Acid Base Balance Part 4: Continuing with Exercise

I find this post incredibly confusing. So, the bottom line is this: various hormones increase in the body at different points during exercise.  Each thing has it’s own unique action on body water balance and sodium AND different actions of a molecule/hormone may be emphasized over others during exercise. And just because something is reported to do X, Y, and Z doesn’t mean it actually does X, Y, and Z during exercise.

To recap from previous post, immediately after starting exercise, blood that was being stored in the capacitance vessels returns to circulation along with fluid from the interstitium.  This contributes to cardiac/heart function. Minutes later plasma volume decreases as fluid is pushed out of the vasculature/plasma into the interstitial space and cells where it performs really important functions such as delivering nutrients, taking away waste, and being converted to sweat for thermoregulation. 

After this initial drop in plasma volume (minutes after exercise started) all the later decreases in plasma are due to the losses in overall body water because of sweat. Initially the water that forms sweat comes from the plasma, but later the cells (ICC space) get progressively more and more dehydrated.

Remember that beautiful way that the kidney increase plasma volume by retaining more sodium than usual, that then drew water into the plasma compartment because of the increased number of solutes?  (This was the result of ALDO, which was the first example I gave you of how water and solute changes could occur in the body to address low plasma and add water back into that compartment).

But during exercise sodium is actually EXCRETED by the kidney, along with potassium.  This excretion (losing) of sodium was called both “significant” and “minimal” with one paragraph in Hinchcliff so I’m not really sure what to think...... this was my first reality check that in the exercising animal the biology is really complicated.

So what is causing this exodus of Na from the body, when theoretically it should be retained so that it can pull water into the plasma?

It’s probably a secondary response to another hormone that is increased in the exercising horse, called Atrial Natriuretic Peptide (ANP).

ANP is the body’s answer to the rapid fluid shift that happen during the beginning of exercise. 

Imagine the cardio system as a closed fluid system.  There’s a pump at one end that can pump more or less fluid through the system.  The diameter of the pipes can change, the number of pipes in use can change, and the total amount of fluid within the pipe system can change.  During the beginning of exercise with the sudden changes of fluid shifts in and out of the system, and the pump engaging faster, you can imagine how difficult it would be to control blood pressure and blood flow and make sure you are getting the right amount of fluid to the right places at the right pressures.

ANP is a hormone produced by the heart that fills this job description.  It regulates blood flow and blood pressure during exercise.  In doing it’s job it inhibits vasopressin, renin (which starts the whole aldosterone cycel), ALDO, and others and is cited by Hinchcliff as the reason sodium is actually excreted by the kidneys during exercise.  It’s a “secondary” effect of ANP doing it’s primary job of regulating the cardio system.

Vasopressin is a defender of plasma volume, fluid and elyte balance.  Although it is initially supressed by ANP at the beginning of exercise, eventually it is able to overcome ANP. In one study, ANP steadily increased to a peak at 40 min and then remained elevated through 60 minutes, but didn’t continue to rise.  However exercise causes an increase in plasma vasopressin that is correlated with both duration and intensity.  ie - the longer and harder the exercise is, the more vasopressin that is in your plasma.

Side note: Vasopressin = AVP (arginine vasopressin) = ADH (anti-diuretic hormone).  I chose to use the name “vasopressin” over my other choices since I already had a bunch of “A” acronyms floating around in this series. But if I accidentally call it AVP or ADH, now you know it’s the same thing. :)

Increased vasopressin during exercise is VERY good during exercise because it causes the body to retain water, and the same things/receptors that signal for more vasopressin to be release also increase thirst.

So.....what would you think if I told you that horses that are over hydrated at the beginning of exercise had high ANP concentration compared to under hydrated horses? Does that make sense now that you know ANP inhibits vasopressin and vasopressin makes you thirty and retain water? 

Vasopressin does more than just retain water, it also affects the cardio system, and does other helpful stuff during exercise like prevent re-sequestration of blood cells into the spleen (horses have a “splenic reserve” of blood cells within their spleen that are released during exercise. Vasopressin makes sure they stay out into the circulation instead of going back into the spleen). 

I have to mention that I did find something that said while vasopressin does increase during steady state submaximal exercise (ie endurance....) it doesn’t affect water clearance.....and the body is using vasopressin during exercise mainly for it’s cardio effects and without changing the water clearance.  But then the next thing I saw a mere page later was “Increase in AVP during exercise stimulates thirst and drinking after exercise causing a decreased in free water clearance by the kidneys (water retention), may influence uptake of Na and water from colon”.....which demonstrates how confusing researching this topic was.

Overall the body tries to protect plasma volume and blood pressure. Sometimes it seems like its contradicting itself - like excreting Na, or suppressing thirst in the first hour of exercise....but in the end water everything works together to make sure that volumes and pressures are maintained.

Urine flow

Urine flow INCREASES during exercise.  This happens in horses AND humans.  Maybe I have an excuse for why I need to use the bathroom so many time during a marathon?

It’s a significant increase in urine production - 45%.  However it’s important to remember that the total extra volume lost in urine is small compared to the loss of water the horse is experiencing in its sweat.

The increase in urine confuses me because this is at odds with the increased level of vasopressin during exercise, which retains water. No idea.  But wanted to throw this out there just because it was mentioned. 

Post exercise urine flow remains increased for about 30 minutes and results in the increase excretion of water, sodium, and potassium.

Post exercise

Post exercise is the ACUTE phase of exercise recovery.  There is also a secondary “Adaptive” phase where the body is actually changing itself to be able to respond to exercise better the next time around.  The two phases are completely different things. 

The acute phase is concerned with replenishment of fluids and electrolytes.  The body doesn’t just replace the fluids and electrolytes and then “hope for the best”.  It actually makes proactive CHANGES in the hormones and body water so that it does better next time.

This was a revelation for me - while it’s natural to think of muscles getting stronger and adapting to training, I hadn’t thought about similar changes happening to the actual water, electrolytes, and exercise hormones that are also working hard during exercise. 

The specific adaptions the body makes in the secondary “adaptive phase” will be the topic of the next post. 

Even though horses can be moderately dehydrated after 50 or 100 miles (moderately dehydrated 1000 pound horse could be missing almost 11 gallons) they can replace this fluid loss overnight on their own! ALDO remains elevated for several hours post exercise and (along with vasopressin) increases the uptake of water and elytes from the gut.  ANP also remains elevated post exercise, probably to help continue regulating the cardiovascular system.


But....
But what about sodium and solutes and what they are doing in exercise?  Wasn’t that the point of these post.  Errr......yes.  But it’s really hard to talk about sodium without introducing ANP, vasopressin, ALDO and all those other “big picture” things that are happening during exercise that are affecting water because in the end THAT’s what is affecting Na.  Remember that Na and water are best friends.  Where one goes, so goes the other.

Even with all these hormones floating around and moving water and sodium around, remember that overall concentration of sodium within the ECC changes relatively little, even after riding 100 miles.  The TOTAL amount of sodium is lower.....because the horse has lost water/sweat and the sodium contained in it.  To replace this water and salt lost during exercise, the body is pulling water in through a increase in thirst and increased absorbtion from the gut, and is pulling in sodium from increased absorbtion from the gut. And voila!  You maintain enough plasma volume to thermoregulate and not have your cardiovascular system crash (hopefully).

Here’s a summary of Na and Water movement during exercise

1. When vasopressin is increased during exercise (remember that it is initially supressed by ANP) it increases WATER and SODIUM uptake from the gut.

2. More water and sodium is needed from the gut because water and sodium is being lost from the interstitial compartment as sweat.

3. Losing water from the interstitial compartment decreases overall water content from the ECC, which includes the plasma.

4. Low plasma pisses the body off and activates all those hormones we just talked about in the last section.......ANP to protect cardiovascular function, vasopressin to help cardiovascular function, increase thirst signals, and continue to demand more water and sodium from the gut.

5. Initially there is an increase (described as “small” “minimal” “not insignificant”) in sodium loss by the kidneys because of the increase of ANP.  ALDO would decrease this loss and actually retain more sodium - but it doesn’t have a significant effect during exercise, just for a couple of hours afterwards (maybe even 24 hours when you are doing the first couple of training rides). Whether or not sodium continues to be secreted at a higher rate past the acute stage of exercise (when ANP reaches it’s peak) is not something I could get a consensus on.

6. Vasopressin should be helping the body to retain water, but the jury seems to be out whether during exercise it actually does this.......

7. There is minimal changes in sodium concentration in the plasma during long term exercise

8. There is a substantial decrease in total sodium CONTENT in the body.  10-20% of the body’s sodium is in the gut.  Which is why #1 and #4 occur.

9. Horse sweat, having the same sodium content as the ECC that it’s being lost from means that there aren’t huge osmolality changes in the plasma because of sweat loss, which means that I’m finding less information on how swings in sodium concentration affect water intake etc. in horses, compared to the human literature.

10. We haven’t discussed potassium in detail yet because it is intimately associated with acid base and electrical activity, kind of like sodium is linked with water, however overall potassium is secreted as a result of all these mechanisms.

11. there are lots of vague “sodium needs to be replaced” and “effective electrolyte replacement” to address the loss of sodium in sweat but the how’s are lacking.  Post exercise intake and the “substantial gut reserve”, as well as the adaptive phase changes we will talk about next time seem to be the natural mechanisms for replacing sodium.

Coming up next: long term adaptation to exercise, potassium and acid-base balance.


A couple of warnings about evaluating literature on this topic
In case you want to do your own digging around.....When looking up literature on the subject of endurance and how electrolytes and water respond it’s really important that you understand what the authors are classifying their different exercise types as.  In general endurance exercise = submaximal or low intensity exercise. The body responds really differently to maximal (thoroughbred racing) exercise, so aren’t appropriate to use if you are researching endurance.  But endurance/submaximal means different things to different studies.

For example, one review I came across listed “submaximal exercise in trained horses”. Sounds perfect eh?  Except when you came across their definition.  Sampling was taken 15 post exercise because that’s after all the initial fluid shifts occur.....but was also chosen because it was 10 minutes before the horses started to become fatigued. 

Ummm.....not as close as I thought.

And then I came across a section that was actually labeled as “endurance” and “submaximal”.  Yeah!........except it was measured in drafts pulling heavy loads on consequetive days. Again, different from what we are looking for.  The lesson here is that you cannot just take titles and abstracts and assume that something with the right key words are going to be relevant to competing a horse across 25 or 50 or 100 miles.