Calcium regulation by lens plasma membrane vesicles.
The role of the plasma membrane in the regulation of lens fiber cell cytosolic Ca2+ concentration has been examined using a vesicular preparation derived from calf lenses. Calcium accumulation by these vesicles was ATP dependent, and was releasable by the ionophore A23187, indicating that calcium was transported into a vesicular space. Calcium accumulation was stimulated by Ca2+ (K1/2 = 0.08 microM Ca2+) potassium (maximally at 50 mM K+), and cAMP-dependent protein kinase; it was inhibited by both vanadate (IC50 = 5 microM) and the calmodulin inhibitor R24571 (IC50 = 5 microM), indicating that this pump was plasma-membrane derived and likely calmodulin dependent. Valinomycin, in the presence of K+, stimulated calcium uptake, suggesting that the calcium pump either countertransports K+, or is regulated in an electrogenic fashion. Inhibition of calcium uptake by selenite and p-chloromercuribenzoate demonstrates the presence of an essential -SH group(s) in this enzyme. Calcium release from calcium-filled lens vesicles was enhanced by Na+, demonstrating that these vesicles also contain a Na:Ca exchange carrier. p-Chloromercuribenzoate and p-chloromercuribenzoate sulfonic acid also promoted calcium release from calcium-filled vesicles, suggesting that this release, like calcium uptake, is in part mediated by a cysteine-containing protein. We conclude that lens fiber cell cytosolic Ca2+ concentration could be regulated by a number of plasma membrane processes. The sensitivity of both calcium uptake and release to -SH reagents has implications in lens cataract formation, where oxidation of lens proteins has been proposed to account for the elevated cytosolic Ca2+ in this condition.
Galvan
A
A
Department of Veterinary Biology, University of Minnesota, St. Paul 55108.
Louis
C F
CF
D002731:4-Chloromercuribenzenesulfonate / Q000494:pharmacology; D000255:Adenosine Triphosphate / Q000494:pharmacology; D000818:Animals; D000001:Calcimycin / Q000494:pharmacology; D002118:Calcium / Q000378:metabolism*; D002147:Calmodulin / Q000037:antagonists & inhibitors / Q000494:pharmacology; D002352:Carrier Proteins / Q000378:metabolism; D002417:Cattle; D002462:Cell Membrane / Q000187:drug effects / Q000378:metabolism*; D002729:Chloromercuribenzoates / Q000494:pharmacology; D000242:Cyclic AMP / Q000494:pharmacology; D003600:Cytosol / Q000378:metabolism; D007093:Imidazoles / Q000494:pharmacology; D007908:Lens, Crystalline / Q000378:metabolism*; D008565:Membrane Proteins / Q000378:metabolism; D011188:Potassium / Q000494:pharmacology; D011494:Protein Kinases / Q000494:pharmacology; D019831:Sodium-Calcium Exchanger; D000254:Sodium-Potassium-Exchanging ATPase / Q000378:metabolism; D014634:Valinomycin / Q000494:pharmacology; D014638:Vanadates / Q000494:pharmacology; D020245:p-Chloromercuribenzoic Acid
D002147:Calmodulin; D002352:Carrier Proteins; D002729:Chloromercuribenzoates; D007093:Imidazoles; D008565:Membrane Proteins; D019831:Sodium-Calcium Exchanger; D014634:Valinomycin; D000001:Calcimycin; D014638:Vanadates; C031938:calmidazolium; D020245:p-Chloromercuribenzoic Acid; D002731:4-Chloromercuribenzenesulfonate; D000255:Adenosine Triphosphate; D000242:Cyclic AMP; D011494:Protein Kinases; D000254:Sodium-Potassium-Exchanging ATPase; D011188:Potassium; D002118:Calcium