STIMULATION OF P-2U PURINERGIC OR ALPHA(1A) ADRENERGIC-RECEPTORS MOBILIZES CA2+ IN LENS CELLS

Purpose. To identify agonists that elevate cytosolic Ca2+ (Ca-i(2+)) i n lens cells and to characterize their mechanism of action. Methods. D igital imaging and the Ca2+-reporting dye fura-2 were used to study th e effects of agonists and their antagonists on Ca-i(2+) in sheep lens primary cell cultures. Results. Exposing cells to adenosine triphospha te (ATP) and epinephrine increased Ca-i(2+), whereas dopamine, 5-hydro xytryptamine acetylcholine, histamine, kassinin, bradykinin, and gluta mate did not elevate Ca2+. The ATP response was mediated by P-2U purin ergic receptors based on inhibition by the P-2 antagonist suramin and the agonist rank potency order ATP = UTP = ATP gamma S > ADP > AMP >> adenosine; adenine, AMP-CPP, and AMP-PCP were inactive. The epinephrin e response was mediated by al adrenergic receptors based on the greate r potency of the alpha(1) adrenergic selective antagonist prazosin com pared to that of the alpha(2) adrenergic selective antagonist yohimbin e. More specifically, the epinephrine response was mediated by the alp ha(1A) adrenergic receptor subtype based on the greater potencies exhi bited by the alpha(1A) subtype selective competitive antagonists WE 41 01 and 5-methylurapidil compared to the alpha(1B) and alpha(1D) select ive antagonists spiperone and BMY 7378, respectively. The agonist-medi ated Ca-i(2+) increase was dependent on intracellular Ca2+ stores and was inhibited by the phospholipase C inhibitor U73122. ATP or epinephr ine could desensitize the cells to either agonist because of both the depletion of intracellular Ca2+ stores and the downregulation of a com mon intermediate in the signal transduction pathway. Conclusions. Ca2 is mobilized from intracellular stores in the sheep lens by ATP and e pinephrine acting through P-2U purinergic and the alpha(1A) adrenergic receptors, respectively. This confirms previous reports of P-2U recep tors in lens and provides the first report of alpha(1A) adrenergic rec eptors in the lens.