The single nephron preparation--cortical collecting tubules.

As Dr. Burg has pointed out, the rabbit has practically the only kidney fromi which nephrons can be dissected conveniently. We have dissected collecting tubules from human kidneys recently, but dissection of the rabbit kidney is far easier and the supply of kidneys certainly more dependable. Two points about the renal anatomy in the rabbit need to be emphasized. First of all, the cortical collecting tubule is formed by the union of five or six distal tubules. The distal tubules are very short in the rabbit and are not convoluted, and because of their small size and short length, we have not been able to study them satisfactorily. In addition the cortical collecting tubule descends unbranched into the outer papilla where several of the tubules join together to form the collecting ducts. As far as we know the cortical collecting tubule and outer medullary collecting tubule function similarly. There are several approaches one can take in the study of isolated collecting tubules. One can study individually perfused tubules, or specific renal tubular membranes. Dr. Burg has described the present state of the art with regard to tubular perfusion. When we study perfused nephrons we are generally interested in measuring the flux of material across the tubular wall in either direction. With regard to the measurement of fluxes in collecting tubules I will consider only water and urea. We measured net water flux in the manner described by Dr. Burg. The tubules were perfused with a Sage pump. We included inulin or albumin in the perfusate as a volume marker. For studies of water flux we found that simply holding the tubule in a moderately constricted distal collecting pipette provided a satisfactory hydraulic seal. In the collecting tubules the changes in net osmotic water flow in response to ADH are very impressive(l). In our studies of urea flux we encountered problems at the collection end, for there,