Role of K(+) channels in frequency regulation of spontaneous action potentials in rat pituitary GH(3) cells

High Capability of Pentagalloylglucose (PGG) in Inhibiting Multiple Types of Membrane Ionic Currents

Pentagalloyglucose (PGG, penta-O-galloyl-β-d-glucose; 1,2,3,4,6-pentagalloyl glucose), a pentagallic acid ester of glucose, is recognized to possess anti-bacterial, anti-oxidative and anti-neoplastic activities. However, to what extent PGG or other polyphenolic compounds can perturb the magnitude and/or gating of different types of plasmalemmal ionic currents remains largely uncertain. In pituitary tumor (GH₃) cells, we found out that PGG was effective at suppressing the density of delayed-rectifier K⁺ current (I_K(DR)) concentration-dependently. The addition of PGG could suppress the density of proton-activated Cl⁻ current (I_PAC) observed in GH₃ cells. The IC₅₀ value required for the inhibitory action of PGG on I_K(DR) or I_PAC observed in GH₃ cells was estimated to be 3.6 or 12.2 μM, respectively, while PGG (10 μM) mildly inhibited the density of the erg-mediated K⁺ current or voltage-gated Na⁺ current. The presence of neither chlorotoxin, hesperetin, kaempferol, morin nor iberiotoxin had any effects on I_PAC density, whereas hydroxychloroquine or 4-[(2-butyl-6,7-dichloro-2-cyclopentyl-2,3-dihydro-1-oxo-1H-inden-5yl)oxy] butanoic acid suppressed current density effectively. The application of PGG also led to a decrease in the area of voltage-dependent hysteresis of I_PAC elicited by long-lasting isosceles-triangular ramp voltage command, suggesting that hysteretic strength was lessened in its presence. In human cardiac myocytes, the exposure to PGG also resulted in a reduction of ramp-induced I_K(DR) density. Taken literally, PGG-perturbed adjustment of ionic currents could be direct and appears to be independent of its anti-oxidative property.

Effectiveness in the inhibition of dapagliflozin and canagliflozin on M-type K+ current and α-methylglucoside-induced current in pituitary tumor (GH3) and pheochromocytoma PC12 cells

Dapagliflozin (DAPA) or canagliflozin (CANA), Na⁺-dependent glucose co-transporter type 2 (SGLT2) inhibitors, were used for treatment of type II diabetes mellitus. Addition of DAPA or CANA suppressed M-type K⁺ current (I_K(M)) in pituitary tumor (GH₃) and pheochromocytoma PC12 cells. The IC₅₀ value for DAPA- or CANA-mediated inhibition of I_K(M) in GH₃ cells was 0.11 or 0.42 μM, respectively. The presence of DAPA (0.1 μM) shifted the steady-state activation of I_K(M) to less depolarized potential without changing the gating charge of the current. During high-frequency depolarizing pulses, I_K(M) magnitude was reduced by DAPA; however, DAPA-induced block of I_K(M) remained effective. The amplitude of neither erg-mediated K⁺ current nor hyperpolarization-activated cation current in GH₃ cells was modified in the presence of 1 μM DAPA. Alternatively, addition of DAPA, CANA, phlorizin or chlorotoxin effectively suppressed α-methylglucoside-(αMG-) induced current (I_αMG) in GH₃ cells, albeit inability of tefluthrin (activator of I_Na) to suppress this current. DAPA shifted the charge-voltage relation of presteady-state I_αMG in a rightward and downward direction with no change in the gating charge of the I_αMG. Under current-clamp recordings, subsequent additions of DAPA, but still in the continued presence of αMG, increased the firing rate of spontaneous action potentials stimulated by αMG. Our results suggested that activity of SGLT was expressed functionally in GH₃ and PC12 cells. Therefore, inhibitory actions of DAPA or CANA on the amplitude and gating of I_K(M) might provide a yet unidentified mechanism through which the SGLT1 or SGLT2 activity were attenuated in unclamped cells occurring in vivo.

Bisoprolol, Known to Be a Selective β₁-Receptor Antagonist, Differentially but Directly Suppresses IK(M) and IK(erg) in Pituitary Cells and Hippocampal Neurons

Bisoprolol (BIS) is a selective antagonist of β₁ adrenergic receptors. We examined the effects of BIS on M-type K⁺ currents (IK(M)) or erg-mediated K⁺ currents (IK(erg)) in pituitary GH3, R1220 cells, and hippocampal mHippoE-14 cells. As GH₃ cells were exposed to BIS, amplitude of IK(M) was suppressed with an IC50 value of 1.21 μM. The BIS-induced suppression of IK(M) amplitude was not affected by addition of isoproterenol or ractopamine, but attenuated by flupirtine or ivabradine. In cell-attached current, BIS decreased the open probability of M-type K⁺ (KM) channels, along with decreased mean opening time of the channel. BIS decreased IK(erg) amplitude with an IC50 value of 6.42 μM. Further addition of PD-118057 attenuated BIS-mediated inhibition of IK(erg). Under current-clamp conditions, BIS depolarization increased the firing of spontaneous action potentials in GH₃ cells; addition of flupirtine, but not ractopamine, reversed BIS-induced firing rate. In R1220 cells, BIS suppressed IK(M); subsequent application of ML-213(Kv7.2 channel activator) reversed BIS-induced suppression of the current. In hippocampal mHippoE-14 neurons, BIS inhibited IK(M) to a greater extent compared to its depressant effect on IK(erg). This demonstrated that in pituitary cells and hippocampal neurons the presence of BIS is capable of directly and differentially suppressing IK(M) and IK(erg), despite its antagonism of β₁-adrenergic receptors.

Functional expression of ERG1 potassium channels in rat alveolar macrophages

Alveolar macrophages (AMs) play a vital role in lung immunity. The recent studies demonstrated that potassium channels were associated with macrophage functions, such as activation, migration and cytokines secretion. However, less is known regarding the expression and function of ERG channels in AMs. Our study showed that ERG1 channel expressed in rat alveolar macrophage, and the expression level was increased when AMs were stimulated with LPS. Furthermore, blockade of ERG1 channels with E4031 down-regulated the mature of ERG1 protein, inhibited NF-κB translocation into the nucleus, and reduced LPS-stimulated IL-6 and IL-1β secretion. These results imply that ERG1 channels are functionally expressed in rat alveolar macrophages and play an important role in inflammatory response.

Calcium and other signalling pathways in neuroendocrine regulation of somatotroph functions

Relative to mammals, the neuroendocrine control of pituitary growth hormone (GH) secretion and synthesis in teleost fish involves numerous stimulatory and inhibitory regulators, many of which are delivered to the somatotrophs via direct innervation. Among teleosts, how multifactorial regulation of somatotroph functions are mediated at the level of post-receptor signalling is best characterized in goldfish. Supplemented with recent findings, this review focuses on the known intracellular signal transduction mechanisms mediating the ligand- and function-specific actions in multifactorial control of GH release and synthesis, as well as basal GH secretion, in goldfish somatotrophs. These include membrane voltage-sensitive ion channels, Na(+)/H(+) antiport, Ca(2+) signalling, multiple pharmacologically distinct intracellular Ca(2+) stores, cAMP/PKA, PKC, nitric oxide, cGMP, MEK/ERK and PI3K. Signalling pathways mediating the major neuroendocrine regulators of mammalian somatotrophs, as well as those in other major teleost study model systems are also briefly highlighted. Interestingly, unlike mammals, spontaneous action potential firings are not observed in goldfish somatotrophs in culture. Furthermore, three goldfish brain somatostatin forms directly affect pituitary GH secretion via ligand-specific actions on membrane ion channels and intracellular Ca(2+) levels, as well as exert isoform-specific action on basal and stimulated GH mRNA expression, suggesting the importance of somatostatins other than somatostatin-14.

Three native somatostatin isoforms differentially affect membrane voltage-sensitive ion currents in goldfish somatotrophs

Message encoding for three isoforms of somatostatin (SS) peptides, SS-14, goldfish brain (gb)SS-28 and [Pro²]SS-14, are expressed in goldfish hypothalamus and pituitary tissues. All three native goldfish SSs are active in reducing basal and stimulated growth hormone (GH) responses in cultured goldfish pituitary cells, although with different potencies and efficacies. In the present study, we examined the effects of these three endogenous SSs on electrophysiological properties of goldfish somatotrophs and their physiological relevance. Voltage-sensitive K+ , Ca²+ and Na+ channels in identified goldfish somatotrophs in primary culture were isolated using whole-cell, amphotericin B-perforated patch-clamp techniques. None of the three SSs affected Na+ currents but all three SSs increased maximal K+ current magnitude, with SS-14 being the most effective. [Pro²]SS14 did not affect Ba²+ currents through voltage-sensitive Ca²+ channels but SS14 decreased the magnitude of early and late Ba²+ currents, whereas gbSS-28 reduced that of the late Ba²+ current. Under current-clamp conditions, SS14 and gbSS28 attenuated evoked action potential magnitudes by 34% and 18%, respectively, although [Pro²]SS14 had no effects. However, all three SSs decreased basal intracellular Ca²+ levels ([Ca²+ ](i)) and suppressed basal GH release. These data suggest that, although the ability of SS-14 and gbSS-28 to decrease basal [Ca²+](i) and GH release can be explained, at least in part, by their attenuating effects on cell excitability and current flow through voltage-sensitive Ca²+ channels, [Pro²]SS14-induced reduction in GH responses and [Ca²+](i) cannot be explained by changes in Ca²+ channel properties.

Mechanism of Spontaneous and Receptor-Controlled Electrical Activity in Pituitary Somatotrophs: Experiments and Theory

Cultured pituitary somatotrophs release growth hormone in response to spontaneous Ca2+ entry through voltage-gated calcium channels (VGCCs) that is governed by plateau-bursting electrical activity and is regulated by several neurohormones, including GH-releasing hormone (GHRH) and somatostatin. Here we combine experiments and theory to clarify the mechanisms underlying spontaneous and receptor-controlled electrical activity. Experiments support a role of a Na+-conducting and tetrodotoxin-insensitive channel in controlling spontaneous and GHRH-stimulated pacemaking, the latter in a cAMP-dependent manner; an opposing role of spontaneously active inwardly rectifying K+ ( Kir) channels and G-protein-regulated Kir channels in somatostatin-mediated inhibition of pacemaking; as well as a role of VGCCs in spiking and large conductance (BK-type) Ca2+-activated K+ channels in plateau bursting. The mathematical model is compatible with a wide variety of experimental data involving pharmacology and extracellular ion substitution and supports the importance of constitutively active tetrodotoxin-insensitive Na+ and Kir channels in maintaining spontaneous pacemaking in pituitary somatotrophs. The model also suggests that these channels are involved in the up- and downregulation of electrical activity by GHRH and somatostatin. In the model, the plateau bursting is controlled by two functional populations of BK channels, characterized by distance from the VGCCs. The rapid activation of the proximal BK channels is critical for the establishment of the plateau, whereas slow recruitment of the distal BK channels terminates the plateau.