Secretory renal proximal tubules in seawater- and freshwater-adapted killifish.

A population of proximal tubules when isolated from the glomerular kidneys of seawater-adapted (SW) and freshwater-adapted (FW) killifish (Fundulus heteroclitus) spontaneously secrete fluid. Regardless of SW or FW adaptation, Na and Cl are the dominant electrolytes in secreted fluid. Mg concentrations in fluid secreted by both tubules are significantly greater than those in the peritubular bath, and Mg concentrations are inversely related to Na concentrations. Proximal tubules from either SW or FW fish exhibit low transepithelial voltage (-1 to -2 mV) and low transepithelial resistances (20-30 omega.cm2) typical of other vertebrate proximal tubules. Transepithelial diffusion potentials for Na, Cl, Mg, and SO4 suggest that the paracellular pathway is Na selective and impermeable to divalent ions. Consideration of transepithelial electrochemical potential differences for Na, Cl, Mg, and SO4 suggests active transport of Mg, SO4, and Cl in proximal tubules isolated from SW- and FW-adapted fish. The similarities in the functional properties of secretory proximal tubules isolated from SW- and FW-adapted killifish are striking and raise questions about the in vivo role of these tubules in the renal adaptations to seawater and freshwater.