Calcium efflux in rat lens: Na/Ca-exchange related to cataract induced by selenite.
Sodium gradient-dependent 45Ca2+ transport occurred across the lens membrane both in the direction of Ca2+ uptake by inside-out vesicles and Ca2+ efflux after Ca2+ loading of right-side-out vesicles. Using the calcium ionophore, A23187, greater than 90% of the Na+ gradient-dependent Ca2+ uptake was estimated to be free Ca2+. A normal Na+ gradient was also required to maintain calcium homeostasis in the intact lens. The Na+ gradient contributed to Ca2+ efflux from lenses pre-loaded in medium containing 15 mM CaCl2. Therefore, a Na/Ca-exchange functions to control Ca efflux in rat lens, in addition to the Ca-ATPase. In the preweanling rat mature nuclear cataracts occurred by 96 h after subcutaneous injection of sodium selenite (30 nmol/g animal wt). A 3-5 fold increase of Ca2+ accompanied cataract formation. The loss of Ca2+ homeostasis can be detected by 48 h after treatment selenite treatment. At this time the initial rate of Na+ gradient-dependent Ca2+ uptake was 30% lower in lens vesicles from selenite-treated rats compared to controls. No significant reduction of Na+,K(+)-ATPase activity was detected. Altered Na/Ca-exchange may contribute directly to the loss of Ca2+ homeostasis that leads to nuclear cataract.
Wang
Z
Z
Department of Biochemistry and Nutrition, Virginia Polytechnic Institute and State University, Blacksburg 24061-0308.
Hess
J L
JL
Bunce
G E
GE
D000818:Animals; D001693:Biological Transport, Active; D002118:Calcium / Q000378:metabolism*; D002352:Carrier Proteins / Q000378:metabolism; D002386:Cataract / Q000139:chemically induced / Q000378:metabolism*; D002462:Cell Membrane / Q000378:metabolism; D006706:Homeostasis; D007908:Lens, Crystalline / Q000378:metabolism*; D051381:Rats; D011919:Rats, Inbred Strains; D012643:Selenium; D012964:Sodium / Q000378:metabolism*; D018038:Sodium Selenite; D019831:Sodium-Calcium Exchanger; D000254:Sodium-Potassium-Exchanging ATPase / Q000378:metabolism
D002352:Carrier Proteins; D019831:Sodium-Calcium Exchanger; D012964:Sodium; D000254:Sodium-Potassium-Exchanging ATPase; D012643:Selenium; D018038:Sodium Selenite; D002118:Calcium