Effects of low [Ca²⁺] and La³⁺ on PAH transport by isolated perfused renal tubules

Effects of low medium calcium concentration and of lanthanum on p-aminohippurate (PAH) transport by isolated perfused snake (Thamnophis spp.) distal-proximal renal tubules were studied. Nominal removal of calcium from bath produced rapid, irreversible disruption of cells and depression of net PAH secretion (J(PAH)). However, nominal removal of calcium from lumen with calcium in bath reversibly depressed J(PAH) and apparent PAH permeability of luminal membrane without altering net fluid absorption (J(v)) or intracellular PAH concentration ([PAH](cell)). In the presence of calcium (1.8 mM), lanthanum (2 mM) in perfusate alone, in bath alone, or in both perfusate and bath reversibly depressed J(PAH), apparent PAH permeabilities of luminal and peritubular membranes, and initial rate of PAH transport into cells without altering [PAH](cell) or J(v). Variations in calcium concentration had little effect on depression of J(PAH) in sodium-free bath. These data suggest that calcium entry into cells, but possibly not cytosolic calcium concentration alone, is important for PAH transport from bath into cells and from cells into lumen and for normal passive permeability of peritubular membrane to PAH and that increased cytosolic calcium concentration is not primarily responsible for depression of J(PAH) in sodium-free medium.