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Involvement of Na+,K+‐ATPase in the Mitogenic Effect of Insulin‐Like Growth Factor‐I on Cultured Rat Astrocytes

Abstract: We have previously reported that insulin/insulin‐like growth factor (IGF)‐I induced the α1 isoform of Na+,K+‐ATPase in cultured astrocytes. In this study the effects of insulin/IGF‐I on Na+,K+‐ATPase activity and cell proliferation were examined in astrocytes cultured under the various conditions, to test the possible involvement of the enzyme activity in the mitogenic action of IGF‐I on astrocytes. Insulin increased Na+,K+‐ATPase activity and stimulated cell proliferation in subconfluent astrocytes (cultured for 7–14 days in vitro). In contrast, these effects were not observed in confluent cells (cultured for 28 days). Furthermore, insulin stimulated neither the enzyme activity nor [3H]thymidine incorporation in astrocytes preincubated in fetal calf serum‐free medium for 2 days (quiescent cells) and treated with dibutyryl cyclic AMP (differentiated cells). The increases in Na+,K+‐ATPase activity and expression of the α1 mRNA preceded the mitogenic effect. 125I‐IGF‐I binding experiment showed that all the cells used here had similar binding characteristics. The insulin‐induced increase in enzyme activity was not affected by 1‐(5‐isoquinolinesulfonyl)‐2‐methylpiperazine (H‐7), and it was observed even in Ca2+‐free medium. The stimulation by IGF‐I of [3H]thymidine incorporation was attenuated by ouabain and a low external K+ level. These findings suggest that stimulation of Na+,K+‐ATPase activity is involved in the mitogenic action of IGF‐I on cultured astrocytes.