Growth hormone-releasing hormone decreases voltage-gated potassium currents in GH4C1 cells

Potassium Current Is Not Affected by Long-Term Exposure to Ghrelin or GHRP-6 in Somatotropes GC Cells

Ghrelin is a growth hormone (GH) secretagogue (GHS) and GHRP-6 is a synthetic peptide analogue; both act through the GHS receptor. GH secretion depends directly on the intracellular concentration of Ca(2+); this is determined from the intracellular reserves and by the entrance of Ca(2+) through the voltage-dependent calcium channels, which are activated by the membrane depolarization. Membrane potential is mainly determined by K(+) channels. In the present work, we investigated the effect of ghrelin (10 nM) or GHRP-6 (100 nM) for 96 h on functional expression of voltage-dependent K(+) channels in rat somatotropes: GC cell line. Physiological patch-clamp whole-cell recording was used to register the K(+) currents. With Cd(2+) (1 mM) and tetrodotoxin (1  μ m) in the bath solution recording, three types of currents were characterized on the basis of their biophysical and pharmacological properties. GC cells showed a K(+) current with a transitory component (I A) sensitive to 4-aminopyridine, which represents ~40% of the total outgoing current; a sustained component named delayed rectifier (I K), sensitive to tetraethylammonium; and a third type of K(+) current was recorded at potentials more negative than -80 mV, permitting the entrance of K(+) named inward rectifier (KIR). Chronic treatment with ghrelin or GHRP-6 did not modify the functional expression of K(+) channels, without significant changes (P < 0.05) in the amplitudes of the three currents observed; in addition, there were no modifications in their biophysical properties and kinetic activation or inactivation.

Ion channels and signaling in the pituitary gland

Endocrine pituitary cells are neuronlike; they express numerous voltage-gated sodium, calcium, potassium, and chloride channels and fire action potentials spontaneously, accompanied by a rise in intracellular calcium. In some cells, spontaneous electrical activity is sufficient to drive the intracellular calcium concentration above the threshold for stimulus-secretion and stimulus-transcription coupling. In others, the function of these action potentials is to maintain the cells in a responsive state with cytosolic calcium near, but below, the threshold level. Some pituitary cells also express gap junction channels, which could be used for intercellular Ca(2+) signaling in these cells. Endocrine cells also express extracellular ligand-gated ion channels, and their activation by hypothalamic and intrapituitary hormones leads to amplification of the pacemaking activity and facilitation of calcium influx and hormone release. These cells also express numerous G protein-coupled receptors, which can stimulate or silence electrical activity and action potential-dependent calcium influx and hormone release. Other members of this receptor family can activate calcium channels in the endoplasmic reticulum, leading to a cell type-specific modulation of electrical activity. This review summarizes recent findings in this field and our current understanding of the complex relationship between voltage-gated ion channels, ligand-gated ion channels, gap junction channels, and G protein-coupled receptors in pituitary cells.

Ghrelin reduces voltage-gated potassium currents in GH3 cells via cyclic GMP pathways

Ghrelin is an endogenous growth hormone secretagogue (GHS) causing release of GH from pituitary somatotropes through the GHS receptor. Secretion of GH is linked directly to intracellular free Ca(2+) concentration ([Ca(2+)]i), which is determined by Ca(2+) influx and release from intracellular Ca(2+) storage sites. Ca(2+) influx is via voltage-gated Ca(2+) channels, which are activated by cell depolarization. Membrane potential is mainly determined by transmembrane K(+) channels. The present study investigates the in vitroeffect of ghrelin on membrane voltage-gated K(+) channels in the GH3 rat somatotrope cell line. Nystatin-perforated patch clamp recording was used to record K(+) currents under voltage-clamp conditions. In the presence of Co(2+) (1 mM, Ca(2+) channel blocker) and tetrodotoxin (1 microM, Na(+) channel blocker) in the bath solution, two types of voltage-gated K(+) currents were characterized on the basis of their biophysical kinetics and pharmacological properties. We observed that transient K(+) current (IA) represented a significant proportion of total K(+) currents in some cells, whereas delayed rectifier K(+) current (IK) existed in all cells. The application of ghrelin (10 nM) reversibly and significantly decreased the amplitude of both IA and IK currents to 48% and 64% of control, respectively. Application of apamin (1 microM, SK channel blocker) or charybdotoxin (1 microM, BK channel blocker) did not alter the K(+) current or the response to ghrelin. The ghrelin-induced reduction in K(+) currents was not affected by PKC and PKA inhibitors. KT5823, a specific PKG inhibitor, totally abolished the K+ current response to ghrelin. These results suggest that ghrelin-induced reduction of voltage-gated K(+) currents in GH3 cells is mediated through a PKG-dependent pathway. A decrease in voltage-gated K(+) currents may increase the frequency, duration, and amplitude of action potentials and contribute to GH secretion from somatotropes.

Stromal cell-derived factor-1alpha directly modulates voltage-dependent currents of the action potential in mammalian neuronal cells

Stromal cell-derived factor-1alpha (SDF-1alpha) is a chemokine whose receptor, CXCR4, is distributed in specific brain areas including hypothalamus. SDF-1alpha has recently been found to play important roles in neurons, although direct modulation of voltage-gated ionic channels has never been shown. In order to clarify this issue, we performed patch-clamp experiments in fetal mouse hypothalamic neurons in culture. SDF-1alpha (10 nm) decreased the peak and rising slope of the action potentials and spike discharge frequency in 22% of hypothalamic neurons tested. This effect was blocked by the CXCR4 antagonist AMD 3100 (1 microm) but not by the metabotropic glutamate receptor antagonist MCPG (500 microm), indicating a direct action of SDF-1alpha on its cognate receptor. This effect involved a depression of both inward and outward voltage-dependent currents of the action potential. We confirmed these effects in the human neuroblastoma cell line SH-SY5Y, which endogenously expresses CXCR4. Voltage-clamp experiments revealed that SDF-1alpha induced a 20% decrease in the peak of the tetrodotoxin-sensitive sodium current and tetraethylammonium-sensitive delayed rectifier potassium current, respectively. Both effects were concentration dependent, and blocked by AMD 3100 (200 nm). This dual effect was reduced or blocked by 0.4 mm GTPgammaS G-protein pre-activation or by pre-treatment with the G-protein inhibitor pertussis toxin (200 ng/mL), suggesting that it is mediated via activation of a G(i/o) protein. This study extends the functions of SDF-1alpha to a direct modulation of voltage-dependent membrane currents of neuronal cells.

Pituitary cell lines and their endocrine applications

The pituitary gland is an important component of the endocrine system, and together with the hypothalamus, exerts considerable influence over the functions of other endocrine glands. The hypothalamus either positively or negatively regulates hormonal productions in the pituitary through its release of various trophic hormones which act on specific cell types in the pituitary to secrete a variety of pituitary hormones that are important for growth and development, metabolism, reproductive and nervous system functions. The pituitary is divided into three sections-the anterior lobe which constitute the majority of the pituitary mass and is composed primarily of five hormone-producing cell types (thyrotropes, lactotropes, corticotropes, somatotropes and gonadotropes) each secreting thyrotropin, prolactin, ACTH, growth hormone and gonadotropins (FSH and LH) respectively. There is also a sixth cell type in the anterior lobe-the non-endocrine, agranular, folliculostellate cells. The intermediate lobe produces melanocyte-stimulating hormone and endorphins, whereas the posterior lobe secretes anti-diuretic hormone (vasopressin) and oxytocin. Representative cell lines of all the six cell types of the anterior pituitary have been established and have provided valuable information on genealogy of the various cell lineages, endocrine feedback control of hormone synthesis and secretions, intrapituitary interactions between the various cell types, as well as the role of specific transcription factors that determine each differentiated cell phenotype. In this review, we will discuss the morphology and function of the cell types that make up the anterior pituitary, and the characteristics of the various functional anterior pituitary cell systems that have been established to be representative of each anterior pituitary cell lineage.

The in vitro regulation of growth hormone secretion by orexins

Orexins, orexigenic neuropeptides, have recently been discovered in lateral hypothalamus and play an important role in the regulation of pituitary hormone secretion. Two subtypes of orexin receptors (orexin-1 and orexin-2) have been demonstrated in pituitaries. In this experiment, the effects of orexins on voltage-gated Ca2+ currents and the GH release in primary cultured ovine somatotropes were examined. Voltage-gated Ca2+ currents were isolated in ovine somatotropes as L, T, and N currents using whole-cell patch-clamp techniques and specific Ca2+ channel blocker and toxin. Application of orexin-A or orexin-B (100 nM) significantly, dose-dependently, and reversibly increased only nifedipine-sensitive L-type Ca2+ current. Inhibitors of PKC (calphostin C, PKC inhibitory peptide) but not inhibitors of PKA (H89, PKA inhibitory peptide) cancelled the increase in the L current by orexins. Co-administration of orexin-A and GHRH (10 nM) showed an additive effect on the L current. Specific intracellular Ca2+-store-depleting reagent, thapsigargin (1 microM), did not affect the orexin-induced increase in the L current. Orexin-B alone slightly increased GH release and co-administration of orexin-A and GHRH synergistically stimulated GH secretion in vitro. It is therefore suggested that orexins may play an important role in regulating GHRH-stimulated GH secretion through an increase in the L-type Ca2+ current and the PKC-mediated signaling pathways in ovine somatotropes.

Orexin-B augments voltage-gated L-type Ca(2+) current via protein kinase C-mediated signalling pathway in ovine somatotropes

Orexins, orexigenic neuropeptides, are secreted from lateral hypothalamus and orexin receptors are expressed in the pituitary. Since growth hormone (GH) secreted from pituitary is integrally linked to energy homeostasis and metabolism, we studied the effect of orexin-B on voltage-gated Ca(2+) currents and the related signalling mechanisms in primary cultured ovine somatotropes using whole-cell patch-clamp techniques. With a bath solution containing TEA-Cl (40 mM) and Tetrodotoxin (TTX) (1 microM), three subtypes of Ca(2+) currents, namely the long-lasting (L), transient (T), and N currents, were isolated using different holding potentials (-80 and -30 mV) in combination with specific Ca(2+) channel blockers (nifedipine and omega-conotoxin). About 75% of the total current amplitude was contributed by the L current, whereas the N and T currents accounted for the rest. Orexin-B (1-100 nM) dose-dependently and reversibly increased only the L current up to approximately 125% of the control value within 4-5 min. Neither a specific protein kinase A (PKA) blocker (H89, 1 microM) nor an inhibitory peptide (PKI, 10 microM) had any effect on the increase in L current by orexin-B. The orexin-B-induced increase in the L current was abolished by concurrent treatment with calphostin C (Cal-C, 100 nM), protein kinase C (PKC) inhibitory peptide (PKC(19-36), 1 microM), or by pretreatment with phorbol-12,13-dibutyrate (PDBu) (0.5 microM) for 16 h (a downregulator of PKC). Orexin-B also increased in vitro GH secretion in a dose-dependent manner. We conclude that orexin-B increases the L-type Ca(2+) current and GH secretion through orexin receptors and PKC-mediated signalling pathways in ovine somatotropes.

Growth hormone-releasing peptide-2 reduces inward rectifying K+ currents via a PKA-cAMP-mediated signalling pathway in ovine somatotropes

Inward-rectifying potassium (Kir) channels are essential for maintaining the resting membrane potential near the K(+) equilibrium and they are responsible for hyperpolarisation-induced K(+) influx. We characterised the Kir current in primary cultured ovine somatotropes and examined the effect of growth hormone-releasing peptide-2 (GHRP-2) on this current and its related intracellular signalling pathways. The Kir current was, in most cases, isolated using nystatin-perforated patch-clamp techniques. In bath solution containing 5 mM K(+), the Kir current was composed of both transient (fast activated) and delayed (slowly activated) components. An increase in the external K(+) concentration from 5 to 25 mM induced an augmentation of approximately 4-fold in the delayed part of the Kir current and both BaCl(2) and CsCl dose-dependently inhibited this current, confirming the presence of the Kir current in ovine somatotropes. Moreover, this specific effect of high K(+) on the Kir current was only observed in the cells that showed positive staining with anti-growth hormone (GH) antibodies, or in GC cells that belong to a rat somatotrope cell line. Application of GHRP-2 (100 nM) reversibly and significantly reduced the Kir current in bath solutions with 5 or 25 mM K(+) in ovine somatotropes. In addition, we found that the reduction in the Kir current mediated by GHRP-2 was totally abolished by the pretreatments with H89 (1 microM) or Rp-cAMP (100 microM) or by intracellular dialysis of a specific protein kinase A (PKA) inhibitory peptide PKI (10 microM). The specific PKC blocker chelerythrine (1 microM) or inhibitory peptide PKC(19-36) (10 microM) did not show any effects on the GHRP-2-induced decrease in the Kir current. These results suggest that the inhibition of Kir current through PKA-cAMP pathways may play an integral role in GHRP-2-induced depolarisation and GH release in ovine somatotropes.

GHRH and IL1beta increase cytoplasmic Ca(2+) levels in cultured hypothalamic GABAergic neurons

GHRH and IL1beta regulate sleep via the hypothalamus. However, actions of these substances on neurons are poorly understood. In this study, we found both GHRH (100 nM) and IL1beta (1.2 pM) acutely increased cytosolic Ca(2+) in 7.6 and 4.0% of cultured hypothalamic neurons tested, respectively, and 1.2% of neurons responded to both. The neurons that responded were mostly GABAergic (96, 81, and 100% for GHRH, IL1beta, and dual-responsive neurons, respectively).

Diverse intracellular signalling systems used by growth hormone-releasing hormone in regulating voltage-gated Ca2+ or K channels in pituitary somatotropes

Influx of Ca2+ via Ca2+ channels is the major step triggering exocytosis of pituitary somatotropes to release growth hormone (GH). Voltage-gated Ca2+ and K+ channels, the primary determinants of the influx of Ca2+, are regulated by GH-releasing hormone (GHRH) through G-protein-coupled intracellular signalling systems. Using whole-cell patch-clamp techniques, the changes of the Ca2+ and K+ currents in primary cultured ovine and human somatotropes were recorded. Growth hormone-releasing hormone (10 nmol/L) increased both L- and T-type voltage-gated Ca2+ currents. Inhibition of the cAMP/protein kinase A (PKA) pathway by either Rp-cAMP or H89 blocked this increase in both L- and T-type Ca2+ currents. Growth hormone-releasing hormone also decreased voltage-gated transient (IA) and delayed rectified (IK) K+ currents. Protein kinase C (PKC) inhibitors, such as calphostin C, chelerythrine or downregulation of PKC, blocked the effect of GHRH on K+ currents, whereas an acute activation of PKC by phorbol 12, 13-dibutyrate (1 micromol/L) mimicked the effect of GHRH. Intracellular dialysis of a specific PKC inhibitor (PKC19-36) also prevented the reduction in K+ currents by GHRH. It is therefore concluded that GHRH increases voltage-gated Ca2+ currents via cAMP/PKA, but decreases voltage-gated K+ currents via the PKC signalling system. The GHRH-induced alteration of Ca2+ and K+ currents augments the influx of Ca2+, leading to an increase in [Ca2+]i and the GH secretion.