For her final “intern question”, Hannah chose a question fundamental to the practice of internal medicine but whose answer remained obscure until very recently.
Why does a urinary tract infection (UTI) burn?
This question, like so many others that emerge during intern year, arose at 3 in the morning while admitting a patient. Other infections don’t “burn”. A sore throat, for example, doesn’t burn even though it would seem we have “burning” innervation to the throat.
In order to understand the explanation it is necessary to first review the innervation of the bladder and urethra. Recall that the bladder is giant detrusor muscle and in order for it be effective it must contract all at once. This means that there must be a feed-forward system where if one muscle fiber within the bladder is contracting, the rest are contracting. As this is occurring, there is a sensory network in the urethra and bladder sensing bladder pressure and urethral flow and, if the bladder is more distended, induce further contractions, or if less distended, tightening the external urethral sphincter. The urethra is also important because it is the entry site for bacteria.
Among the cells that are part of this network are cells called urethral brush cells, named because they are structured like brush cells in the respiratory epithelium. Amazingly, they were only recently described, in 2014. But these amazing cells do more than just sense urine flow. Specifically, they have transient receptor potential cation channel (TRP) cells and transient receptor potential cation channel subfamily V member 1 (TRPV1) channels which are a part of the signal transduction pathway for bitter and umami flavors.
But, and this may be the piece that explains the famous “burning” sensation, TRPV1 is also responsible for the sensation induced by capsaicin. Capsaicin, of course, is notorious for causing a “burning” sensation.
But do UTIs taste good?
As noted above, TRPV1 channels are part of the signal transduction pathway for bitter and umami flavors. Interestingly, glutamate (one of the two “umami” triggers, as discussed in Episode 9) is positively linked to the pathogenic potential of proteus in the urine and pseudomonas makes biofilms that trigger the “bitter” taste. Heat-inactivated uropathogenic E coli trigger the same cascade and if a bitter compound is placed in the urethra it enhances activity of the bladder detrusor! Finally, TRPV1 has been found on much of the urothelium and is responsible for tasting vanilloid flavors. There are a remarkable number of tastes circulating in the setting of a urinary tract infection.
Take Home Points
- The urinary tract has immense sensory innovation that is crucial for normal urination.
- In urinary tract infection (UTI), a variety of factors may trigger transient receptor potential (TRP) signaling.
- TRP signaling results in the typical sensory/taste cascade that we associate with “burning” but instead, during a UTI, may be what alerts us to the presence of a uropathogen and promotes detrusor activity.
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Credits & Citation
◾️Episode written by Hannah Abrams
◾️Show notes written by Tony Breu and Hannah Abrams
◾️Audio edited by Clair Morgan of nodderly.com
Abrams HR, Cooper AZ, Breu AC. Why does a UTI burn? The Curious Clinicians Podcast. June 23, 2021