G-protein activation decreases isoflurane inhibition of N-type Ba2+ currents

Abstract

Background: G-protein activation mediates inhibition of N-type Ca2+ currents. Volatile anesthetics affect G-protein pathways at various levels, and activation of G-proteins has been shown to increase the volatile anesthetic potency for inhibiting the electrical-induced contraction in ileum. The authors investigated whether isoflurane inhibition of N-type Ba2+ currents was mediated by G-protein activation. Methods: N-type Ba2+ currents were measured in the human neuronal SH-SY5Y cell line by using the whole cell voltage-clamp method. Results: Isoflurane was found to have two effects on N-type Ba2+ currents. First, isoflurane reduced the magnitude of N-type Ba2+ currents to a similar extent (IC50 ∼ 0.28 mM) in the absence and presence of GDPβS (a nonhydrolyzable GDP analog). Interestingly, GTPγS (a nonhydrolyzable GTP analog and G-protein activator) in a dose-dependent manner reduced the isoflurane block; 120 μM GTPγS completely eliminated the block of 0.3 mM isoflurane and reduced the apparent isoflurane potency by ∼2.4 times (IC50 ∼ 0.68 mM). Pretreatment with pertussis toxin or cholera toxin did not eliminate the GTPγS-induced protection against the isoflurane block. Furthermore, isoflurane reduced the magnitude of voltage-dependent G-protein-mediated inhibition of N-type Ba2+ currents, and this effect was eliminated by pretreatment with pertussis toxin or cholera toxin. Conclusions: It was found that activation of G-proteins in a neuronal environment dramatically reduced the isoflurane potency for inhibiting N-type Ba2+ currents and, in turn, isoflurane affected the G-protein regulation of N-type Ba2+ currents.