Insulin-mediated upregulation of T-type Ca2+ currents in GH3 cells is mediated by increased endosomal recycling and incorporation of surface membrane Cav3.1 channels

Growth factors and hormones have both short- and long-term regulatory effects on the functional expression of voltage gated Ca2+ (Ca-v) channels. In particular, it has been reported that chronic treatment with insulin upregulates T-type channel membrane expression, leading to an increase in current density in clonal pituitary GH(3) cells. Though this regulatory action may result from alterations in gene expression, recent studies have demonstrated also that endosomal trafficking provides a mechanism for dynamic changes in Ca-v channel membrane density. Therefore, in the present work we sought to determine whether the actions of insulin on T-type channel functional expression are mediated by transcriptional and/or post-transcriptional mechanisms. Using real-time RT-PCR and semi-quantitative western blot we found no changes after treatment in the transcript and protein levels of Ca(v)3.1 the T-type channel isoform preferentially expressed in the GH(3) cells. Consistent with this, transcriptional studies using a luciferase reporter assay suggested that insulin treatment does not affect the Ca(v)3.1 promoter activity. In contrast, patch-clamp recordings on HEK-293 cells stably expressing Ca(v)3.1 channels showed a significant increase in current density after treatment, suggesting that the effects of insulin may require post-transcriptional regulation. In line with this, disruption of the endosomal recycling pathway using Brefeldin A and a dominant negative mutant of the small GTPase Rab11a prevented the stimulatory effects of insulin on Ca(v)3.1 channels in HEK-293 cells. These results may help explain the effects of insulin on T-type channels and contribute to our understanding of how endosomal recycling impacts the functional expression of Ca-v channels. (C) 2012 Elsevier Ltd. All rights reserved.