Regional differences in functional receptor distribution and calcium mobilization in the intact human lens

To investigate regional differences in Ca(2+) mobilization kinetics in the intact human lens produced by exposure to agonists of tyrosine-kinase and G-protein-coupled receptors and to characterize the major receptor subtypes involved in Ca(2+) signaling in the different regions.

Whole human lenses were placed anterior side down in a plastic chamber and perifused with artificial aqueous humor (AAH) at 30 degrees C. After fura-2 incorporation, cytosolic Ca(2+) levels were monitored by using epifluorescence techniques in either the equatorial or central anterior epithelial cells of the intact lens. Agonists dissolved in AAH were applied to the lens in successive short pulses.

Central anterior lens epithelial cells produced a large response to 10 microM acetylcholine (ACh) and histamine; only a small response to adenosine triphosphate (ATP); and no response to 10 microM adrenalin, 10 ng/ml epithelial growth factor (EGF) or TGF alpha, or 50 ng/ml platelet-derived growth factor (PDGF)-AB. Conversely, the equatorial cells produced a strong response to 10 microM ATP and histamine, 10 ng/ml EGF (or TGF alpha), and 50 ng/ml PDGF-AB, but failed to respond to 10 microM ACh or 10 microM adrenalin. The EGF-induced response in the equatorial cells was blocked completely by tyrphostin (AG1478), a specific inhibitor of the EGF receptor tyrosine kinase. Carbachol, a nonhydrolyzable analogue of ACh, and pilocarpine, the M1 muscarinic receptor-specific agonist, both produced the same trend of response amplitude elicited by ACh in each region of the lens. The potency order of purinergic agonist-induced Ca(2+) mobilization at the equator was consistent with the P2Y(2) receptor subtype. The histamine-induced response was abolished by 10 microM triprolidine, a specific H(1) receptor antagonist, but remained unaffected by the specific H(2) and H(3) antagonists, ranitidine and thioperamide, respectively. CONCLUSIONS. There is a spatial heterogeneity in functional receptor activity in different regions of the whole lens. The important growth factor receptors for EGF and PDGF are functionally active only in the equatorial cells of the mature human lens. This study further shows that the ACh, histamine, and ATP-induced responses arise from the activation of M1 muscarinic, H(1) histamine, and P2Y(2) purinergic receptors, respectively.