =25 ng/mL were administered 500 microg of TRH, and baseline and peak RIA-PRL concentrations were determined. Aliquots of the baseline sample and the sample representing the peak RIA-PRL were used for measuring BA-PRL by means of the Nb2 lymphoma cell bioassay. For each sample, BA/RIA-PRL ratios were determined. Positive correlations were found between peak RIA-PRL and baseline BA/RIA-PRL ratios (P<0.05) and also between baseline and peak BA/RIA-PRL ratios (P<0.001). Negative correlations were found between baseline RIA-PRL and both baseline and peak BA/RIA-PRL ratios (P<0.001 and P<0.05, respectively). We conclude that (1) the lactotroph response to TRH in women with normal RIA-PRL may, in part, be governed by the amount of biologically active prolactin at baseline and (2) the relative proportion of BA-PRL to RIA-PRL produced at baseline is maintained at peak response. Finally, in light of the greater availability of bioactive prolactin in women with exaggerated TRH responses, our findings support the use of bromocriptine in those patients with such responses and ovulatory dysfunction.