45Ca uptake by retinal pigment epithelial cells.

Uptake of 45Ca was studied in isolated frog retinal pigment epithelial cells. 45Ca accumulation was found to be a saturable, temperature-dependent event. Kinetic analysis of this accumulation revealed two transport systems with apparent km of 2.0 and 0.3 mM. We found the presence of a Na-Ca exchanger mechanism that releases Ca2 under depolarized conditions. Light induced an increase of 45Ca uptake due to activation of the Na-K-ATPase and consequent decrease of extracellular potassium concentration.