Inhalational anesthetic actions on voltage-gated ion currents of bovine adrenal chromaffin cells.

The effects of the inhalational anesthetics halothane and isoflurane on voltage-gated ionic currents in bovine adrenal chromaffin cells were examined using patch-clamp techniques. Halothane (1.5% atmospheres, 0.90 mM in solution) and isoflurane (2.5% atmospheres, 0.78 mM in solution) diminished the Ca(2+)-dependent K+ current (IK(Ca)) "hump" by 52 +/- 3% (n = 16 cells) and 40 +/- 4% (n = 6), respectively. These concentrations of halothane and isoflurane had virtually no effect on the rapid inward Na+ current and exerted only minor effects on outward K+ currents in the absence of external Ca2+. The effectiveness of halothane (0.90 mM) was reduced by increasing the external Ca2+ concentration ([Ca2+]o); IK(Ca) was decreased by 64 +/- 4% (n = 4) with 1 mM [Ca2+]o but by only 37 +/- 4% (n = 4) with 10 mM [Ca2+]o. Voltage-activated Ca2+ current (ICa), isolated by intracellular perfusion with Cs+ and tetraethylammonium in the presence of 15-30 microM external tetrodotoxin, was insensitive to 0.90 mM halothane and partially reduced by 0.78 mM isoflurane (24 +/- 7%, n = 9). Halothane at 1.4 mM decreased ICa by approximately 30 +/- 5% (n = 7), with a slight enhancement of the rate of activation and no change in the voltage dependence of activation. Higher levels of halothane produced a clear enhancement of the rates of both activation and deactivation of ICa. Detailed examination of the Na+ current showed that excessive levels of halothane (approximately 3 mM) decreased the peak amplitude by 26 +/- 7% (n = 6) and enhanced the rates of activation and inactivation, while shifting the voltage dependence of activation by 8.3 +/- 2.3 mV (n = 3) and the steady state inactivation by 6 mV towards hyperpolarized levels. Excised membrane patch-clamp measurements were performed to assess the direct effect of halothane on the large conductance "BK" channels underlying IK(Ca). Halothane (0.90 mM) with approximately 900 nM free Ca2+ at the cytoplasmic side of the patch decreased the probability of opening (Po) by 42 +/- 10% (n = 8), with no alteration in the open channel amplitude. With the cytoplasmic [Ca2+] reduced to approximately 100 nM, halothane diminished po by 65 +/- 8% (n = 8), suggesting that Ca2+ can antagonize the effect of halothane on BK channels, consistent with whole-cell experiments. Although these results show that several channel types in chromaffin cells are affected by inhalational agents, BK channels appear to be more sensitive than other voltage-dependent channels.(ABSTRACT TRUNCATED AT 400 WORDS)