Bimatoprost and prostaglandin F2α selectively stimulate intracellular calcium signaling in different cat iris sphincter cells

Bimatoprost is a synthetic analog of prostaglandin F2α ethanolamide (prostamide F2α), and shares a pharmacological profile consistent with that of the prostamides. Like prostaglandin F2α carboxylic acid, bimatoprost potently lowers intraocular pressure in dogs, primates and humans. In order to distinguish its mechanism of action from prostaglandin F2α, fluorescence confocal microscopy was used to examine the effects of bimatoprost, prostaglandin F2α and 17-phenyl prostaglandin F2α on calcium signaling in resident cells of digested cat iris sphincter, a tissue which exhibits contractile responses to both agonists. Constant superfusion conditions obviated effective conversion of bimatoprost. Serial challenge with 100 nm bimatoprost and prostaglandin F2α consistently evoked responses in different cells within the same tissue preparation, whereas prostaglandin F2α and 17-phenyl prostaglandin F2α elicited signaling responses in the same cells. Bimatoprost-sensitive cells were consistently re-stimulated with bimatoprost only, and prostaglandin F2α sensitive cells could only be re-stimulated with prostaglandin F2α. The selective stimulation of different cells in the same cat iris sphincter preparation by bimatoprost and prostaglandin F2α, along with the complete absence of observed instances in which the same cells respond to both agonists, strongly suggests the involvement of distinct receptors for prostaglandin F2α and bimatoprost. Further, prostaglandin F2α but not bimatoprost potently stimulated calcium signaling in isolated human embryonic kidney cells stably transfected with the feline- and human-prostaglandin F2α FP-receptor and in human dermal fibroblast cells, and only prostaglandin F2α competed with radioligand binding in HEK-feFP cells. These studies provide further evidence for the existence of a bimatoprost-sensitive receptor that is distinct from any of the known prostaglandin receptor types.