MECHANISM OF CALCIUM-INDUCED DISINTEGRATIVE GLOBULIZATION OF RAT LENSFIBER CELLS

Purpose. To study the role of calcium and calcium-dependent processes in the disintegrative globulization of isolated single rat lens cortic al fibers. Methods. The authors isolated viable and morphologically in tact single fiber cells from rat lens cortex and studied the effect of I mM [Ca2+](o) on the globulization of fiber cells from the outer and inner cortex. They investigated the effects of the calcium-channel bl ocker, verapamil; an inhibitor of calcium transport, lanthanum; variou s protease inhibitors; Na+-free and K+-free media; calcium ionophore, A23187; and calcium chelator, BAPTA, on the globulization of fiber cel ls exposed to 1 mM [Ca2+](o). Results. Perfusion with Ringer's solutio n containing 1 mM [Ca2+](o), caused disintegration and globulization o f the isolated fibers in 32.3 +/- 1 minute, and the addition of 10 mu M A23187 to the superfusing solution reduced the time to complete glob ulization (t(g)) to 19.4 +/- 0.3 minutes. However, the addition of pro tease inhibitors, leupeptin, calpain inhibitor I, E-64, or pepstatin ( 0.5 mM each) to the superfusing solution, increased t, to 105 +/- 3.5, 84.2 +/- 7.8, 60.7 +/- 3.5, and 48.3 +/- 3.1 minutes, respectively. T he t(g) also increased (96.4 +/- 3.5 minutes) when the fibers were pre incubated with BAPTA-AM or when they were exposed to 1 mM [Ca2+](o) in Na+- or K+-free Ringer's solution (t(g) = 66.7 +/- 5.3 and 58.9 +/- 3 .9 minutes, respectively) or in Ringer's solution containing 1 mM [Ca2 +](o) + 50 mu M verapamil (t(g) = 49.3 +/- 3.8 minutes). Single fibers isolated from the outer cortex of the lens were less susceptible to e xtracellular calcium than those isolated from the inner cortex. Conclu sions. Increased calcium influx and the attendant elevation of [Ca2+]( i) are necessary for disintegrative globulization of lens fiber cells. Calcium influx appears to be mediated partially by the L-type calcium channels and the background calcium leak. Protection by protease inhi bitors suggests that membrane fragmentation, caused by elevated [Ca2+] (i), results from proteolytic damage to the fiber cytoskeleton. Beside s underscoring the central role of calcium homeostasis in preserving t he morphologic integrity of the cortical fibers, this study suggests a possible cellular mechanism for the formation of supranuclear catarac t.