Renal tubular secretion and reabsorption of organic bases in the dog.

The dependence of the renal excretion of relatively weak organic bases on urine pH has been repeatedly demonstrated for both man (1-3) and the dog (4). Many investigators have considered this as an example of non-ionic diffusion (5), according to which the renal tubule is permeable to the non-ionic form but impermeable to the ionized particle. The non-ionic form tends to reach an equilibrium concentration on either side of the tubular epithelium. Thus, the concentration of total base, both ionized and non-ionized, is determined by the hydrogen ion gradient across the membrane. If the urine is more acid than -the plasma, the urinary concentration of total base will be higher than in plasma; the opposite will obtain in a urine more alkaline than plasma. When clearance data have been evaluated in these terms, the mechanism of non-ionic diffusion has tbeen considered adequate to explain a wide range -of excretion rates varying from net tubular secretion to net tubular reabsorption (6).1