The effect of elevated progesterone levels on intraocular pressure in lions (Panthera leo)

Recently, we reported that intraocular pressure (IOP) in juvenile male lions (Panthera leo) is significantly higher than in juvenile lionesses. Whilst we could not ascertain the basis for this gender-related difference, we suspected that they were the result of fluctuations in levels of sex hormones. Because 19 of the 22 lions described in our previous report had to be re-anaesthetized, we repeated tonometry in these animals, to try and correlate between IOP and the levels of progesterone, oestrogen and/or testosterone. Based on elevated (>5 ng ml(-1)) levels of progesterone, lionesses were divided into a luteal (n = 8) and a non-luteal (n = 13) group. In the luteal group, mean IOP was 27.07+/-2.15 mm Hg, significantly (P = 0.001) higher than in the non-luteal group (21.61+/-2.70 mm Hg). Oestrogen, testosterone, anesthesia and age had no significant effect on IOP. It is suggested that elevated progesterone levels associated with the luteal phase in lionesses cause increased resistance to aqueous humor outflow, leading to IOP elevation. Similar ocular hypertension has been observed in rabbits following exogenous progesterone application, but this is the first report on the hypertensive effect of endogenous progesterone on IOP.