Ion channels and signaling in the pituitary gland

Abstract

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.

Figure 1

Schematic representation of pituitary gland development. Inset, Formation of Rathke’s pouch and early pituitary development. Gray areas, Neuroepithelium; red areas, oral epithelium. Main panel, Cell lineage development and selected transcriptional factors involved during mouse pituitary organogenesis. Prop-1 and Pit-1, Pituitary-specific transcription factors; Sf1, steroidogenic factor-1; GATA2, zinc finger transcription factor; Tbx19 (or Tpit), member of the T-box family of transcription factors; I, POMC-producing cells; II, α-GSU-producing cells; and III, PRL/GH-producing cells.

Figure 2

Na_v and Ca_v channels. Top, Structural TM folding model of Na_v and Ca_v channels. In this and the following models, α-helices are illustrated as cylinders and the extracellular and intracellular chains of amino acids as continuous lines. The positively charged 4S domain illustrates the voltage sensor, and the S5 and S6 domains contribute to the formation of the channel pore. Bottom, TTX and saxitoxin (SAX) sensitivity of Na_v-α subunits. Rectangles show the subunits whose presence was identified in pituitary cells at the mRNA level. TTX-sensitive Na_v currents have been identified in all endocrine pituitary cells.

Figure 3

Classification of Ca_v channel α-subunits. HVA, High-voltage activated; LVA, low-voltage activated; DHP, dihydropyridines; IVA, w-agatoxin; GVIA, w-conotoxin; SNT, SNK-482. HVA and LVA currents have been detected in all endocrine pituitary cells. Rectangles indicate the mRNA transcripts for Ca_v-α subunits in pituitary cells. Immunocytochemical studies showed the presence of Ca_v1.1, 1.2, 1.3, 2.1, 2.2, 2.3, and 3.1-α subunits in these cells.

Figure 4

K_ir channels play important roles in the control of resting membrane potential and agonist-induced inhibition of spontaneous electrical activity in pituitary cells. Left, Structural TM folding model of K_ir channels. Right, The 15 known members of K_ir channels are divided into three groups, based on their regulation. Rectangles indicate K_ir-α subunits identified in pituitary cells. The presence of K_ir3.1 and 3.2 has also been confirmed by Western blot analysis.

Figure 5

K_v channels reduce excitation in pituitary cells. Top, Structural TM folding model of K_v channels (left) and tetrameric organization of α-subunits (right). Bottom, Families of K_v channels. Rectangles indicate K_v-α-subunits for which mRNAs were identified in pituitary cells.

Figure 6

Two types of K_Ca channels are expressed in pituitary cells. Top left, SK channels have similar TM organization as K_v channels, but are not voltage-regulated. Top right, BK (maxi) channels have an additional TM domain (S0) and are regulated by both voltage and calcium. Bottom, Phylogenetic tree for K_Ca channels. The K_Ca1.1 mRNA transcript was found in pituitary cells, and the presence of SK and BK currents in pituitary cells was confirmed using specific blockers of these channels.

Figure 7

Cyclic nucleotide-modulated channels are nonselective cation channels. Top, Structural TM folding model of CNG channels and HCN channels. Bottom, Phylogenetic tree of CNG and HCN channels. The mRNA transcripts for all four HCN α-subunits and CNGA1 α-subunit were identified in pituitary cells, as well as the functional HCN current.

Figure 8

Model simulation of the transition from spiking to bursting with the addition of BK current. A, Application of a voltage-independent hyperpolarizing current slows down spiking, but does not convert the spiking to bursting (thin bar, small current; thick bar, larger current). B, Application of a rapidly activating BK-type K⁺ current converts spiking to bursting.

Figure 9

Dependence of the cyclic nucleotide signaling pathway on Ca²⁺ influx through Ca_v channels. Calcium stimulates the activity of several adenylyl cyclases, PDE1, and nitric oxide synthase (NOS) in a calmodulin (CaM)-dependent manner (continuous lines). It also inhibits some adenylyl cyclase isozymes and sGS directly (dotted lines). Changes in the resting membrane potential affect the multidrug resistance protein (MRP)-mediated cyclic nucleotide efflux. PKG, Protein kinase G; LPS+IFN, lipopolysaccharide and interferon.

Figure 10

ATP acts as an extracellular messenger in pituitary cells. The extracellularly released ATP is hydrolyzed by two enzyme families, ectonucleotide pyrophosphate/PDE and ectonucleoside triphosphate diphosphohydrolase, generating ADP and AMP, whereas AMP is efficiently hydrolyzed by the ecto-5′-nucleotidase family of enzymes, generating adenosine. ATP is an agonist for two TM domain P2X receptors (P2XRs) and 7TM domain P2YRs, whereas ADP activates a few P2YRs but no P2XRs. Adenosine also acts as an agonist for four GPCRs, called adenosine receptors (ARs). Rectangles indicate receptors expressed in pituitary cells.

Figure 11

Pituitary cells express GPCRs that engage the cAMP signaling pathway. Top left, A small number of neurohormones act through GPCRs coupled to G_s heterotrimeric proteins, and their α-subunit binds to AC, leading to stimulation of cAMP production. cAMP acts as an intracellular messenger by directly activating HCN and CNG channels, and indirectly through PKA. Binding of cAMP to PKA leads to dissociation of the catalytic (C) from regulatory (R) subunits. The messenger function of cAMP is terminated by PDEs and by efflux of cAMP mediated by cyclic nucleotide pumps (not shown). Top right, Coupling of several GPCRs to G_i/o and/or G_z provide an effective mechanism for inhibition of adenylyl cyclase activity by their α-subunits. In addition to inhibition of the cAMP signaling pathway, these receptors also modulate electrical activity through βγ dimers acting on K_ir and Ca_v channels (not shown).

Figure 12

Expression of Ca²⁺-mobilizing GPCRs in endocrine pituitary cells. Top rectangle, Ca²⁺-mobilizing GPCRs expressed in mammalian pituitary cells. Activation of these receptors by hypothalamic or intrapituitary hormones leads to dissociation of heterotrimeric G proteins, and their α-subunit (and in some cases βγ-subunits) stimulates PLC. This enzyme serves as an amplifier by producing two intracellular messengers: IP₃ and DAG. IP₃ binds to its receptors in the ER and evokes Ca²⁺ release, called Ca²⁺ mobilization. During sustained agonist occupancy, Ca²⁺ mobilization is accompanied by Ca²⁺ flux into the cell.

Figure 13

Model simulation of the pulse-decay-plateau Ca²⁺ response to activation of the G_q/11 pathway. A, Activation of the pathway results in production of IP₃, which binds to IP₃Rs and releases Ca²⁺ from the ER into the cytosol. This results in a rapid rise in [Ca²⁺]_i and subsequent decay to an elevated plateau. In this and all other modeling figures, the IP₃ concentration is constant during simulated application. B, Partial depletion of the ER Ca²⁺ store due to release of Ca²⁺ through IP₃R. The slow [Ca²⁺]_ER decay underlies the slow decay of [Ca²⁺]_i. This model and others used in the article can be downloaded as freeware from www.math.fsu.edu/∼bertram/software/pituitary.

Figure 14

Model simulation of one mechanism for Ca²⁺ oscillations that can be produced by G_q/11 activation. A, Oscillations are due to the fast activation and slow inactivation of IP₃Rs. The variable “h” is the fraction of receptors that are not inactivated. B, Ca²⁺ oscillations persist as long as there is sufficient Ca²⁺ flux into the cell. When influx is eliminated (J_in = 0), the oscillation amplitude and frequency decline, and eventually oscillations cease.

Figure 15

Model simulation of coupled IP₃R and membrane oscillations. A, Bursting electrical oscillations are produced due to periodic hyperpolarizations that result from the activation of Ca²⁺-activated K⁺ (K_Ca) current. B, IP₃R-mediated oscillations in [Ca²⁺]_i periodically activate K_Ca current so that each peak of [Ca²⁺]_i produces a silent phase of the burst.

Figure 16

GPCR-modulated channel activity in pituitary cells. GPCRs expressed in pituitary cells regulate numerous voltage-gated channels through intracellular messengers. Top panel shows agonists that activate GPCRs in mammalian cells. Red letters indicate the principal regulators of pituitary functions, and black letters indicate agonists that modulate pituitary cell function. Note that only somatotrophs utilize all three signaling pathways and that lactotroph function is regulated by numerous G_i and G_q/11-coupled receptors. Melatonin receptors are present only in neonatal pituitary gonadotrophs. Bottom panel shows channels expressed in pituitary cells in which gating is affected by an intracellular messenger. The nature of Na_b current is unknown at the present time, as well as the Na⁺-conducting channel that is phosphorylated by PKA. It has also not been clarified whether inhibition of erg and M current accounts for agonist-induced depolarization of cells and sustained stimulation. The operation of the STIM/Orai pathway in pituitary cells has not been established.