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Circadian rhythm is interesting physiological mechanism to maintain homeostasis of whole body including eyes. Especially IOP regulation during 24 hours is one of the important factors to control IOP in glaucoma. Regulation of diurnal variation in the peripheral tissues has not been clarified, but it is known that the aqueous concentration of PGF2α indicates a circadian rhythm with a reverse phase of IOP variation in rabbit eyes. Crowston et al. (685) investigated the diurnal variation of IOP in FP-/-, FP+/-, and wild type mouse using rebound tomometer. Among these three genotypes, there was no difference in IOP during the day.
The presence of FP receptor itself does not have any physiological effect in IOP regulation, whereas the presence of FP receptor is indispensable to lower IOPThis fact was also confirmed using a microneedle method, which can measure IOP more accurately. No contribution of FP receptor to IOP regulation in mouse has been also confirmed in our group. These results are interesting thatthe presence of FP receptor itself does not have any physiological effect in IOP regulation, whereas the presence of FP receptor is indispensable to lower IOP. What is the intrinsic physiological role of this FP receptor in a static state? Has this been just waiting for the exogenous stimulation by FP agonists, such as prostaglandin analogues for thousands of years? This is unlikely. Rather, it is likely that the FP receptor has developed to suit some tasks.