Effects of isoflurane on the expressed Ca v2.2 currents in Xenopus oocytes depend on the activation of protein kinase C δ and its phosphorylation sites in the Ca v2.2α 1 subunits

Abstract

The effects of isoflurane on the modulation of two neuronal voltage-gated calcium channels (Ca v; Ca v2.1 and 2.2) by protein kinase C (PKC) isozymes βII, ε or δ and their combination were examined. Ca v2.1α1 or Ca v2.2α1 with β1b and α2δ subunits were expressed in Xenopus oocytes and the currents ( I Ba) were recorded by two-electrode voltage clamp. Isoflurane (0.70 mM) decreased both Ca v2.1 and 2.2 currents by 20–35% and also caused translocation of PKCδ to the membrane. Compared to the wild type (WT), isoflurane caused greater inhibition of Ca v2.2 currents in the absence of stimulatory PKC sites (Thr-422, Ser-1757, Ser-2108, Ser-2132) and in the presence of inhibitory PKC site (Ser-425). In contrast, isoflurane caused less inhibition of I Ba in the oocytes expressing S425A, the inhibitory site mutant, compared to WT. PKCδ by itself did not modulate Ca v2.2 currents, but potentiated these currents in the presence of isoflurane. PKCε increased Ca v2.2 currents either alone or in combination with isoflurane. Ca v2.1 currents were not modulated by phorbol-12-myristate, 13-acetate (PMA) or acetyl-β-methylcholine (MCh), activators of PKC. Yet the presence of isoflurane caused PMA (but not MCh) to enhance Ca v2.1 currents. PKCβII and PKCε isozymes activated by PMA, did not alter Ca v2.1 currents. However, in the presence of isoflurane, these two isozymes together potentiated Ca v2.1 currents. The variable responses of Ca v2.1 currents to PKCβII and PKCε and Ca v2.2 currents to PKCδ in the presence of isoflurane may be due to increased affinity or accessibility of these isozymes to their Ser/Thr PKC sites of Ca vα1 subunits.