[125I]calcitonin gene-related peptide binding in membranes of the ciliary body-iris block

Calcitonin gene-related peptide (CGRP) is a mediator of intraocular inflammatory responses, but it may also affect aqueous humour dynamics. The aim of the present work was to characterize CGRP binding sites in the eyes of various mammals. The binding of radiolabelled human CGRP to membranes from the ciliary body-iris (c+i) block of porcine eye showed characteristics expected of an interaction with a receptor site: it was reversible, saturable and displaced by rat CGRP and calcitonin. Studies with CGRP fragments demonstrated the importance of rather long carboxy-terminal sequences of the CGRP molecule for high-affinity binding to the receptor. Rat islet amyloid polypeptide (IAPP), which has about 50% structural similarity to CGRP, displaced radioligand binding nearly as efficiently as CGRP, while human IAPP was about twenty-fold less potent. No displaceable CGRP binding could be reliably demonstrated by the present method in c+i membranes from cat, rabbit and bovine eyes, thus indicating differences in the number or localization of CGRP receptors between different mammalian species.