Norepinephrine inhibits a toxin resistant Ca2+ current in carotid body glomus cells: Evidence for a direct G protein mechanism

Previous studies have demonstrated that endogenous norepinephrine (NE) inhi bits carotid body (CB) sensory discharge, and the cellular actions of NE ha ve been associated with inhibition of Ca2+ current in glomus cells. The pur pose of the present study was to elucidate the characteristics and mechanis m of NE inhibition of whole cell Ca2+ current isolated from rabbit CB glomu s cells and to determine the type(s) of Ca2+ channel involved. NE ( 10 mu M ) inhibited 24 +/- 2% (SE) of the macroscopic Ca2+ current measured at the end of a 25 ms pulse to 0 mV and slowed activation of the current. The alph a(2) adrenergic receptor antagonist, SK&F 86466, attenuated these effects. Inhibition by NE was fast and voltage-dependent i.e., maximal at -10 mV and then diminished with stronger depolarizations. This is characteristic of G protein beta gamma subunit interaction with the alpha(1) subunit of certai n Ca2+ channels, which can be relieved by depolarizing steps. A depolarizin g step (30 ms to +80 mV) significantly increased (14 +/- 1%) current in the presence of NE, whereas it had no effect before application of NE (1 +/- 1 %). To further test for the involvement of G proteins, NE was applied to ce lls where intracellular GTP was replaced by GDP-beta S. NE had little or no effect on Ca2+ current in cells dialyzed with GDP-beta S. To determine whe ther NE was inhibiting N- and/or P/Q-type channels, we applied NE in the pr esence of omega-conotoxin MVIIC (MVIIC). In the presence of 2.5 mu M MVIIC, NE was equally potent at inhibiting the Ca2+ current (23 +/- 4% vs. 23 +/- 4% in control), suggesting that NE was not exclusively inhibiting N- or P/ Q-type channels. NE was also equally potent (30 + 2% vs. 26 +/- 4% in contr ol) at inhibiting the Ca2+ current in the presence of 2 mu M nisoldipine, s uggesting that NE was not inhibiting L-type channels. Further, NE inhibited a significantly larger proportion (47 +/- 6%) of the resistant Ca2+ curren t remaining in the presence of NISO and MVIIC. These results suggest that N E inhibition of Ca2+ current in rabbit CB glomus cells is mediated in most part by effects on the resistant, non L-, N-, or P/Q-type channel and invol ves a direct G protein beta gamma interaction with this channel.