Ion Channels of Pituitary Gonadotrophs and Their Roles in Signaling and Secretion

Abstract

Gonadotrophs are basophilic cells of the anterior pituitary gland specialized to secrete gonadotropins in response to elevation in intracellular calcium concentration. These cells fire action potentials (APs) spontaneously, coupled with voltage-gated calcium influx of insufficient amplitude to trigger gonadotropin release. The spontaneous excitability of gonadotrophs reflects the expression of voltage-gated sodium, calcium, potassium, non-selective cation-conducting, and chloride channels at their plasma membrane (PM). These cells also express the hyperpolarization-activated and cyclic nucleotide-gated cation channels at the PM, as well as GABA_A, nicotinic, and purinergic P2X channels gated by γ-aminobutyric acid (GABA), acetylcholine (ACh), and ATP, respectively. Activation of these channels leads to initiation or amplification of the pacemaking activity, facilitation of calcium influx, and activation of the exocytic pathway. Gonadotrophs also express calcium-conducting channels at the endoplasmic reticulum membranes gated by inositol trisphosphate and intracellular calcium. These channels are activated potently by hypothalamic gonadotropin-releasing hormone (GnRH) and less potently by several paracrine calcium-mobilizing agonists, including pituitary adenylate cyclase-activating peptides, endothelins, ACh, vasopressin, and oxytocin. Activation of these channels causes oscillatory calcium release and a rapid gonadotropin release, accompanied with a shift from tonic firing of single APs to periodic bursting type of electrical activity, which accounts for a sustained calcium signaling and gonadotropin secretion. This review summarizes our current understanding of ion channels as signaling molecules in gonadotrophs, the role of GnRH and paracrine agonists in their gating, and the cross talk among channels.

Keywords: calcium signaling; electrical activity; gonadotrophs; gonadotropin-releasing hormone; ligand-gated channels; luteinizing hormone secretion; voltage-gated channels.

Figure 1

The expression of voltage-gated and ligand-gated ion channels in identified rat pituitary gonadotrophs. (A) Voltage-gated sodium current traces elicited by 100 ms voltage steps from holding potentials (HPs) of −117 mV. (B) Voltage-gated calcium current traces elicited by 400 ms voltage-steps. (C) Voltage-gated potassium current traces elicited by 1.5 s voltage steps. (D) Cells expressing HCN current display inward rectification in response to hyperpolarizing current pulses of −5 pA that are suppressed by 1 mM Cs⁺, a blocker of HCN channels. (E) Spontaneous electrical activity can be observed in about 50% of cultured gonadotrophs. (F) 8-Br-cAMP stimulates electrical activity. (G) γ-aminobutyric acid (GABA)-induced current recorded using gramicidin-perforated patch from cells held at different membrane potentials. (H) GABA-stimulated electrical activity. (I) Concentration-dependent effects of acetylcholine (ACh) on the amplitude of nicotinic current. (Top) Representative traces. (Bottom) Mean ± SEM values. (J) ACh-induced electrical activity. (K) Concentration-dependent effects of ATP on peak P2XR current response. (L) ATP-induced firing of action potentials. Derived from data shown in Ref. (16, 41, 50, 52, 63, 67); no permission is required from the copyright holder.

Figure 2

Influence of Ca²⁺ mobilization on excitability of pituitary gonadotrophs. (A,B) Gonadotropin-releasing hormone (GnRH)-induced calcium oscillations in rat pituitary gonadotrophs (A) and non-oscillatory calcium signals in immortalized αT3-1 pituitary gonadotrophs and testicular Leydig cells (B). (C,D) GnRH-induced membrane potential (C) and small-conductance K⁺ (SK) current (D) oscillations in rat gonadotrophs. (E,F) Increase in the amplitude of GnRH-induced SK current oscillations by depolarization (E) and activation of P2X2R channels by extracellular ATP (F) in rat gonadotrophs. Current oscillations were initiated by 0.1 nM GnRH (E) and 1 nM GnRH (F). Calcium recordings were done in intact Indo-1-loaded cells (A,B), whereas electrophysiological recordings were done in nystatin-perforated cells voltage-clamped (D–F) or in current-clamped cells (C). V_m, membrane potential; I, SK current; HP, holding potential. Derived from data shown in Ref. (36, 50, 63, 91); no permission is required from the copyright holder.