Page 94 - 72_03

JESÚS PINTOR AN. R. ACAD. NAC. FARM.

re-epithelialisation, the effects may be justified by the activation of
different P2Y receptors subtypes, according to our pharmacological
studies. It is clear that the pharmacology of P2 receptors is rather
complicated due to the lack of selective antagonists. A possible
picture of the receptors involved in the acceleration of re-
epithelialisation or its delay is maybe possible by studying the
behaviour of several purinergic agonists (26).

The increase in the rate of re-epithelialization has been observed
when Ap4A, and UTP were applied. This profile matches quite well
with that described by Lazarowski et al. (27), in which Ap4A and
UTP are the best agonists on the cloned P2Y2 receptor. A similar
P2Y2 profile has been previously described for corneal wound healing
in the in vivo experiments described in the previous paragraph. With
corneal epithelial cells in culture, it has been possible to detect some
dinucleotides which clearly reduce the rate of healing. Together with
the study of mononucleotides it was possible to obtain a profile for
those nucleotides reducing the rate of re-epithelialisation. A profile
with the ranking order Ap5A > Ap3A ? UDP suggests the involvement
of a P2Y6 receptor. This fact is altering the idea of the P2Y6 receptor
being a pyrimidinoceptor, sensitive to UTP and UDP. Nevertheless,
studies performed with diadenosine polyphosphates and P2Y6
receptors heterologously expressed in 1321N1 cells, demonstrate that
both Ap3A and Ap5A are agonists of the P2Y6 receptor although the
concentration required for the receptor stimulation are higher than
those of the best agonist, UDP (28). Also recently, the design of novel
dinucleoside polyphosphates with uridine as nucleoside moiety
(UpnU), demonstrate that some of them are quite effective activating
the P2Y6 receptor (29).

The involvement of metabotropic P2 receptors in corneal wound
healing has been reported by other groups (30-32). The presence of
P2Y2, P2Y4 and P2Y11, on corneal epithelial cells seem to be clear
from a pharmacological point of view. Discussion arises when the
presence of P2Y1 or P2Y6 is investigated. The assay of UDP together
with the enzyme hexokinase suggests the presence of a P2Y6 receptor
in these cells (30).

Concerning the second messenger system underlying the
activation of those receptors, it seems that both intracellular Ca2+

528