Today I decided to kill some time waiting for K to get home from band practice by reading the Articles in Press for AJP: Regulatory, Integrative, and Comparative Physiology. For one thing, I am laughing heartily at the title of this paper: Colorectal and Rectocolonic Reflexes in Canines: Involvement of Tone, Compliance and Anal Sphincter Relaxation. Yes, I know, it is incredibly sophomoric of me, but I can’t help laughing at the fact that someone spent that much time researching dog poop. Perhaps someone even wrote their thesis on dog poop. I think I might be jealous of that person, actually. Please don’t think that I have an attitude of diminishing the importance of researching colorectal reflexes in canines; I am perfectly capable of taking this paper seriously and judging it on its own merits and not the fact that it is about dog poop. I’m just a little stir crazy today. I suppose that since I put so much energy into laughing at it, I now have to take the time to read it in the interest of fairness and atonement. Oh well.
One article I did take the time to read, though, is Race, Sex and the Regulation of Urine Osmolality-Observations Made During Water Deprivation by Hancock et al. Hancock and colleagues got an almost equal mix of white and black men and women to agree to 24 hours of water deprivation, during which time they measured urine and plasma osmolality, vasopressin levels, urine volume, and a few other things. I read it, and it had me thinking some thinky thoughts, so I figured I’d write down my thinky thoughts and share.
They found that black people had more concentrated urine than white people at baseline and after water deprivation, as well as a lower urine flow rate both at baseline and after deprivation, despite having similar levels of vasopressin. Vasopressin, among other things, is responsible for putting water channels in the nephron epithelium for water reabsorption. Generally, the more vasopressin you have present, the greater potential for water reabsorption in the kidneys, resulting in urine that has less fluid and more solutes (it isn’t as simple as that, but that’s one of the factors involved). Since vasopressin isn’t causing the observed difference between white people and black people, the second most likely culprit is an increased counter current multiplier gradient caused by more active sodium pumps on the ascending loop of Henle. Shortly: In the loop of Henle, a gradient is established by actively pumping sodium back into the body in the ascending loop so that water will follow via osmosis in the descending loop. The more sodium you pump back into the body, the more water gets reabsorbed. The authors and I reached the same conclusion in that respect, and apparently there is existing literature to suggest that black people have more active sodium pumps in the loop of Henle than white people. It would have been nice if they had also measured aldosterone levels, since aldosterone is responsible for adding the sodium pumps.
However, one question they didn’t ask is why black people would have this evolutionary propensity for concentrated urine. The ecological physiologist in me is screaming that this is a ‘habitat’ aridity issue, although it is certainly more complicated than that. While a great part of Africa is desert, and a desert environment would definitely put selective pressure on all mechanisms for water conservation, there are also great parts of Africa which are not desert. And, indeed, Middle Eastern populations that are technically considered Caucasian would have evolved in desert environments as well. That isn’t to say that historical differences in habitat aridity aren’t a major player in this difference, but it probably isn’t the only reason, since the black and white populations weren’t actually that homogenous.
Men had higher vasopressin levels than women during the first 12 hours post-deprivation, which the authors claim explains why men have been shown in other studies to generally have more concentrated urine than women (which wasn’t seen in this study at baseline or during the first 12 hours, which the authors attribute to small sample size). However, after 12 hours, women had more concentrated urine than men. Not only that, but after 12 hours, men actually had a negative correlation between vasopressin levels and urine osmolality. This is unexpected, and as the authors state, the significance is unclear. I do not personally know why women would need to have more dilute urine than men at baseline, although the authors attribute this to the fact that women are more sensitive to hyponatremia. It makes sense to me that women would be more adapted towards water conservation when water is resource limited, due to the high resource costs of reproduction.
The voice of this paper is a bit overly simplistic, which could be problematic but I actually appreciated it, since I’m interested in renal physiology but I’m actually quite the n00b and wouldn’t have understood a bunch of renal jargon. The scope was also really limited. I would have liked to have seen more ethnic groups represented in a study like this. It is hard to draw evolutionary conclusions from just two populations. This tells us a bit about the difference about white people and black people, but nothing about Hispanic or Asian populations. I also take issue with the authors’ decision to refer to ‘blacks’ and ‘whites’ instead of calling them black people and white people. Perhaps this is just me being overly political (I mean, scientific writing values conciseness over political correctness), but I really hate it when people turn adjectives into nouns like that. I take similar issue to the fact that they referred to men and women as ‘males’ and ‘females’. I guess you can’t win them all.
Michael L. Hancock, II, Daniel Georges Bichet, George J. Eckert, Lise Bankir, Mary Anne Wagner, and J. Howard Pratt (2010). Race, Sex and the Regulation of Urine Osmolality-Observations Made During Water Deprivation Am J Physiol Regul Integr Comp Physiol