On the function of the mammalian renal papilla and the peristalsis of the surrounding pelvis.

This is an informal personal review of the development over time of my ideas about the concentrating mechanism of the mammalian renal papilla. It had been observed that animals with a need to produce a concentrated urine have a long renal papilla. I saw the function of the long papilla in desert rodents as an elongation of the counter-current concentrating mechanism of the inner medulla. This model led me to overlook contrary evidence. For example, in many experiments, the final urine has a higher osmolality than that of the tissue at the tip of the papilla. In addition, we had observations of the peristalsis of the renal pelvis surrounding the papilla. The urine concentration falls if the peristalsis is stopped. I was wrong; together, these lines of evidence show that the renal papilla is not just an elongation of the inner medulla. We are left without a full explanation of the concentrating mechanism of the mammalian renal papilla. It is hoped that other researchers will tackle this interesting problem.

Effect of peristaltic contractions of the renal pelvic wall on solute concentrations of the renal inner medulla in the hamster.

The mechanism of solute accumulation in the renal inner medulla remains an unresolved issue. Experiments were carried out in hamsters to address the possibility that the peristaltic contractions of the renal pelvic wall surrounding the inner medulla play a role in the inner medullary concentrating process. The right renal pelvis was subjected to one of four manipulations (surgical removal of the pelvic wall, paralysis of the pelvic wall with xylocaine, inhibition of pelvic contractions by direct application of heat, or sham treatment) followed by analysis of the inner medullary solute concentrations in the right kidney vs. the untouched left kidney. Removal of the pelvic wall resulted in a marked reduction in inner medullary osmolality, confirming prior observations. Paralysis of the pelvic wall with xylocaine produced a similar decrease in inner medullary osmolality, despite the fact that urine flow was maintained. In contrast, sham treatment (surgical exposure of the right renal pelvic wall without any further manipulation) did not decrease inner medullary osmolality. To test whether the decrease in urinary osmolality following xylocaine treatment could have been due to a side effect of the drug, pelvic peristaltic contractions were eliminated in another way, by direct application of heat to denature the smooth muscle of the pelvic wall. This procedure also significantly decreased inner medullary osmolality. We conclude that elimination of the contractions of the renal pelvic wall in the hamster significantly impairs inner medullary concentrating ability.

The renal pelvis: machinery that concentrates urine in the papilla.

Two decades ago, Bodil Schmidt-Nielsen and Bruce Graves documented the rhythmic contractions of the renal pelvis in a remarkable video, visually demonstrating how peristaltic waves empty the papilla and how the subsequent elastic recoil draws water from the collecting duct and into the tethered-open ascending vasa recta. Thus a periodic hydrostatic gradient generates an axial osmotic gradient. This review recapitulates the video and offers a link to figures and scenes digitized from the original tape.