Under physiological conditions, PRL secretion is regulated precisely by various stimulating and inhibiting factors. Hyperprolactinemia may arise as a primary consequence of a PRL-secreting pituitary adenoma. Secondary hyperprolactinemia (SH) may emerge in patients with hypothalamic disease, hypophyseal stalk compression, or suprasellar extension of a (nonlactotrope) pituitary adenoma. The latter may reflect diminished delivery of dopamine or other inhibitory factors to normal lactotropes. We hypothesized that diurnal and ultradian rhythms of PRL secretion would differ in secondary (e.g. hypothalamic) and primary (e.g. tumoral states) hyperprolactinemia (PH), assuming that the underlying pathophysiologies differ. To test this clinical postulate, we investigated the patterns of 24-h PRL release in eight patients with SH associated with functional hypothalamo-pituitary disconnection and in eight patients with PH attributable to microprolactinoma. Data in each group were compared with values in healthy gender-matched controls. PRL time series were obtained by repetitive 10-min blood sampling, followed by high-precision immunofluorometric assay. PRL concentration profiles were analyzed by the complementary tools of model-free discrete peak detection, waveform-independent deconvolution analysis, cosinor regression, and the approximate entropy metric to quantitate pulsatile, basal, 24-h rhythmic, and pattern-dependent (entropic) PRL secretion. Patients with tumoral hyperprolactinemia (PH) showed a 2-fold higher 24-h mean serum PRL concentration than patients with SH (62 ± 13 μg/L vs. 30 ± 6.9 μg/L, respectively, P = 0.029). Estimated PRL pulse frequency (events/24 h) was similar in the two patient groups (18.5 ± 0.7 vs. 17.6 ± 0.8; P = 0.395) but elevated over that in euprolactinemic controls (P < 0.0001 for both). Deconvolution analysis disclosed a mean daily PRL secretion rate of 790 ± 170 μg in PH patients vs. 380 ± 85μg in SH patients (P = 0.030). Nonpulsatile PRL secretion comprised nearly 70% of total secretion in both patient groups and 50% in controls (P < 0.0001). Cosinor analysis revealed similar acrophases in all three study cohorts. The mean skewness of the statistical distribution of the individual PRL sample secretory rates was reduced, compared with controls (P < 10 -5 for each), but equivalent in SH and PH patients (0.83 ± 0.12 vs. 0.78 ± 0.08, respectively), denoting a loss of the normal spectrum of low- and higher-amplitude secretion rates. Approximate entropy, a regularity statistic, was markedly elevated in both patient groups over controls (P < 10 -6 for each) and was slightly higher in PH patients in SH patients (1.639 ± 0.029 vs. 1.482 ± 0.067, P = 0.048). In summary, patterns of PRL secretion in PH and SH states exhibit an equivalently increased frequency of PRL pulses, a comparably marked rise in nonpulsatile (basal) PRL secretion. Despite overlap, the regularity of PRL release patterns is disrupted even more profoundly in PH (tumoral), compared with SH. Assuming that the orderliness of serial PRL output monitors normal integration within a feedback-controlled neurohormone axis, then the more disorderly patterns of tumoral PRL secretion point to greater regulatory disruption in PH. The latter may reflect abnormal secretory behavior associated with lactotrope neoplastic transformation and/or isolation of the tumor cell mass from normal hypothalamic controls.
ASJC Scopus subject areas
- Endocrinology, Diabetes and Metabolism
- Clinical Biochemistry
- Biochemistry, medical