Rhythmic and Nonrhythmic Modes of Anterior Pituitary Hormone Release in Man

Abstract

Variations in plasma hormone concentrations have been studied extensively in various conditions of health and disease in the human and experimental animals. A wide range of environmental, metabolic, nutrient, and hormonal stimuli are known to modulate normal patterns of anterior pituitary hormone release and/or alter the metabolic clearance of the endocrine effector substance. The ability to evaluate quantitatively not only variations in plasma hormone concentrations but also regulated features of hormone secretory events has been acquired recently through the use of so-called deconvolution techniques [20, 26, 32, 36, 41, 48, 54, 56, 60, 62, 72, 71, 74]. Deconvolution represents a procedure in which available plasma hormone concentration measurements are interpreted mathematically as the specific consequence of definable secretory events and hormone-specific metabolic clearance. The examination of calculated in vivo secretory events is of particular importance pathophysiologically, when a clinician and investigator wish to assess regulation of the actual secretory behavior of the endocrine gland. In contrast, evaluation of plasma hormone concentrations in the conventional manner discloses information about the hormonal milieu to which the target gland is exposed. Although the majority of available clinical investigative work has focused on variations in plasma hormone concentrations (and hence, the signal made available to the target tissue), recent advances in analytical tools have allowed detailed studies of the secretory rhythms inherent in the regulated output of the endocrine gland. Accordingly, here we will emphasize not rhythms in pituitary hormone concentrations in plasma, but rhythms in the secretory behavior of the anterior pituitary gland under a range of normal and pathological conditions in man.