Influence of external calcium and glucose on internal total and ionized calcium in the rat lens

Free calcium in the rat lens, measured by ion-sensitive electrodes, is 1.8 microM while the total, measured by atomic absorption, is of the order of 600 microM. The measured free calcium concentration (pCa) varies with the depth below the surface. It is lowest in the region 100-400 micron below the capsule and again in the nucleus, while the intervening perinuclear cortex has a relatively high free calcium. In young rats (less than 16 weeks) the free calcium in the posterior and anterior cortical regions is the same, while in the older lenses the free calcium is lower in the anterior and the regional variation is greater. Rat lenses incubated in a medium of similar ionic composition to aqueous humour for 15-24 h maintained a low level of free calcium. The maintenance of low internal calcium (both free and total) was dependent on external glucose and on removing glucose the intracellular free calcium increased from 5 to 15 microM while the total calcium increased from 600 to over 1000 microM. Following incubation in high calcium (10 mM), the free and total calcium increased to 40 and 3000 microM respectively. Omitting glucose from the high-calcium solution led to a further increase in both free and total calcium to 400 and 10000 microM respectively. The incubated control lenses maintained their normal sodium and potassium levels and resting potential, while removing glucose gave rise to an increase in sodium, a decrease in potassium and a depolarization of the membrane potential. Increasing external calcium also depolarized the membrane potential, but there was no change in internal sodium and potassium.(ABSTRACT TRUNCATED AT 250 WORDS)