Glucocorticoid modulation of calcium currents in growth hormone 3 cells

Upregulation of voltage-gated calcium channel cav1.3 in bovine somatotropes treated with ghrelin

Activation of the growth hormone (GH) secretagogue receptor (GHS-R) by synthetic GH releasing peptides (GHRP) or its endogenous ligand (Ghrelin) stimulates GH release. Though much is known about the signal transduction underlying short-term regulation, there is far less information on the mechanisms that produce long-term effects. In the current report, using an enzyme-linked immunosorbent assay for GH detection and whole-cell patch-clamp recordings, we assessed the long-term actions of such regulatory factors on voltage-activated Ca²⁺ currents in bovine somatotropes (BS) separated on a Percoll gradient and detected by immunohistochemistry. After 24 h of treatment with Ghrelin (10 nM) or GHRP-6 (100 nM) enhanced BS secretory activity; GH secretion stimulated by GHS through the activation of GHS-R because treatment with the antagonist of GHS-R (D-Lys3-GHRP-6, 10 μM) blocked the GH secretion, and the effect was dose and time dependent (24, 48, and 72 h). GH secretion stimulated by GHRP-6 was abolished by nifedipine (0.5 μM), a blocker of L-type HVA Ca²⁺ channels, and KN-62 (10 μM), an inhibitor of Ca²⁺/CaM-KII. After 72 h in culture, all recorded BS exhibited two main Ca²⁺ currents: a low voltage-activated (LVA; T-type) and a high voltage-activated (HVA; mostly dihydropyridine-sensitive L-type) current. Interestingly, HVA and LVA channels were differentially upregulated by Ghrelin. Chronic treatment with the GHS induced a significant selective increase on the Ba²⁺ current through HVA Ca²⁺ channels, and caused only a small increase of currents through LVA channels. The stimulatory effect on HVA current density was accompanied by an augment in maximal conductance with no apparent changes in the kinetics and the voltage dependence of the Ca²⁺ currents, suggesting an increase in the number of functional channels in the cell membrane. Lastly, in consistency with the functional data, quantitative real-time RT-PCR revealed transcripts encoding for the Ca_v1.2 and Ca_v1.3 pore-forming subunits of L-type channels. The treatment with Ghrelin significantly increased the Ca_v1.3 subunit expression, suggeting that the chronic stimulation of the GHS receptor with Ghrelin or GHRP-6 increases the number of voltage-gated Ca²⁺ channels at the cell surface of BS.

Up-regulation of high voltage-activated Ca(2+) channels in GC somatotropes after long-term exposure to ghrelin and growth hormone releasing peptide-6

Activation of the growth hormone (GH)-secretagogue receptor (GHS-R) by synthetic GH-releasing peptides (GHRP) or its endogenous ligand (ghrelin) stimulates GH release. Though much is known about the signal transduction underlying short-term regulation, there is far less information on mechanisms that produce long-term effects. In the current report, using whole-cell patch-clamp recordings, we assessed the long-term actions of such regulatory factors on voltage-activated Ca(2+) currents in GH-secreting cells derived from a rat pituitary tumour (GC cell line). After 96 h in culture, all recorded GC somatotropes exhibited two main Ca(2+) currents: a medium voltage-activated (MVA; T/R-type) and a high voltage-activated (HVA; mostly dihydropyridine-sensitive L-type) current. Interestingly, L- and non-L-type channels were differentially up-regulated by GHRP-6 and ghrelin. Chronic treatment with the GHS induced a significant selective increase on Ba(2+) current through HVA Ca(2+) channels, and caused only a modest increase of currents through MVA channels. Consistent with this, in presence of D-(Lys(3))-GHRP-6, a specific antagonist of the GHS-R, the increase in HVA Ca(2+) channel activity after chronic treatment with the GHS was abolished. The stimulatory effect on HVA current density evoked by the secretagogues was accompanied by an augment in maximal conductance with no apparent changes in the kinetics and the voltage dependence of the Ca(2+) currents, suggesting an increase in the number of functional channels in the cell membrane. Lastly, in consistency with the functional data, quantitative real-time RT-PCR revealed that the expression level of transcripts encoding for the Ca(V)1.3 pore-forming subunit of the L-type channels was significantly increased after chronic treatment of the GC cells with ghrelin.

LY294002 inhibits glucocorticoid-induced COX-2 gene expression in cardiomyocytes through a phosphatidylinositol 3 kinase-independent mechanism

Glucocorticoids induce COX-2 expression in rat cardiomyocytes. While investigating whether phosphatidylinositol 3 kinase (PI3K) plays a role in corticosterone (CT)-induced COX-2, we found that LY294002 (LY29) but not wortmannin (WM) attenuates CT from inducing COX-2 gene expression. Expression of a dominant-negative mutant of p85 subunit of PI3K failed to inhibit CT from inducing COX-2 expression. CT did not activate PI3K/AKT signaling pathway whereas LY29 and WM decreased the activity of PI3K. LY303511 (LY30), a structural analogue and a negative control for PI3K inhibitory activity of LY29, also suppressed COX-2 induction. These data suggest PI3K-independent mechanisms in regulating CT-induced COX-2 expression. LY29 and LY30 do not inhibit glucocorticoid receptor transactivity. Both compounds have been reported to inhibit Casein Kinase 2 activity and modulate potassium and calcium levels independent of PI3K, while LY29 has been reported to inhibit mammalian Target of Rapamycin (mTOR), and DNA-dependent Protein Kinase (DNA-PK). Inhibitor of Casein Kinase 2 (CK2), mTOR or DNA-PK failed to prevent CT from inducing COX-2 expression. Tetraethylammonium (TEA), a potassium channel blocker, and nimodipine, a calcium channel blocker, both attenuated CT from inducing COX-2 gene expression. CT was found to increase intracellular Ca(2+) concentration, which can be inhibited by LY29, TEA or nimodipine. These data suggest a possible role of calcium instead of PI3K in CT-induced COX-2 expression in cardiomyocytes.

Downregulation of voltage-gated sodium channels by dexamethasone in clonal rat pituitary cells

The effect of chronic dexamethasone (DEX) treatment (4-5 days) on Na(+) channel expression was examined in a clonal strain of rat pituitary cells secreting growth hormone (GH) and prolactin (GH3 cells). Using whole-cell patch clamp recording, we found that DEX (1 microM) induces an 80% decrease in Na(+) current density. No concomitant changes in current kinetics or voltage dependence of Na(+) channel function were detected. Instead, the decrease in current density was accompanied by a similar reduction in maximal Na(+) conductance, suggesting the loss of Na(+) channels from the plasma membrane. Accordingly, saxitoxin binding assays carried out on intact cells showed that the average number of Na(+) channels per cell is markedly decreased by DEX. Thus, this glucocorticoid inhibits the cell surface expression of Na(+) channels when chronically applied to GH3 cells.

L-type calcium channel activity regulates sodium channel levels in rat pituitary GH3 cells

1. The effects of chronic pharmacological modulation of L-type Ca2+ channel activity on the cell surface expression of Na+ channels were examined in GH3 cells. 2. Prolonged inhibition (4-5 days) of L-channels with nimodipine caused a 50-60 % decrease in the peak amplitude of whole-cell Na+ currents recorded with the patch-clamp technique. On the contrary, prolonged exposure to the L-channel agonist Bay K 8644 induced an approximately 2.5-fold increase in peak Na+ current. In both cases, there were only minor changes in cell capacitance and no significant changes in Na+ channel gating properties. 3. Measurements of the specific binding of radiolabelled saxitoxin to intact cells showed that nimodipine treatment reduced the number of cell surface Na+ channels, whereas treatment with Bay K 8664 produced the opposite effect. The dual regulation of Na+ channel abundance explained the mentioned changes in Na+ current amplitude. 4. Plasma membrane Na+ channels had a half-life of approximately 17 h both in control cells and in cells treated with Bay K 8644, as estimated from the rate of decay of peak Na+ current after inhibition of protein synthesis with cycloheximide. Actinomycin D, an inhibitor of gene transcription, and also cycloheximide, occluded the stimulatory effect of Bay K 8644 on Na+ current density when measured over a 24 h period. 5. These findings indicate that the entry of Ca2+ through L-type channels influences in a positive way the number of functional Na+ channels in GH3 cells, and suggest that Ca2+ influx stimulates either Na+ channel gene expression or the expression of a regulatory protein that promotes translocation of pre-assembled Na+ channels into the plasma membrane.

Dexamethasone-mediated inhibition of calcium transients and ACTH release in a pituitary cell line (AtT-20)

In the corticotroph-like murine pituitary tumor cell line, AtT-20, adrenocorticotropic hormone release is triggered by corticotropin-releasing hormone and is attenuated by the synthetic adrenal steroid dexamethasone. The precise mechanisms by which dexamethasone inhibits secretion are under investigation. We examined whether dexamethasone can modulate release via regulation of calcium homeostasis. More specifically, we have evaluated the effects of dexamethasone on calcium current, intracellular calcium concentration, and adrenocorticotropic hormone release. Using perforated patch-clamp and calcium imaging with fura PE3/AM, we found that dexamethasone decreases calcium current and intracellular calcium levels. The inhibition of current by dexamethasone is not, however, altered by the calcium channel antagonists nifedipine (L-type) or omega-agatoxin IVA (P/Q-type), despite the presence of these calcium channel subtypes in AtT-20 cells and the exclusive coupling of adrenocorticotropic hormone release to the L-type channel in these cells. We also evaluated the temporal relationship between dexamethasone-mediated inhibition of secretion and calcium influx. Whereas a prolonged (2 h) incubation with dexamethasone inhibits corticotropin-induced release by approximately 40%, a rapid (10 min) incubation (a time interval sufficient for dexamethasone-mediated inhibition of calcium transients) does not inhibit release. These data suggest, therefore, that dexamethasone does, indeed, modulate calcium homeostasis in AtT-20 cells, but that this effect is not responsible for its inhibition of secretion.

Replication of bovine respiratory syncytial virus in bovine and ovine peripheral blood lymphocytes and monocytes and monocytic cell lines

The present study compared the replication of bovine respiratory syncytial virus (BRSV) in bovine and ovine peripheral blood mononuclear cells, ovine and bovine monocytic cell lines and ovine alveolar macrophages. Low titres of virus were detected in ovine and bovine lymphocytes and monocytes 24-96 h post-exposure to the virus but there was no apparent replication of the virus in ovine alveolar macrophages during the culture period. The virus replicated to higher but statistically insignificant titres in ovine and bovine peripheral blood monocytes than in lymphocytes, with lymphocytes yielding peak titres significantly earlier. The secondary cell lines obtained from ovine liver and bone marrow also supported the replication of BRSV to high titres. The titres of BRSV in ovine and bovine lymphocytes and monocytes were significantly lower than in secondary cell lines. The addition of human recombinant tumour necrosis factor alpha after exposure to the virus or pre-incubation of ovine or bovine monocytic cells with either human recombinant interleukin 2 or phorbol myristate acetate before exposure to BRSV, did not significantly affect virus titre. Pre-incubation of cells with indomethacin or actinomycin significantly lowered virus titre (p < 0.05).

Effect of dexamethasone on voltage-gated Ca2+ channels and cytosolic Ca2+ in rat chromaffin cells

The present study examined whether the synthetic glucocorticoid dexamethasone (DEX) can modulate voltage-gated Ca2+ channel (VGCC) activity, and as a consequence agonist-induced increases in cytosolic Ca2+, in cultured rat adrenal medullary chromaffin (RAMC) cells. Exposure to 1 microM DEX for 48 h significantly increased peak VGCC current (delta +140%). DEX treatment also significantly potentiated the increases in cytosolic Ca2+ in response to submaximal stimulatory concentrations of KCl (delta +64%) and nicotine (delta +32%). The Ca2+ channel agonist BAY K-8644 increased both VGCC current (delta +109%) and potentiated the KCl-stimulated increase in cytosolic Ca2+ (delta +35%) to a comparable extent to that seen with DEX. These data suggest that DEX treatment increases VGCC activity, and that this increased Ca2+ influx leads to potentiation of agonist-induced increases in cytosolic Ca2+ in RAMC cells.

Dexamethasone rapidly increases calcium channel subunit messenger RNA expression and high voltage-activated calcium current in clonal pituitary cells

Glucocorticoid hormones increase voltage-gated Ca(2)+ current density in clonal pituitary cells. To test whether these steroids might stimulate expression of Ca(2)+ channel genes, messenger RNase protection assays were used to measure alpha IC and alpha ID RNAs that encode pore-forming subunits of L-type Ca2+ channels. We show here that dexamethasone rapidly increases alpha IC messenger RNA expression without affecting alpha ID messenger RNA level. This up-regulation of channel messenger RNA is also produced by natural glucocorticoids and is blocked by the glucocorticoid antagonist Ru48386. The up-regulation of the channel subunit messenger RNA expression is associated with an increase in high voltage-activated Ca(2)+ current density. Thus, glucocorticoids may produce a long-term effect on Ca(2)+ homeostasis in clonal pituitary cells by differentially regulating expression of Ca(2)+ channel subunit genes.