Up-regulation of High Voltage-activated Ca2+ Channels in GC Somatotropes After Long-term Exposure to Ghrelin and Growth Hormone Releasing Peptide-6

Abstract

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.

Ion Channels and Signaling in the Pituitary Gland

“…An immunocytochemical study also suggested that pituicytes express Ca v 1.2, 2.1, 2.2, 2.3, and 3.1 subunits (108). In GH somatotrophs, ghrelin and GH-releasing peptide-6 enhanced the expression of the Ca v 1.3 pore-forming ␣-subunit (109). In L␤T2 gonadotrophs, leptin increases L-type Ca v channel expression and GnRH-stimulated LH release (110).…”

Ghrelin reduces voltage-gated calcium currents in GH3 cells via cyclic GMP pathways

“…channel expression was increased and Ca 2? channel activity was enhanced in GC cells by long-term treatment (96 h) with ghrelin and GHRP-6 [10]. There maybe difference between long-term and short-term treatments.…”

“…Dominguez et al reported that high voltage-activated Ca 2? channel activity was enhanced in GC rat somatotrope cell line after long-term treatment (96 h) of cells by ghrelin and GHRP-6 [10]. Short-term modification of Ca 2?…”

Upregulation of Voltage-Gated Calcium Channel Cav1.3 in Bovine Somatotropes Treated with Ghrelin

“…Though much is known about the signal transduction underlying short-term regulation, but there is far less information on mechanisms that produce long-term effects. Although the molecular mechanisms of GH release are ill defined, some studies have shown that GHS-R sustained activation may be linked to increased voltage-gated Ca 2+ channel surface expression, resulting in enhanced electrical and secretory activity in clonal pituitary somatotropes [9, 25]. This latter type of regulation is likely to play a role in long-lasting forms of plasticity in the pituitary gland [26].…”

“…In the same way, the upregulation of voltage-gated HVA L-type calcium channel (Ca v 1.3) [9] and an upregulation of voltage-gated Na + channels by long-term activation of the GHS-R in this cell line have been observed [8]. Likewise, in goldfish pituitary cells Ghrelin-induced GH release involves voltage-sensitive calcium channels [10, 11].…”