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

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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 Ca2+ 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 Ca2+ 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 Ba2+ current through HVA Ca2+ channels, and caused only a modest increase of currents through MVA channels. Consistent with this, in presence of D-(Lys3)-GHRP-6, a specific antagonist of the GHS-R, the increase in HVA Ca2+ 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 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 that the expression level of transcripts encoding for the CaV1.3 pore-forming subunit of the L-type channels was significantly increased after chronic treatment of the GC cells with ghrelin.

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Acknowledgements

This work was supported by a grant from The National Council for Science and Technology (Conacyt), Mexico, to E.M. B.D. was the recipient of a doctoral fellowship from Conacyt. We thank Dr. Mario Bermúdez for help with quantitative PCR analysis and interpretation and BS. Lissette Ponce for assistance in the preparation of the manuscript.

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Correspondence to Eduardo Monjaraz.

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Dominguez, B., Avila, T., Flores-Hernandez, J. et al. Up-regulation of High Voltage-activated Ca2+ Channels in GC Somatotropes After Long-term Exposure to Ghrelin and Growth Hormone Releasing Peptide-6. Cell Mol Neurobiol 28, 819–831 (2008). https://doi.org/10.1007/s10571-007-9234-1

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