Modulation of calcium channels by norepinephrine in internally dialyzed avian sensory neurons

Abstract

Modulation of voltage-dependent Ca channels by norepinephrine (NE) was studied in chick dorsal root ganglion cells using the whole-cell configuration of the patch-clamp technique. Cells dialyzed with K+ and 2-10 mM EGTA exhibited Ca action potentials that were reversibly decreased in duration and amplitude by NE. Ca channel currents were isolated from other channel contributions by using: (a) tetrodotoxin (TTX) to block gNa, (b) internal K channel impermeant ions (Cs or Na/N-methylglucamine mixtures) as K substitutes, (c) external tetraethylammonium (TEA) to block K channels, (d) internal EGTA to reduce possible current contribution from Ca-activated channels. A marked decline (rundown) of Ca conductance was observed during continual dialysis, which obscured reversible NE effects. The addition of 2-5 mM MgATP to the intracellular solutions greatly retarded Ca channel rundown and permitted a clear assessment of modulatory drug effects. The inclusion of an intracellular creatine phosphate/creatine phosphokinase nucleotide regeneration system further stabilized Ca channels, which permitted recording of Ca currents for up to 3 h. NE reversibly decreased both steady state Ca currents and Ca tail currents in Cs/EGTA/MgATP-dialyzed cells. A possible role of several putative intracellular second messengers in NE receptor-Ca channel coupling was investigated. Cyclic AMP or cyclic GMP added to the intracellular solutions at concentrations several orders of magnitude higher than the Kd for activation of cyclic nucleotide-dependent protein kinases did not block or mask the expression of the NE-mediated decrease in gCa. Addition of internal EGTA to a final concentration of 10 mM also did not affect the expression of the NE response. These results suggest that neither cyclic AMP nor cyclic GMP nor Ca is acting as a second messenger coupling the NE receptor to the down-modulated Ca channel population.