Effects of disassembly of actin microfilaments on the AVP-induced regulation of sodium channel densities in frog skin epithelium

Vasopressin (AVP) stimulates the absorption of Na+ across frog skin epithelium by increasing the number of open apical channels (N(o)) mainly through a large expansion of the total number of channels (N(T)) at the membrane. This study investigates with blocker-induced noise analysis the potential role of actin in the regulation of AVP-induced changes in channel densities. Particularly we examined the idea that actin regulates the insertion of channels into apical cell membranes, consistent with the model for its hydrosmotic action. Treating cells with cytochalasin B (CB) for 2 h to disassemble the prominent subapical actin network did not significantly alter the stimulation of the I(SC) by AVP. Importantly the treatment had no significant effect on the AVP-induced activation of Na+ channels, nor on the increases in N(T) indicating that an intact actin network is not required for the natriferic actions on the channels. This result is disparate from well known effects of CB on the AVP-induced hydrosmotic response and we assume that these distinct responses are produced by different mechanisms. Other mechanisms need to be considered to explain the mechanism whereby new Na+ channels are recruited to the apical membrane. In particular, mechanisms of intracellular trafficking involved in the redistribution of epithelial Na+ channels remain unresolved and need to be refined.