Purinergic signaling in the rabbit ciliary body epithelium

In the past forty years, a wealth of information has accumulated that points to the presence of adenosine and adenine nucleotides in the anterior segment of the eye and a number of hypotheses have been introduced to describe the possible role of these agents in the regulation of aqueous humor flow. However, in the absence of a generally accepted model for the cellular and molecular mechanisms of aqueous humor formation by the ciliary body epithelium, efforts to identify the signal transduction pathway(s) responsible for regulation of the ion and water transport have not been successful. This article briefly reviews the evidence for (i). the presence in aqueous humor of adenine nucleotides, cyclic adenosine monophosphate and adenosine, their metabolic product, (ii). the possible role of these agents in the regulation of aqueous humor dynamics, and (iii). the expression of ecto-nucleotidases, receptors, and second messengers that may mediate such regulation. Finally, a model for the regulation of aqueous humor formation by adenosine and ATP is proposed.