The exchange in intact squid axons

Abstract

The exchange in intact axons displays a number of features in common with other systems, but a number of interesting points remain to be examined. Both forward (Nao-Cai) and reverse (Cao-Nai) exchange are sensitive to changes in membrane potential, but potassium depolarization can also stimulate Cao-Nai exchange by chemical activation at a monovalent cation-binding site. By monitoring lithium uptakes into intact axons, activating cations do not appear to be transported on the exchange, but this deserves further examination under more stringent conditions. Cao-Nai exchange in intact axons appears activated by monovalent cations to a greater extent compared to dialyzed axons that exhibit little, if any, shift in the Km for Cao. The catalytic effect of Cai on Cao-Nai exchange seen in dialyzed axons proves elusive to study in intact axons, with or without introduction of Ca chelators. Experiments using ruthenium red suggest that free calcium can be dissociated from Cao-Nai exchange fluxes; this finding is also important to those studies monitoring exchange activity using Ca indicators. The possibility that Ca chelators may effect changes in the kinetics of Na-Ca exchange is a subject that needs further investigation.