Effect of basement membrane and colloid osmotic pressure on renal tubule cell volume

Renal tubule cell volume is thought to be kept constant by a cation pump. When active transport is blocked, intracellular impermeant solutes cause cells to swell. Cell size is then determined by transmembrane hydrostatic and colloid osmotic forces. We studied the importance of passive transmembrane forces in determining cell size in isolated rabbit proximal straight tubules (PST). We blocked active solute transport with ouabain and evaluated subsequent changes in cell size by measuring outer diameter of nonperfused tubules. Tubules in a ouabain and 6 g/100 ml protein bath swelled only 40% above control. However, removal of the tubule basement membrane with collagenase dissipated a transmembrane hydrostatic pressure and caused more swelling. Final cell volume was determined largely by bath protein concentration. Tubules in ouabain and collagenase swelled enormously in hyponcotic protein, moderately in isoncotic protein, and could be shrunk below control in hyperoncotic protein. Intracellular colloid osmotic pressure was estimated to exceed 38 cmH20. We conclude that hydrostatic and colloid osmotic forces are major determinants of cell size in isolated PST treated with ouabain.