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

Abstract

Previous studies have demonstrated that endogenous norepinephrine (NE) inhibits carotid body (CB) sensory discharge, and the cellular actions of NE have been associated with inhibition of Ca2+ current in glomus cells. The purpose of the present study was to elucidate the characteristics and mechanism of NE inhibition of whole cell Ca2+ current isolated from rabbit CB glomus cells and to determine the type(s) of Ca2+ channel involved. NE (10 microM) 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 alpha2 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 betagamma subunit interaction with the alpha1 subunit of certain Ca2+ channels, which can be relieved by depolarizing steps. A depolarizing 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 cells where intracellular GTP was replaced by GDP-betaS. NE had little or no effect on Ca2+ current in cells dialyzed with GDP-betaS. To determine whether NE was inhibiting N- and/or P/Q-type channels, we applied NE in the presence of omega-conotoxin MVIIC (MVIIC). In the presence of 2.5 microM 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 control) at inhibiting the Ca2+ current in the presence of 2 microM nisoldipine, suggesting that NE was not inhibiting L-type channels. Further, NE inhibited a significantly larger proportion (47 +/- 6%) of the resistant Ca2+ current remaining in the presence of NISO and MVIIC. These results suggest that NE 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 involves a direct G protein betagamma interaction with this channel.