Calcium uptake, release and ryanodine binding in melanosomes from retinal pigment epithelium.

High levels of calcium have been reported in pigmented tissues of the vertebrate eye, such as retinal pigment epithelium (RPE). Melanin granules also have high calcium concentrations, suggesting that melanin granules may be a calcium reservoir. Here we characterized the uptake and release of calcium in a pure melanosomal fraction obtained from frog RPE. Melanosomes take up 45Ca by a saturable system with an apparent KM of 0.5 mM. About 40% of 45Ca accumulation was insensitive to low temperature. 45Ca uptake was not affected by verapamil, nifedipine, dantrolene, vanadate, thapsigargin or cyclopiazonic acid, but it was reduced by 50% by ruthenium red, and increased by the ionophore A23187 and nigericin. Release of 45Ca-loaded was stimulated by caffeine and inositol 1,4,5 trisphosphate (IP3). Caffeine stimulated release of calcium was blocked by either ryanodine or ruthenium red, but calcium released by IP3 was not affected by heparin. No binding of 3H-IP3 was observed. The 3H-ryanodine binding sites exhibited a KB of 1.3 nM and a Bmax of 12.1 fmol/mg protein. Thus, our results suggest that melanosomes may function as intracellular organelles that regulate calcium concentration in RPE.