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CO2 transport in FISH, as well as us lowly humans.

> >Date: Fri, 14 Jan 2000 09:45:11 -0500
> >From: "Peter G. Aitken" <peter at pgacon_com>
> >
> >As I detail in another post, CO2 does NOT bind to hemoglobin.
> ...
> >Date: Fri, 14 Jan 2000 09:46:24 -0800
> >From: Dave Gomberg <gomberg at wcf_com>
> >
> >At 03:48 AM 1/14/2000 -0500, Sherman wrote:
> >>Someone said recently that CO2 was transported out of the system by
> >>hemoglobin.
> >
> >I believe this is bushwa (at least in humans).
> ...
> >From: "Booth, Karla" <BOOTHK at HESKA_com>
> >Date: Fri, 14 Jan 2000 17:18:26 -0700
> >
> >The comment is wrong because hemoglobin does carry CO2 - that is how
> >CO2 is transported from your body, a natural waste from metabolism.  Your
> >body is always handling CO2 and O2 transport and the amount transported is
> >dependent on the concentration of O2, hydrogen ion concentration (pH),
> >concentration of 2,3 DPG, and concentration of CO2 at the site.  In your
> >lungs with high O2 a release of CO2 is favored and O2 is bound - in
> >exercising muscles, veins, etc where the concentration of CO2 is high, pH is
> >low (and I think 2,3 DPG is high) a release of O2 occurs and CO2 is bound.
> >It is a very complicated mechanism - not just a high concentration of one
> >gas causes that gas to be bound and carried by hemoglobin.
> >
> >Carbon monoxide is so dangerous because it is irreversibly bound by
> >hemoglobin.  Once it binds to Hb, that molecule no longer can transport
> >oxygen so with continuing CO input - you die.
>
> Since Karla has a PhD in Biochemistry (among other reasons :-), I'll go
> with her statement.
> George Booth, Ft. Collins, Colorado (booth at frii_com)
>     http://www.frii.com/~booth/AquaticConcepts/

O.K. I am going to chime in on this one, since there are lots of educated
people out there all giving bits of the correct story, but no one seems to be
referring to FISH physiology in any of their posts.  Additionally, some people seem
to be answering off the top of their head.  That's great if you can do it,
but I'm a lowly fish ecologist, so I will try to cite my information where I can.
A good portion of this info. is from Hoar (1983) and Evans (1993).  Note, if you
don't want to
hear about the physiology of CO2 transport....skip this LONG post.

First, for vertebrates in general, CO2 transport is carried out by 3 mechanisms.
Approx., 81% is through several different blood buffer systems,
11% is through carbamino compounds (including but not limited
to hemoglobin), and the remaining 8% is carried in simple solution
(Hoar, 1983)(note these are averaged values).  Since CO2 plays such
a major role in the blood buffer system, its transport and acid base
regulation are inseparable and too extensive to cover in a
post, even on APD ;-) .  I will try to summarize the actual transport
and exchange.

In mammals, carbamino compounds carry a relatively small % of the
blood CO2; however, they play a key role in respiration.
This is primarily due to the fact that the amino (NH2) groups in Hb
(as well as other blood proteins) link with CO2 in a
"non-enzymatic reaction".  This is important since this reaction is so
rapid and because carbaminohemoglobin (use that in Scrabble) forms
a reversible reaction with O2:
               O2 + HHbCO2 <---> HHbO2 + CO2
This reaction allows CO2 to be rapidly taken up or released
without significant changes in blood pH (i.e. without affecting
the blood buffer system).

In the body the deoxyhemoglobin is relatively alkaline and combines
with increasing amounts of carbonic acid (which is increasing...refer
to George/Karla's post).  See, it's all intertwined with pH regulation ;-).
In the lungs the mechanisms are reversed and CO2 is readily
discharged (Hoar, 1983).

Additionally, since hemoglobin (Hb) is a potassium salt it also acts as one
of the major blood buffers by combining with carbonic acid in the following
reaction:
            KHb + H2CO3 <--> KHCO3 + HHb

Now the problem with comparing fish to humans or other mammals
in terms of CO2 transport, really makes the apples and oranges
analogy a little too tame, more like apples and carrots.
The reason for this is that respiration and pH regulation are obviously
very different.  Aquatic animals have very low blood levels of CO2
(bicarbonate) compared to terrestrial animals.  The key reason is that fish
must circulate large volumes of water over their gills to get O2, and water
dissolves CO2 about 200 times more rapidly than O2.  Thus, CO2 diffuses
rapidly into the water via the gills so that there is only a small difference
between the levels of CO2 in arterial blood and the surrounding water (Hoar, 1983).

In fish, CO2 transport is also carried out via the 3 mechanisms I mentioned
above; however, the ratios and importance in gas exchange of each are different.
Physically dissolved CO2 usually constitutes <5% of the total CO2 in the blood of
fish.
CarbaminoCO2 (including HHbCO2) is relatively unimportant in fish
(Perry and McDonald, 1993).  Perry and McDonald (1993) speculate that it is due to
the acetylation of the terminal amino (NH2) groups on the alpha chains of hemoglobin

(take from that what you will).  The largest fraction (90-95%) of total CO2 in the
blood
of fish exists as HCO3- .  HCO3- is carried both in the red blood cells (rbc), and
the
blood plasma (majority).

The general mechanism is as follows [summarized from Evans (1993)]:
CO2 diffuses from the tissues into the plasma.  During venous transit, the
CO2 is converted to HCO3-  via catalyzation by carbonic anhydrase
(enzyme) within the rbc.  This HCO3- exits the rbc in exchange for plasma
Cl- (chloride shift, and I'm not getting into that either).  As blood flows through
the gills, HCO3- again enters the rbc in exchange for intracellular Cl-, and is then

dehydrated to CO2 with carbonic anhydrase.  This CO2 then diffuses into
the plasma and then into the water via the gill epithelium.  The excretion
of CO2 is largely a simple diffusion mechanism; thus, CO2 content
of the blood is similar to the surrounding water.

So people are correct in saying that hemoglobin plays a direct and significant
role in CO2 transport in mammals, but NOT when referring to fish.  Granted,
Hb still plays a key role since H+ ions for the dehydration of HCO3- in the
rbc are supplied by buffer groups on hemoglobin during oxygenation
(but I'll spare you that detail as well ;-).

Hope this helps a bit.  You can deduce from this that increased CO2
in the water will not directly prevent hemoglobin in fish from carrying O2, since
Hb does not play a significant role in CO2 transport in fish.  However, at
high enough levels CO2 in the water will reduce the diffusion of CO2 from
the blood, resulting in acidosis among a long list of other consequences.  I do not
know what behaviour this would illicit, but I do not find it impossible that
the fish would increase respiration, not to obtain more O2, but to get rid
of excess CO2.  Since CO2 is so soluble in water, It makes sense that
it would take quite a high concentration to impede diffusion from the gills.

That's the way I see it!  If this lowly ecologist has interpreted anything
incorrectly (quite probable) feel free to post corrections or [like you all wouldn't
;-) ]

Literature Cited

Evans, D.H., ed. 1993.  The Physiology of Fishes.  CRC Press,
     Boca Raton, Ann Arbor, London, Tokyo: 592pp.

Hoar, W.S. 1983. General and Comparative Physiology. Third Edition
    Prentice-Hall, Inc., Englewood Cliffs, New Jersey: 851pp

Perry, S.F., and G. McDonald.  1993.  Gas Exchange: 251-278. In
    Evans, D.H., ed. 1993.  (see above).