Guanine nucleotides regulate beta-adrenergic activation of Na-H exchange independently of receptor coupling to Gs.

Abstract

We have previously shown that the beta-adrenergic receptor (beta-AR) stimulates activity of the ubiquitous Na-H exchanger (NHE-1) independently of changes in cAMP accumulation and independently of a cholera toxin-sensitive stimulatory GTP-binding protein (Gs). To further investigate the potential role of a GTP-binding protein in coupling the beta-AR to NHE-1, we have used a recently available nonhydrolyzable GTP analog, "caged" guanosine 5'-O-(3-thiotriphosphate) (GTP gamma S), to study time-dependent effects of GTP on NHE-1 in intact cells. By monitoring intracellular pH (pHi) in cells loaded with the fluorescent pH-sensitive dye, 2,7-biscarboxyethyl-5(6)-carboxyfluorescein, we determined NHE-1 activity in primary cultures of canine enteric endocrine cells, which express an endogenous beta-AR, and in mouse L cells stably transfected with either the wild type hamster beta 2-AR or a mutant construct of the hamster beta 2-AR containing a deletion in amino acid residues 222-229. This D(222-229)beta 2-AR is functionally uncoupled from Gs and adenylylcyclase. In all three cell types, NaF and GTP gamma S induced an increase in activity of the exchanger, determined by assessing the rate of pHi recovery from an acute intracellular acid load (dpHi/dt). This increase in pHi recovery was dependent on extracellular Na+ and sensitive to the amiloride analog ethylisopropylamiloride. GTP gamma S, but not NaF, also increased beta-adrenergic stimulation of resting NHE-1 activity. The alkalinization in response to isoproterenol was reversed by propranolol in the absence, but not the presence, of GTP gamma S and was completely blocked by GDP beta S. The ability of guanine nucleotides to regulate beta-adrenergic activation of NHE-1 in cells expressing the mutant D(222-229)beta 2-AR suggests that functional coupling of the beta-AR to NHE-1 may be mediated by a GTP-binding protein other than Gs.