Date Published: July , 2011
Publisher: Blackwell Publishing Ltd
Author(s): Bodil Schmidt-Nielsen, Bent Schmidt-Nielsen.
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.
My interest in studying the peristalsis of the renal papilla stems from a chance observation. I had invited Karl Ullrich from Berlin to the US to collaborate with me. As we were preparing an animal for micro-puncture after we had injected lissamine green to colour the urine, we noticed that the peristalsis moved the blue-green urine in waves through the collecting ducts in the renal papilla. This intrigued me and I decided to study the peristalsis further. Another observation showed the importance of the peristalsis. In a collaborative study with Carl Gottschalk on the counter-current mechanism, his assistant would prepare antidiuretic animals for micro-puncture studies. For a better access, she would remove the pelvic wall surrounding the papilla. To our surprise, this procedure resulted in increased urine production with a lower osmolality in the prepared animals. At that point, we did not understand the meaning of the observation. Later, together with Bruce Graves, I started studies of the peristalsis.
[This section is a mildly edited transcript of the video. It should be noted that the hypothesis shown in the video and in the following transcript is incomplete. In particular, an accounting of the forces moving water is far from complete. I suggest that this is a fertile area for future research].
The peristaltic mechanism proposed above is currently incomplete. Our knowledge of the papillary concentrating mechanism is insufficient to explain its action. However, the data clearly show that the papilla is an important part of the overall concentrating mechanism.