ALPHA-2-ADRENERGIC RECEPTORS ACCELERATE NA+/H+ EXCHANGE IN NEUROBLASTOMA-X GLIOMA-CELLS

Abstract

The regulation of cytoplasmic pH (pHi) was examined in neuroblastoma .times. glioma hybrid cell-line cells (NG108-15 cells) using 2,7-biscarboxyethyl-5(6)-carboxyfluorescein. The pHi of NG108-15 cells suspended in nominally HCO3--free, Na+-containing buffer could be reduced by the external application of acetate. The recovery of pHi to its resting vale was blocked by the removal of extracellular Na+, by the addition of extracellular H+, and by the addition of analogs of amiloride selective for inhibition of Na+/H+ exchange. The rate of recovery of pHi from acid load exhibited an ionic selectivity of Na+ > Li+ .mchgt. K+, and no recovery was observed in N-methyl-D-glucamine+. Tetrodotoxin and 4,4''-diisothiocyanostilbene-2,2''-disulfonic acid had no effect on early pHi recovery. These data suggest that Na+/H+ exchange accounts primarily for the recovery of pHi in NG108-15 cells under our experimental conditions. Na+/H+ exchange in NG108-15 cells was accelerated by .alpha.2-adrenergic receptors. Thus, (.sbd.)epinephrine, but not (+)epinephrine, elicited an intracellular alkalinization which was blocked by the .alpha.2-adrenergic receptor selective antagonist yohimbine but not by the .alpha.1-adrenergic receptor antagonist, prazosin, nor the .beta.-adrenergic antagonist, propranolol. Norepinephrine, clonidine, and the clonidine analog, UK-14304, also cause alkalinization of NG108-15 cells, whereas isoproterenol, a .beta.-adrenergic receptor agonist, and phenylephrine, a selective .alpha.1-adrenergic receptor agonist, did not. Manipulations that blocked Na+/H+ exchange blocked the ability of .alpha.2-adrenergic agonists to alkalinize the interior of NG108-15 cells without blocking the ability of these agonists to attenuate cAMP accumulation. These finding provide the first direct evidence of modulation of Na+/H+ exchange activity by a receptor linked to inhibition of adenylate cyclase and offer a possible mechanism whereby .alpha.2-adrenergic receptors might influence cellular activity apart from changes in cyclic nucleotide metabolism.