Contribution of different Ca2+-activated K+ channels to the first phase of the response to TRH in clonal rat anterior pituitary cells

Characterization of Convergent Suppression by UCL-2077 (3-(Triphenylmethylaminomethyl)pyridine), Known to Inhibit Slow Afterhyperpolarization, of erg-Mediated Potassium Currents and Intermediate-Conductance Calcium-Activated Potassium Channels

UCL-2077 (triphenylmethylaminomethyl)pyridine) was previously reported to suppress slow afterhyperpolarization in neurons. However, the information with respect to the effects of UCL-2077 on ionic currents is quite scarce. The addition of UCL-2077 decreased the amplitude of erg-mediated K⁺ current (I_K(erg)) together with an increased deactivation rate of the current in pituitary GH₃ cells. The IC₅₀ and K_D values of UCL-2077-induced inhibition of I_K(erg) were 4.7 and 5.1 μM, respectively. UCL-2077 (10 μM) distinctly shifted the midpoint in the activation curve of I_K(erg) to less hyperpolarizing potentials by 17 mV. Its presence decreased the degree of voltage hysteresis for I_K(erg) elicitation by long-lasting triangular ramp pulse. It also diminished the probability of the opening of intermediate-conductance Ca²⁺-activated K⁺ channels. In cell-attached current recordings, UCL-2077 raised the frequency of action currents. When KCNH2 mRNA was knocked down, a UCL-2077-mediated increase in AC firing was attenuated. Collectively, the actions elaborated herein conceivably contribute to the perturbating effects of this compound on electrical behaviors of excitable cells.

Common and diverse elements of ion channels and receptors underlying electrical activity in endocrine pituitary cells

The pituitary gland contains six types of endocrine cells defined by hormones they secrete: corticotrophs, melanotrophs, gonadotrophs, thyrotrophs, somatotrophs, and lactotrophs. All these cell types are electrically excitable, and voltage-gated calcium influx is the major trigger for their hormone secretion. Along with hormone intracellular content, G-protein-coupled receptor and ion channel expression can also be considered as defining cell type identity. While many aspects of the developmental and activity dependent regulation of hormone and G-protein-coupled receptor expression have been elucidated, much less is known about the regulation of the ion channels needed for excitation-secretion coupling in these cells. We compare the spontaneous and receptor-controlled patterns of electrical signaling among endocrine pituitary cell types, including insights gained from mathematical modeling. We argue that a common set of ionic currents unites these cells, while differential expression of another subset of ionic currents could underlie cell type-specific patterns. We demonstrate these ideas using a generic mathematical model, showing that it reproduces many observed features of pituitary electrical signaling. Mapping these observations to the developmental lineage suggests possible modes of regulation that may give rise to mature pituitary cell types.

Control of anterior pituitary cell excitability by calcium-activated potassium channels

In anterior pituitary endocrine cells, large (BK), small (SK) and intermediate (IK) conductance calcium activated potassium channels are key determinants in shaping cellular excitability in a cell type- and context-specific manner. Indeed, these channels are targeted by multiple signaling pathways that stimulate or inhibit cellular excitability. BK channels can, paradoxically, both promote electrical bursting as well as terminate bursting and spiking dependent upon intrinsic BK channel properties and proximity to voltage gated calcium channels in somatotrophs, lactotrophs and corticotrophs. In contrast, SK channels are predominantly activated by calcium released from intracellular IP3-sensitive calcium stores and mediate membrane hyperpolarization in cells including gonadotrophs and corticotrophs. IK channels are predominantly expressed in corticotrophs where they limit membrane excitability. A major challenge for the future is to determine the cell-type specific molecular composition of calcium-activated potassium channels and how they control anterior pituitary hormone secretion as well as other calcium-dependent processes.

Electrophysiological differences between fshb- and lhb-expressing gonadotropes in primary culture

Synthesis and release of FSH and LH are differentially regulated by GnRH, but the mechanisms by which this regulation is achieved are not well understood. Teleost fish are powerful models for studying this differential regulation because they have distinct pituitary cells producing either FSH or LH. By using pituitary cultures from Atlantic cod (Gadus morhua), we were able to investigate and compare the electrophysiological properties of fshb- and lhb-expressing cells, identified by single-cell quantitative PCR after recording. Both cell types fired action potentials spontaneously. The relative number of excitable cells was dependent on reproductive season but varied in opposing directions according to season in the 2 cell types. Excitable and quiescent gonadotropes displayed different ion channel repertoires. The dynamics of outward currents and GnRH-induced membrane responses differed between fshb- and lhb-expressing cells, whereas GnRH-induced cytosolic Ca²⁺ responses were similar. Expression of Ca²⁺-activated K⁺ channels also differed with cell type and showed seasonal variation when measured in whole pituitary. The differential presence of these channels corresponds to the differences observed in membrane response to GnRH. We speculate that differences in ion channel expression levels may be involved in seasonal regulation of hormone secretion as well as the differential response to GnRH in LH- and FSH-producing gonadotropes, through differences in excitability and Ca²⁺ influx.

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.

Blockage of intermediate-conductance-Ca(2+) -activated K(+) channels inhibits progression of human endometrial cancer

Potassium (K(+)) channels have been implicated in proliferation of some tumor cells. However, whether K(+) channels are important to the pathogenesis of endometrial cancer (EC) remains unknown. In the present study, we report that intermediate-conductance Ca(2+)-activated K(+) (IKCa1) channels play a critical role in the development of EC. The expression of IKCa1 at both mRNA and protein levels in EC tissues was greatly increased than that in atypical hyperplasia and normal tissues. Treatment of EC cells with clotrimazole and TRAM-34, two agents known to inhibit IKCa1 channels, suppressed the proliferation of EC cells and blocked EC cell cycle at G(0)/G(1) phase. Similarly, downregulation of IKCa1 by siRNA against IKCa1 inhibited EC cell proliferation and arrested its cell cycle at G(0)/G(1) phase. A clotrimazole-sensitive K(+) current was induced in EC cells in response to the increased Ca(2+). The current density induced by Ca(2+) was greatly reduced by clotrimazole, TRAM-34, charybdotoxin or downregulation of IKCa1 by the siRNA against IKCa1. Furthermore, TRAM-34 and clotrimazole slowed the formation in nude mice of tumor generated by injection of EC cells. Our results suggest that increased activity of IKCa1 channel is necessary for the development of EC.