Activation of Amiloride‐Sensitive Sodium Transport in C6 Glioma Cells

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Abstract
We have characterized, in C6 cells, an amiloride-sensitive Na+ entry pathway that can exchange for H+. In this report we demonstrate that this cation-exchange system can be induced within 24–36 h by either serum removal or by dibutyryl cyclic AMP; however, these modes of induction are not additive and are manifest only after activation by serum. In these glioma cells we found that activation by serum can be mimicked in part by specific serum factors, i.e., epidermal growth factor and bradykinin. We attempted to characterize this activation process further using several cell biologic probes. We had previously shown that that activation process involves a calcium-dependent step with full activation obtained in the presence of the calcium iono-phore A23187. The activation by serum was inhibited by preincubation with colchicine but not with dihydrocyto-chalasin B, suggesting a cytoskeletal involvement in the activation process. Activation by epidermal growth factor and bradykinin was found to be unaffected by colchicine, suggesting that other factors must be present in serum that confer sensitivity to colchicine. Incubation of the cells with phorbol myristoyl acetate results in the activation of amiloride-sensitive transport, suggesting that stimulation of protein kinase C may be integral to the activation process. Unlike the effects of serum, activation by phorbol myristoyl acetate is not inhibited by colchicine, indicating that this drug works in a way that bypasses the cytoskeletal-dependent step. Since diacyl-glycerol is the presumed endogenous activator of protein kinase C., we studied the effects of dioleylglycerol. This intermediate of phosphoUpid turnover was found to increase specifically the amiloride-sensitive sodium pathway. We feel therefore that Na+-H+ exchange in glial cells as well as in other cells may be tightly linked to phospholipid turnover and to calcium flux and that these two processes are integral to glial cell acidification and ion homeostasis.

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