Up‐regulation of L‐ and non‐L, non‐N‐type Ca2+ channels by basal and stimulated protein kinase C activation in insulin‐secreting RINm5F cells

[摘要]

We studied the effect of protein kinase C (PKC) inhibition and activation on voltage-dependent Ca²⁺ channels in rat insulinoma RINm5F cells. PKC down-regulation by chronic (24 h) treatment with the PKC activator phorbol 12-myristate 13-acetate (PMA) reduced by about 60% the Ba²⁺ currents through L- and non-L, non-N-type high-voltage-activated Ca²⁺ channels, indicating that PKC tonically up-regulates the two main Ca²⁺ channel subtypes of RINm5F cells under basal conditions. Consistently, PKC activation by acute PMA application caused only a modest increase (average 23%) of Ba²⁺ currents in a minority of cells (24%). L- and non-L, non-N-type channels were differentially up-regulated by either basal or stimulated PKC activation. Acute up-regulation was predominant on L-type channels and caused an I/V shift of the Ba²⁺ currents in the hyperpolarizing direction. Non-L, non-N-type channels were less affected by acute PMA application, possibly reflecting a more effective tonic PKC up-regulatory action. Unexpectedly, the increase of Ba²⁺ currents during acute PMA application was followed by a progressive current decrease, which was also observed in isolation in another 24% of the cells and could be ascribed to PKC-induced ATP depletion, rather than to a direct effect of PKC on Ca²⁺ channels. We also provide evidence that PKC-mediated phosphorylation is not involved in the G-protein-mediated noradrenergic modulation of Ca²⁺ channels in RINm5F cells.

[学科分类] 生物化学/生物物理

[关键词] Ca2+ channel;Protein kinase C;Channel phosphorylation;Noradrenaline;RINm5F cells;Insulinoma