The presence of the P2Y2 (P(2U)-purinergic) receptor on the apical surface of airway tissue raises the possibility that aerosolized
UTP might be used therapeutically to induce Cl- secretion in individuals with
cystic fibrosis. However, the duration of the effects of
UTP may be limited by enzymatic degradation. We therefore have analyzed the metabolism 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 surface of HNE cells revealed the presence of an
ecto-nucleotidase(s) that hydrolyzed [3H]
UTP and [3H]
UDP with t1/2 values (at 1 microM
nucleotide) of 14 and 27 min, respectively. An ecto-
nucleoside diphosphokinase activity also was observed, which promoted conversion of [3H]
UDP into [3H]
UTP in the presence of
ATP. The effects of
UDP on [3H]
inositol phosphate accumulation, intracellular
calcium levels ([Ca2+]i), and Cl- secretory rates (I(Cl-)) were quantitated in HNE cells in the presence of
hexokinase and
glucose to ensure that no
UTP (or
ATP) contaminated
UDP solutions during the assays. Although
UDP does not activate the human
P2Y2 receptor, mucosal addition of
UDP promoted [3H]
inositol phosphate accumulation with an EC50 of 190 nM. Mucosal addition of
UTP stimulated [3H]
inositol phosphate accumulation with an EC50 of 280 nM. The maximal effects of mucosal
UDP on [3H]
inositol phosphate, [Ca2+]i, and I(Cl-) responses were approximately one-half of those observed with mucosal
UTP. Serosal application of
UTP promoted a 50% greater [3H]
inositol phosphate and
calcium response than did mucosal application of
UTP. In contrast,
UDP had no effect when added to the serosal medium. Repetitive mucosal applications of
UDP to HNE cells resulted in a progressive loss, i.e., desensitization, of the [Ca2+]i and I(Cl-) response to
UDP, whereas the corresponding responses to
UTP remained unchanged. Our results provide evidence for the existence of a
UDP receptor on HNE cells that is pharmacologically distinct from the
P2Y2 receptor. The relative stability of
UDP on the airway surface and the apparent predominant mucosal expression of this putative
UDP receptor make it a potential target for
cystic fibrosis treatment.