UDP ACTIVATES A MUCOSAL-RESTRICTED RECEPTOR ON HUMAN NASAL EPITHELIAL-CELLS THAT IS DISTINCT FROM THE P2Y(2) RECEPTOR

The presence of the P2Y(2) (P-2U-purinergic) receptor on the apical su rface of airway tissue raises the possibility that aerosolized UTP mig ht be used therapeutically to induce Cl- secretion in individuals with cystic fibrosis, However, the duration of the effects of UTP may be l imited by enzymatic degradation, We therefore have analyzed the metabo lism of UTP and its metabolite UDP on polarized human nasal epithelium (HNE), and have compared the pharmacological activities of these two uridine nucleotides. HPLC analysis of medium bathing the mucosal surfa ce of HNE cells revealed the presence of an ecto-nucleotidase(s) that hydrolyzed [H-3]UTP and [H-3]UDP with t1/2 values (at 1 mu M nucleotid e) of 14 and 27 min, respectively, An ecto-nucleoside diphosphokinase activity also was observed, which promoted conversion of [H-3]UDP into [H-3]UTP in the presence of ATP. The effects of UDP on [H-3]inositol phosphate accumulation, intracellular calcium levels ([Ca2+](i)), and Cl- secretory rates (ICl-) were quantitated in HNE cells in the presen ce of hexokinase and glucose to ensure that no UTP (or ATP) contaminat ed UDP solutions during the assays, Although UDP does not activate the human P2Y(2) receptor, mucosal addition of UDP promoted [H-3]inositol phosphate accumulation with an EC(50) of 190 nM, Mucosal addition of UTP stimulated [H-3]inositol phosphate accumulation with an EC(50) of 280 nM, The maximal effects of mucosal UDP on [H-3]inositol phosphate, [Ca2+](i), and ICl- responses were approximately one-half of those ob served with mucosal UTP. Serosal application of UTP promoted a 50% gre ater [H-3]inositol phosphate and calcium response than did mucosal app lication of UTP. In contrast, UDP had no effect when added to the sero sal medium, Repetitive mucosal applications of UDP to HNE cells result ed in a progressive loss, i.e., desensitization, of the [Ca2+](i) and ICl- response to UDP, whereas the corresponding responses to UTP remai ned unchanged, Our results provide evidence for the existence of a UDP receptor on HNE, cells that is pharmacologically distinct from the P2 Y(2) receptor, The relative stability of UDP on the airway surface and the apparent predominant mucosal expression of this putative UDP rece ptor make it a potential target for cystic fibrosis treatment.