Ca²⁺ and Ba²⁺ effects on basolateral tetraethylammonium transport in isolated snake renal proximal tubules

Previous work on snake renal tubules suggested that basolateral transport of tetraethylammonium (TEA) is symmetrical. To examine regulation of this transport step more closely, we determined the effects of (1) reductions in the extracellular Ca²⁺ concentration and basolateral Ca²⁺ entry, and (2) the presence of extracellular Ba²⁺ on TEA uptake and efflux across the basolateral membrane of isolated snake renal proximal tubules. Removal of extracellular Ca²⁺ reduced initial TEA uptake and enhanced TEA efflux. Blocking Ca²⁺ entry also reduced initial TEA uptake. Extracellular Ba²⁺ depolarized the basolateral membrane and reduced both TEA uptake and efflux. Inhibition of basolateral TEA uptake with a reduced membrane potential supports previous data indicating that uptake involves potential-driven facilitated diffusion. Inhibition of basolateral TEA efflux by Ba²⁺ even with a reduced membrane potential not only supports previously obtained data indicating that efflux is not influenced by the potential difference and that basolateral TEA transport is asymmetrical, but also suggests that TEA uptake and efflux may occur by separate pathways.