Stimulation of P2U purinergic or alpha 1A adrenergic receptors mobilizes Ca2+ in lens cells.

PURPOSE

To identify agonists that elevate cytosolic Ca2+ (Cai2+) in lens cells and to characterize their mechanism of action.

METHODS

Digital imaging and the Ca(2+)-reporting dye fura-2 were used to study the effects of agonists and their antagonists of Cai2+ in sheep lens primary cell cultures.

RESULTS

Exposing cells to adenosine triphosphate (ATP) and epinephrine increased Cai2+, whereas dopamine, 5-hydroxytryptamine, acetylcholine, histamine, kassinin, bradykinin, and glutamate did not elevate Cai2+. The ATP response was mediated by P2U purinergic receptors based on inhibition by the P2 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 epinephrine response was mediated by alpha 1 adrenergic receptors based on the greater potency of the alpha 1 adrenergic selective antagonist prazosin compared to that of the alpha 2 adrenergic selective antagonist yohimbine. More specifically, the epinephrine response was mediated by the alpha 1A adrenergic receptor subtype based on the greater potencies exhibited by the alpha 1A subtype selective competitive antagonists WB 4101 and 5-methylurapidil compared to the alpha 1B and alpha 1D selective antagonists spiperone and BMY 7378, respectively. The agonist-mediated Cai2+ increase was dependent on intracellular Ca2+ stores and was inhibited by the phospholipase C inhibitor U73122. ATP or epinephrine could desensitize the cells to either agonist because of both the depletion of intracellular Ca2+ stores and the downregulation of a common intermediate in the signal transduction pathway.

CONCLUSIONS

Ca2+ is mobilized from intracellular stores in the sheep lens by ATP and epinephrine acting through P2U purinergic and the alpha 1A adrenergic receptors, respectively. This confirms previous reports of P2U receptors in lens and provides the first report of alpha 1A adrenergic receptors in the lens.