Selective Stimulation of Calcium (Ca2+) Signaling in Different Cat Iris Sphincter Cells by Bimatoprost and PGF2-Alpha

Abstract

Abstract: : Purpose: Bimatoprost, a synthetic lipid prostamide that potently lowers intraocular pressure in dogs, primates and humans, has recently been introduced into the clinic for the management of increased intraocular pressure (IOP). Bimatoprost exhibits prostamide-like pharmacological activities, and has been shown to be functionally and pharmacologically distinct from PGF_2α. Moreover, it does not meaningfully interact with any of the known prostaglandin (PG) receptors. Methods: Confocal fluorescence microscopy was employed to compare the effects of bimatoprost and PGF_2α on stimulation of calcium (Ca²⁺) signaling in resident cells of the cat iris sphincter. Tissues obtained from freshly enucleated feline eyes were first digested with 0.75 mg/ml collagenase B and subsequently digested with 0.5 mg/ml collagenase B/0.35 mg/ml pronase. Digested tissue fragments were loaded with Fluo-4 AM dye, and a time-based series of images acquired with argon laser excitation at the 488 nm line and 510 - 570 nm emission filtration under constant perfusion conditions. Results: 100 nM or 1 µM of bimatoprost or PGF_2α consistently evoked Ca²⁺ signaling responses in different cells within the same tissue preparation, and repetitive dosing with either agonist alone elicited responses in the same cells. Sequential dosing with both agonists in the same tissue preparation did not result in any observed instances in which the same cell responded to both bimatoprost and PGF_2α,; randomly altering the order of bimatoprost and PGF_2α sequential dosing provided no evidence of cross-desensitization or cross-priming of responses in the same cell. Conclusions: These results demonstrate that bimatoprost and PGF_2α selectively evoke Ca²⁺ signaling in different cells in the same cat iris sphincter tissue preparation. The lack of observed instances in which the same cells respond to both bimatoprost and PGF_2α suggests that each of these agonists act at different receptor sites.

Keywords: receptors: pharmacology/physiology • lipids • microscopy: confocal/tunneling