Calcium and protein in tears: diurnal variation.

With the use of an automatic titrator for calcium and a microcolorimetric assay for protein, diurnal variations in tear calcium and total protein values were studied in nine subjects during a ten-day period. Tear calcium and total protein concentrations were found to be individualized functions that were dependent on the sampling time and sampling day. Prolonged eye closure resulted in an approximate twofold increase in both tear calcium and total protein concentrations. The concentrations of both species remained relatively constant during the day between 8 AM and 5 PM, exhibiting few substantive variations. Diurnal changes in tear calcium and protein values closely paralleled one another. It was found that the tear concentrations of these two species could be related with a simple linear function, which suggests that calcium binding and transport by tear protein may be occurring.