Roles of Purinergic P2X Receptors as Pacemaking Channels and Modulators of Calcium-Mobilizing Pathway in Pituitary Gonadotrophs

Abstract

Anterior pituitary cells release ATP and express several subtypes of purinergic P2 receptors, but their biophysical properties and roles in spontaneous and receptor-controlled electrical activity have not been characterized. Here we focused on extracellular ATP actions in gonadotrophs from embryonic, neonatal, and adult rats. In cells from all three age groups, the Ca2+-mobilizing agonist GnRH induced oscillatory, hyperpolarizing, nondesensitizing, and slow deactivating currents. In contrast, ATP induced nonoscillatory, depolarizing, slowly desensitizing, and rapidly deactivating current, indicating that these cells express cation-conducting P2X channels but not Ca2+-mobilizing P2Y receptors. The amplitudes of P2X current response and the rates of receptor desensitization were dependent on ATP concentration. The biophysical and pharmacological properties of P2X currents were consistent with the expression of P2X2 subtype of channels in these cells. ATP-induced rapid depolarization of gonadotrophs lead to initiation of firing in quiescent cells, an increase in the frequency of action potentials in spontaneously active cells, and a transient stimulation of LH release. ATP also influenced GnRH-induced current and membrane potential oscillations and LH release in an extracellular Ca2+-dependent manner. These inositol 1,4,5-triphosphate-dependent oscillations were facilitated, slowed, or stopped, depending of ATP concentration, the time of its application, and the level of Ca2+ content in intracellular stores. These results indicate that, in gonadotrophs, P2X receptors could operate as pacemaking channels and modulators of GnRH-controlled electrical activity and secretion.

Activation and Regulation of Purinergic P2X Receptor Channels

“…It is also present in the hypothalamus Stojilkovic, 2009) and retina (Greenwood et al, 1997) as well as in the peripheral nervous system Robertson et al, 1996;Simon et al, 1997;Vulchanova et al, 1997;Xiang et al, 1998;Zhong et al, 1998;Zhong et al, 2000Zhong et al, , 2001Calvert and Evans, 2004;Ma et al, 2004;Cockayne et al, 2005;Ma et al, 2005). These receptors are also expressed in nonneuronal cells, including pituitary cells (Stojilkovic and Koshimizu, 2001;Zemkova et al, 2006;Stojilkovic et al, 2010a), the adrenal medulla (Vulchanova et al, 1996), skeletal cells (Ryten et al, 2001;Jiang et al, 2005b), cardiac cells (Hansen et al, 1999a), smooth muscle cells (Lee et al, 2000), endothelial and epithelial cells Hansen et al, 1999b;Birder et al, 2004), and lymphocytes (Di Virgilio et al, 2001).…”

“…When tested in the same experiment, RB-2 was more potent than TNP, which was more potent than suramin (Liu et al, 2001a). Native rP2X2Rs expressed in pituitary gonadotrophs are also inhibited by RB-2 (Zemkova et al, 2006). P2X1 and P2X2 subunits can generate heteromeric receptors expressed at the plasma membrane (Aschrafi et al, 2004).…”

Expression of P2Y receptors in the rat anterior pituitary

“…With molecular biology techniques, P2X 2 , P2X 3 , P2X 4 , and P2X 7 mRNA transcripts were detected in a mixed population of anterior pituitary cells. Lactotrophs and immortalized GH3 pituitary cells expressed transcripts for P2X 3 , P2X 4 , and P2X 7 subunits, somatotrophs expressed P2X 2 receptor, and thyrotrophs and corticotrophs also express P2X receptors [7] Single-cell patch-clamp analysis in gonadotrophs from embryonic, neonatal, and adult rats revealed that P2X 2 receptors are operative and involved in release of luteinizing hormone (LH) and regulation of gonadotrophinreleasing hormone (GnRH) controlled electrical activity and secretion [8]. Lactotrophs express P2X 4 receptors, which facilitate Ca 2+ influx and hormone secretion [9].…”

Purinergic signalling in endocrine organs

“…Another study also described P2X7 receptors on GH3 cells and showed that they mediated increase in [Ca 2+ ] i and depolarisation [101]. ATP, operating via P2X2 receptors controls the pacemaker activity, voltagegated Ca 2+ influx and basal LH release in gonadotrophs [613]. A valuable review discusses the complexity of purinergic signalling in lactotrophs, which express multiple purinoceptors and also reports the presence of P2X receptors in thyrotrophs and corticotrophs, although the subtypes were not identified ( [510]; Fig.…”