Multi-responsiveness of single anterior pituitary cells to hypothalamic-releasing hormones: A cellular basis for paradoxical secretion

Abstract

The classic view for hypothalamic regulation of anterior pituitary (AP) hormone secretion holds that release of each AP hormone is controlled specifically by a corresponding hypothalamic-releasing hormone (HRH). In this scenario, binding of a given HRH (thyrotropin-, growth hormone-, corticotropin-, and luteinizing hormone-releasing hormones) to specific receptors in its target cell increases the concentration of cytosolic Ca 2؉ ([Ca 2؉ ] i ), thereby selectively stimulating the release of the appropriate hormone. However, ''paradoxical'' responses of AP cells to the four wellestablished HRHs have been observed repeatedly with both in vivo and in vitro systems, raising the possibility of functional overlap between the different AP cell types. To explore this possibility, we evaluated the effects of HRHs on [Ca 2؉ ] i in single AP cells identified immunocytochemically by the hormone they stored. We found that each of the five major AP cell types contained discrete subpopulations that were able to respond to several HRHs. The relative abundance of these multi-responsive cells was 59% for lactotropes, 33% for thyrotropes, and in the range of 47-55% for gonadotropes, corticotropes, and somatotropes. Analysis of prolactin release from single living cells revealed that each of the four HRHs tested were able to induce hormone release from a discrete lactotrope subpopulation, the size of which corresponded closely to that in which [Ca 2؉ ] i changes were induced by the same secretagogues. When viewed as a whole, our diverse functional measurements of multi-responsiveness suggest that hypothalamic control of pituitary function is more complicated than previously envisioned. Moreover, they provide a cellular basis for the so-called ''paradoxical'' behavior of pituitary cells to hypothalamic hypophysiotropic agents.The pituitary gland is a major neuroendocrine modulator that regulates many peripheral glands and tissues through the secretion of anterior pituitary (AP) hormones. These hormones include prolactin (PRL), growth hormone (GH), thyrotropin (TSH), adrenocorticotropin, and the gonadotropins [follicle-stimulating hormone (FSH) and luteinizing hormone (LH)]. The secretory activity of the gland is controlled, in turn, by signals derived from the hypothalamus in the form of hypothalamic-releasing hormones (HRHs) such as thyrotropin-releasing hormone (TRH), growth hormone-releasing hormone (GHRH), corticotropin-releasing hormone (CRH), and gonadotropin-releasing hormone (LHRH). The classic view for hypothalamic control of AP hormone secretion holds that each major HRH modulates the secretion of a single pituitary hormone (1), but there are some notable exceptions to this rule. For example, TRH is acknowledged to be a physiological modulator of both TSH and PRL secretion, just as LHRH controls the release of both LH and FSH. In addition to these acknowledged exceptions, there have been sporadic reports about ''paradoxical'' responses to various HRHs. Such observations of AP hormone release elicited by a noncorresp...

GH and cortisol rebound rise during and following a somatostatin infusion: studies in dogs with the use of a GH-releasing peptide

“…Alternatively, to explain our findings, it cannot be ruled out that EP51216 was acting directly at the pituitary level, since 'paradoxical' responses of freshly dispersed or cultured anterior pituitary cells to hypothalamic-releasing hormones have been reported recently (Villalobos et al 1997, Roudbaraki et al 1999. Multi-responsiveness to hypothalamic releasing hormones would thus be a genuine property of normal anterior pituitary cells (Villalobos et al 1997).…”

“…Multi-responsiveness to hypothalamic releasing hormones would thus be a genuine property of normal anterior pituitary cells (Villalobos et al 1997).…”

Functional measurements of [Ca2+] in the endoplasmic reticulum using a herpes virus to deliver targeted aequorin

“…In the [Caz+], experiments performed in the infected PC12 cells (Fig. 4B), an abbreviated immunofluorescence protocol was carried out at the end of the [Ca2+], measurements [25]. (BHQ) 10 PM in KRB (Krebs-Ringer modified buffer: 125 mM NaCl, 5 mM KCl, 1 mM Na,PO,, 1 mM MgCl,, 5.5 mM glucose, 20 mM HEPES, pH 74) supplemented with 3 mM EGTA.…”

“…Cells were loaded with 4 @VI Fura-and single-cell [Ca"+], measurements and time-resolved digital image analysis were performed as previously described [25].…”

“…They respond to thyrotropin-releasing hormone (TRH) with a rapid increase in [Ca2+lc due both to release of Ca2+ from intracellular stores and to stimulated Ca2+ influx from the extracellular medium [29]. They possess voltagedependent Caz+-channels that activate on depolarization of the plasma membrane, producing a fast peak in cytosolic Ca2+ [17,25]. In Figure 5 we compared the effects of both TRH and depolarization with 50 mM K+ on [Caz+],, and [Ca'+], in the GH, rat pituitary cell line (Fig.…”