ROLE OF G-PROTEINS IN ALPHA(1)-ADRENERGIC INHIBITION OF THE BETA-ADRENERGICALLY ACTIVATED CHLORIDE CURRENT IN CARDIAC MYOCYTES

alpha(1)-Adrenergic receptor stimulation can inhibit the Cl- current a ctivated by beta-adrenergic receptor agonists in guinea-pig ventricula r myocytes. We investigated the role of G proteins in mediating this t ype of alpha-adrenergic response. The combined alpha- and beta-adrener gic agonist norepinephrine (NE) activated the Cl- current with an EC50 value of 53 nM. Preincubation of myocytes with PTX decreased the EC50 value for NE activation of the Cl- current to 5.9 nM, and addition of the alpha(1)-adrenergic receptor antagonist prazosin did not cause an y further change in sensitivity to NE. These results suggest that the alpha(1)-adrenergic inhibition of beta-adrenergic responses is mediate d through a PTX-sensitive G protein. However, PTX pretreatment also in creased the sensitivity of the Cl- current to the selective beta-adren ergic agonist isoproterenol (Iso), which indicates that the PTX treatm ent increases the sensitivity to beta-adrenergic stimulation alone and that this could account for the PTX-induced change in sensitivity to NE. Consistent with this idea, the selective alpha(1)-adrenergic recep tor agonist methoxamine was still able to inhibit the Cl- current acti vated by Iso in PTX-treated myocytes. However, the sensitivity to meth oxamine was significantly decreased. In control cells, the Cl- current activated by 30 nM Iso was inhibited by methoxamine with an EC50 valu e of 8.3 mu M, but in PTX-treated cells, the EC50 value was 284 mu M. The EC50 for methoxamine inhibition was similarly increased when the C l- current was activated by 300 nM Iso. These data suggest that the ef fects of PTX on alpha(1)-adrenergic responses can actually be explaine d by changes in the sensitivity to beta-adrenergic stimulation. To ver ify the role for a G protein in mediating the inhibitory alpha(1)-adre nergic response, we examined the effect of methoxamine on the Cl- curr ent activated in cells dialyzed with the nonhydrolyzable GTP analogue guanosine-5'-O-(3-thio)triphosphate. Pre-exposure to methoxamine resul ted in an attenuated response upon subsequent exposure to Iso alone. W e conclude that alpha(1)-adrenergic inhibition of beta-adrenergic resp onses is mediated by a G protein-dependent mechanism that appears to b e PTX-insensitive.