Calcium in the toad ( Bufo marinus) cornea: Binding by the stroma

Abstract

1. 1. The Ca concentration in the toad ( Bufo marinus) cornea was 2.6 mmol/kg wet wt compared at 1.0 mmol/l in the bathing aqueous humor and 2.8 mmol/kg wet wt in the separated corneal stromal layer. Cell Ca content was calculated to be about 1.8 mmol/kg wet wt. 2. 2. About 80% of the total Ca appears to be sequestered or bound to tissue components most of which (68% of the total) is associated with the stroma (2.2 mmol/kg wet wt stroma). 3. 3. About 85–90% of the Ca in the stroma is readily exchangeable with external 45Ca. 4. 4. The loss of accumulated 45Ca from the stroma was measured in vitro. This efflux of the isotope was enhanced by multivalent ions and was greatest when Ca 2+ or La 3+ was present in the external media. Other alkaline earth metal ions were not as effective. The relative effectiveness of this displacement of 45Ca was Ca = La > Sr > Ba > Mg. 5. 5. The results suggest that the Ca 2+ is bound by the amphibian stroma at sites that have a preference or specificity for this divalent ion as compared to the other alkaline earth metals. 6. 6. The possible functional role of this bound Ca is discussed.