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Abstract

The effects of norepinephrine on resting intracellular pH (pH;) and on the rate of recovery of pH; following an imposed intracellular acid load were investigated in acutely dissociated adult rat hippocampal CA1 neurons loaded with the fluorescent hydrogen ion indicator, 2',7'-bis-(2-carboxyethyl)-5(6)-carboxyfluorescein. During perfusion with HC037C02-free, HEPES-buffered media, the dominant acid extrusion mechanism utilized by these neurons was an amiloride-insensitive variant of the Na7H+ exchanger. In HC037C02-buffered saline, acid extrusion was supplemented by the activity of a 4,4'-diisothiocyanatostilbene-2,2'-disulphonic acid-sensitive, Na+ - dependent HCOy/Q" exchanger which also contributed to the maintenance of resting pH: . Norepinephrine 10 uM evoked a rise in resting pH; that was long-lasting and increased the rate of pH; recovery from an imposed intracellular acidification in both the absence and presence of external HCGy. The effects of norepinephrine were attenuated by the non-selective R-adrenergic receptor antagonist, propranolol, and were mimicked both by the mixed (3-adrenergic receptor agonist isoproterenol and by the selective p,- and p2- adrenergic receptor agonists dobutamine and terbutaline, respectively, cc-adrenoceptor agonists affected neither steady-state pLf nor the rate of pHj recovery from an imposed acid load. The norepinephrine-induced increase in resting pH; and in the rate of pH; recovery from an imposed acid load were independent of changes in intracellular free [Ca2+] and were mimicked by the acute application of cholera toxin, forskolin or SpcAMPS in the absence of norepinephrine. Conversely, the effects of norepinephrine were occluded by 2',5'-dideoxyadenosine, Rp-cAMPS and H-89. The results indicate not only that norepinephrine can augment the activity of the Na7H+ exchanger through the activation of p-adrenergic receptors, Gs a , cAMP, and protein kinase A but also that the second messengers themselves are able to alter the activity of the Na7H+ exchanger. The ability of norepinephrine to alter pH, via modulation of the activity of the dominant acid extrusion mechanism present in rat hippocampal CA1 neurons may constitute a mechanism to dissipate rapidly intracellular acid loads produced by the actions of other neurotransmitters such as glutamate and may subserve some of the reported P-adrenergic receptor-mediated effects of norepinephrine in the CA1 region of the hippocampus.