Fluid composition of basolateral space of kidney cells in culture and its modification by intracellular cAMP.

LLC-PK1 kidney cells, like other epithelial cells, form domes due to Na+ and water movements from the apical to the basolateral side of the epithelium when cultured on a nonpermeable support. The composition of the fluid trapped under these domes can therefore be considered to reflect the nature of the fluid reabsorbed through the epithelium. Collecting the basolateral fluid by micropuncture and analyzing it by electron microprobe revealed that it was isosmotic with the external medium and that the concentrations of Na+, Cl-, Ca2+, Mg2+, and phosphate were nearly the same in both. In contrast, the K+ concentration was found to be 40% lower under the domes than in the bath. Changing the osmolality of the apical medium showed that the epithelium was leaky for water, although it maintained an ionic gradient for at least 20 min. Fifteen minutes after addition of dibutyryl adenosine 3',5'-cyclic monophosphate or 3-isobutyl-1-methylxanthine (5 X 10(-4) M) to the external medium, the phosphate concentration in the basolateral fluid was increased. This rise peaked at 1 h and diminished thereafter. The significance of these observations is discussed in regard to transepithelial permeability and to the transport properties of LLC-PK1 cells.