Localization of carbonic anhydrase IV in a specific capillary bed of the human eye.

Carbonic anhydrase (CA) activity plays an important role in controlling aqueous humor production in the eye and in regulating intraocular pressure. Prior studies identified the soluble isozymes CA II and CA I in the human eye and also suggested a distinct membrane-associated CA. We used an antibody to CA IV, the membrane-anchored isozyme from human lung, to study CA IV in eye tissues and to compare its distribution with that of CA II. We found intense immunostaining for CA IV associated with endothelial cells of one specific uveal capillary bed, the choriocapillaris. CA IV was not detected in endothelial cells of the contiguous capillaries of the iris or in endothelial cells of other vessels. Immunoreactivity for CA IV was also intense in epithelial and fiber cells of the lens but was not detectable in the neuroretina, the ciliary process (except for capillaries), and the cornea, all sites where immunostaining with anti-CA II antibody was intense. These studies indicate that the membrane-associated CA in human eye, which was suspected from histochemical studies, is CA IV. Defining the physiological role of this ocular isozyme remains a challenge.