Challenges and intriguing problems in comparative renal physiology.

The comparative approach has proved important many times in understanding renal function and continues to offer possible approaches to unsolved problems today, in three general areas. (1) Quantification of glomerular ultrafiltration. In contrast to the complex capillary network in the mammalian glomerulus, the glomerulus of the superficial loopless (reptilian-type) avian nephrons consists of a single capillary loop. This structure, in an avian species where it can be approached directly, should for the first time permit accurate determinations of the pressure profiles and the capillary area involved in glomerular ultrafiltration in an animal with high arterial pressure. (2) Fluid reabsorption by proximal renal tubules. In some reptilian proximal renal tubules, isolated and perfused in vitro, isosmotic fluid reabsorption can occur at control rates when lithium replaces sodium or when some other substance replaces sodium or chloride or both in the perfusate and bathing medium simultaneously. Reabsorption at the control rates, regardless of the composition of the perfusate and bathing medium, can be at least partially inhibited by cold and cyanide, but not by blockers of Na(+)-K(+)-ATPase. It is also independent of the buffer system used, but it is reduced about 20% by removal of colloid from the peritubular fluid. During the substitutions, the surface area of the proximal tubule cells increases dramatically and might permit some insignificant force to be more effective in the reabsorptive process. Understanding the process involved in this, apparently unique coupling of solute and fluid transport, certainly would be very valuable in understanding coupled transport of solutes and water across epithelia in general. (3) Urate secretion by proximal renal tubules. Urate is the major excretory end product of nitrogen metabolism in birds, most reptiles, and a few amphibians. It undergoes net secretion by the renal tubules. It has been possible to learn much about the physiology of net secretion in reptiles and birds and this process appears to be similar to the much less significant secretory flux in some mammals. However, nothing is known about the molecular basis of the transport system and understanding these processes may provide important information for mammals as well as reptiles and birds. These are some examples of intriguing possibilities for comparative studies of renal physiology.