NOREPINEPHRINE INHIBITS A CA2+ CURRENT IN RAT SYMPATHETIC NEURONS VIA A G-PROTEIN

The effects of norepinephrine on a Ca2+ current from acutely isolated and short-term (24 h) cultured adult rat superior cervical ganglion neurons were studied using the whole-cell variant of the patch-clamp technique. Norepinephrine produced a rapid, reversible and concentration-dependent reduction of the Ca2+ current. Accurately timed applications of norepinephrine (3-mu-M) showed that the development of Ca2+ current inhibition was delayed by up to 11 s after application of norepinephrine. Internal 500-mu-M guanylyl-imidodiphosphate (Gpp(NH)p) or guanosine-5'-O-(3-thiotriphosphate) (GTP-gamma-S) decreased the Ca2+ current amplitude and induced a biphasic rising phase of the Ca2+ current. Under these conditions, the reduction of Ca2+ current amplitude by 3-mu-M norepinephrine was virtually abolished when compared with cells dialysed with GTP-containing internal solutions. Internal dialysis with solutions containing 2 mM guanosine-5'-O-(2-thiodiphosphate) (GDP-beta-S) increased the Ca2+ current amplitude and reduced the inhibition produced by 3-mu-M norepinephrine compared to cells dialysed with control internal solution. Treatment with 200 ng/ml pertussis toxin for 12-16 h greatly reduced the norepinephrine-induced Ca2+ current inhibition. Internal dialysis with solutions containing 500-mu-M cyclic adenosine 3',5'-monophosphate (cyclic AMP) and 3-isobutyl-1-methylxanthine had no significant effect on either the Ca2+ current inhibition by norepinephrine or the Ca2+ current amplitude. These results suggest that norepinephrine blocks a Ca2+ current in adult rat superior cervical ganglion neurons via a pertussis toxin-sensitive G-protein which is independent of intracellular cyclic AMP.