THE CIRCADIAN-RHYTHM IN INTRAOCULAR-PRESSURE AND ITS RELATION TO DIURNAL OCULAR GROWTH CHANGES IN CHICKS

Recent investigations have shown that growing chicken eyes elongate du ring the day and shorten during the night. We asked whether the chick, like a number of other animals, exhibits a rhythm in intraocular pres sure (IOP) and whether this rhythm might be associated with this rhyth m in elongation. We find that the intraocular pressure in normal eyes is high during the day and low in the middle of the night, similar to the rhythm in ocular elongation. The amplitude of this rhythm in IOP i s approximately 8 mm Hg; it persists in constant darkness, albeit with a reduced amplitude, implying that the rhythm has a circadian compone nt. Form deprivation by translucent diffusers does not affect the ampl itude of the rhythm in IOP, but makes the phase of the rhythm more var iable, such that the trough no longer consistently occurs at night. We find that the magnitude of the ocular compliance (the change in lengt h induced by change in intraocular pressure) is consistent with the po ssibility that the diurnal changes in IOP might, through mechanical st retch, account for much of the diurnal changes in length. However, in individual eyes, we find consistent phase differences between the rhyt hms in IOP and ocular elongation. Therefore, we propose that the rhyth m in IOP influences ocular elongation in ways other than by simply inf lating the eye, for example, by influencing underlying rhythms in scle ral extracellular matrix production. We conclude that the rhythm in IO P plays a role in the regulation of the growth of the eye. (C) 1998 Ac ademic Press Limited.