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

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Abstract
Free Ca in the rat lens, measured by ion-selective electrodes, is 1.8 .mu.M; the total, measured by atomic absorption, is of the order of 600 .mu.M. The measured free Ca concentration (pCa) varies with the depth below the surface. It is lowest in the region 100-400 .mu.m below the capsule and again in the nucleus, while the intervening perinuclear cortex has a relatively high free Ca. In young rats (< 16 wk), the free Ca in the posterior and anterior cortical regions is the same; in the older lenses, the free Ca is lower in the anterior, and the regional variation is greater. Rat lenses incubated in a medium of ionic composition similar to aqueous humor, for 15-24 h, maintained a low level of free Ca. The maintenance of low internal Ca (both free and total) was dependent on external glucose; on removing glucose, the intracellular free Ca increased from 5 to 15 .mu.M, while the total Ca increased from 600 to > 1000 .mu.M. Following incubation in high Ca (10 mM), the free and total Ca increased to 40 and 3000 .mu.M, respectively. Omitting glucose from the high-Ca solution led to a further increase in both free and total Ca to 400 and 10,000 .mu.M, respectively. The incubated control lenses maintained their normal Na and K levels and resting potential, while removing glucose gave rise to an increase in Na, a decrease in K and a depolarization of the membrane potential. Increasing external Ca also depolarized the membrane potential, but there was no change in internal Na and K. Omitting glucose from this solution depolarized the membrane potential still further, and disrupted the Na and K levels. The permeability ratio, .alpha. (PNa/PK), increased from 0.053 in control solutions to 0.190 in high Ca, and raising the Ca and omitting glucose caused a further increase to 0.935. Only the control lenses remained clear after incubation, and the greatest degree of light scatter was associated with lenses that were incubated in raised Ca solutions with glucose omitted.