REGULATION OF BLOOD pH: REVISITING THE “LACTATE PARADOX””

Claudio Marconi

IBFM-Sect. of Muscle Physiology and Proteome, National Research Council, Milano, Italy;

[ADP] oxidative phosphorilation

PCr + ADP + H+ ATP + Cr

LOHMANN REACTION

In normoxia, the transition from rest to exercise is characterized by rapid changes in muscle and blood [H+]

CK

0

10

20

30

40

50G

lyco

lytic

AT

P su

pply

, %

0

10

20

30

40

50G

lyco

lytic

AT

P su

pply

, %

PERCENTAGE OF ATP SUPPLY FROM GLYCOLYSIS (LACTATE GENERATION UNDER AEROBIC CONDITIONS)

wrist flexor tibialis ant quadriceps tailshaker

rattlesnake

ATP by an excess of pyruvate supply

ATP by glycogen shunt

The total metabolic proton production increases with time as the rate of PCrbreakdown declines and anaerobic glycolysis plays a progressively greater role.

In hypoxia, the mechanism involved in proton generation are the same as in normoxia, but anaerobic glycolysis appears to be regulated differently.

CO2 HYDRATION

CO2 + H2O [H2CO3] H+ + HCO3-

Henderson-Hasselbalch

pH = pK + log[HCO3

-]0.03 PCO2.

pK = 6.12

normal buffer line

HYPERVENTILATION PCO 2= 20

torr

PCO 2= 40 to

rr

urinary excretion of HCO3-

N

acute hypoxia

incomplete accl.

complete accl.

Cl reabsorption-HCO3 excretion-

[HCO3]p(mM)

-25

20

15

7.3 7.4 7.5 7.6 7.7 pH

BUFFER POWER IN BLOOD

β = Δ [H+]/ΔpH

-plasma proteins -19% (due to Ht >)-hemoglobin +20% -phosphate ions =-2,3 DPG +97% (due to Hb >, DPG/Hb>)

TOTAL NON BICARBONATES +35%

At 5,000 mβ

0

100

200

300

Buffer power, mM H+per unit pH change

0 2000 4000 6000

Altitude, m

total

Hb

2,3-DPG

plasma proteins

inorganic phosphates

TOTAL NON-BICARBONATE BUFFER

INCREASING NON-BICARBONATE BUFFER POWER HAS TWO CONTRASTING EFFECTS:

The pH shift secondary to a given change in the acid-base balance is less

The amount of acid or base needed to compensate that change is increased

The two corrections may not be equivalent

EVALUATION OF THE EFFECTS OF AN INCREASED NON-BICARBONATE

BUFFER POWER ON THE BICARBONATE β

0

10

20

30

40

50

60

70Bicarbonates, mM

6.9 7.0 7.1 7.2 7.3 7.4 7.5 7.6 7.7 7.8 7.9

pH

β=252.1

β=186.7

Start a

bb'

c

PCO2=40 mmHg

PCO2=21 mmHg (at 5,000 m)

Uncompensated

Fullycompensated

Increased non-bicarbonate β at altitude contributes to decrease alkalosis derived from hyperventilation

Metabolic acidosis

As the main producer and consumer of large amounts of lactic acid,

muscle plays a key role in the body acid-base balance

• proteins (imidazol groups of histidine residues)• inorganic phosphates and bicarbonate

Muscle buffer capacity depends on theconcentration of:

During heavy exercise lactic acid accounts for 85-95% of the total H+ load

Muscle buffer capacity ranges from 25 to 40 mmol H+· l-1 H2O · pH-1

and depends on: type, intensity, time course of muscle activity, and training.

• 8 weeks of sprint training increases β by 38% (likely as a consequence of increased levels of dipeptides such as carnosine, which is mainly present in white muscle)

• Endurance training has no effect on β• Altitude acclimatization alters β, mainly because of

changes in muscle protein concentration.

Regulation of muscle pH at rest

• At rest pH ranges from 7.10 to 7.15• It depends on the balance between:

H+ metabolic production and H+ inflow, Transport of H+ out (Na+/H+ exchange) or HCO3

- in (HCO3- /Cl- exchange)

-180 -60 60 180 300 420 540 6600

1

2

3

Time (sec)

VO2 (

L/m

in)

Regulation of muscle pH at exercise

ATP + CrPCr + ADP + H+CK

CO2 and HCO3-

Lactic acid

05

1015202530

-5 0 5 10 15

6,66,76,86,97,07,17,2

-5 0 5 10 15

Rest Ex

(min)

pH

[La]mmol·kg-1

Recovery

05

1015202530

-5 0 5 10 15

6,66,76,86,97,07,17,2

-5 0 5 10 15

Rest Ex

(min)

pH

[La]mmol·kg-1

Recovery

Quadriceps muscle

Schematic model of pH regulation

a: prevailing mechanism in moderate exercise (it may be responsible for the greater accumulation of H+ than La- in the extracellular space after short bouts of supramaximal exercise)b: prevailing mechanisms in maximal exercise

H+CO2 + H2OHCO3

-

Lactic acid Lactate

Fuels Metabolism Intracellular buffers

Passive fluxes Transport systems

La-

H+

Na+

H+a

Na+

HCO3-

Cl-H+

bb MCT

ACID-BALANCE DURINGSUPRAMAXIMAL EXERCISE

[Lab] = 25 mMpHb < 7.0HCO3

- buffers ~75% of protonsNon-bicarbonate buffer system plays an increasing role for [Lab] > 15 mM

[Lam] = 40 mM pHm = 6.6

MCT : a family of monocarboxylate La-/H+ transporters

• MCT are membrane proteins and are pH sensitive• MCT1 is more expressed in slow-twitch fibers

(they need a fast MCT transport because theyproduce La- for long time periods) and myocardial fibers than in fast-twitch fibers.

• MCT4 is not correlated to fiber type• It is suggested that MCT1 is specialized for

uptake, whereas MCT4 is specialized for efflux.

MCT and HYPOXIAThere are relatively few data supporting a role of MCT in regulating the acid-base balance in hypoxia.

ACID-BASE BALANCE AT EXERCISE DURING ALTITUDE ACCLIMATIZATION

pHa in acclimatized lowlanders:literature survey

7.3

7.4

7.5

7.6

7.7

7.8

0 2000 4000 6000 8000 10000

Altitude, m

Cerretelli unpublishedLahiri 1967Winslow 1984Winslow estimatedSutton 1988, chamberBender 1989Kayser 1993

pHa in acclimatized lowlanders:literature survey

7.3

7.4

7.5

7.6

7.7

7.8

0 2000 4000 6000 8000 10000

Altitude, m

Cerretelli unpublishedLahiri 1967Winslow 1984Winslow estimatedSutton 1988, chamberBender 1989Kayser 1993

7.3

7.4

7.5

7.6

7.7

7.8

0 2000 4000 6000 8000 10000

Altitude, m

Cerretelli unpublishedLahiri 1967Winslow 1984Winslow estimatedSutton 1988, chamberBender 1989Kayser 1993

Samaja et al Acta Physiol. Scand. 1997 * P<0.05 from Caucasians

7.40

7.45

7.50

7.55pHa

3400 5050 6450

Caucasiansafter 3 weeks

SherpasCaucasians

0

10

20

30

40PaCO 2, mmHg

3400 5050 6450

* * * * **

0102030405060

PaO2, mmHg

3400 5050 6450

Altitude, m

Samaja et al Acta Physiol. Scand. 1997 * P<0.05 from Caucasians

7.40

7.45

7.50

7.55pHa

3400 5050 6450

Caucasiansafter 3 weeks

SherpasCaucasians

0

10

20

30

40PaCO 2, mmHg

3400 5050 6450

* * * * **

0102030405060

PaO2, mmHg

3400 5050 6450

Altitude, m

Hb-O2 equilibrium curve

PO2 , mmHg

SO2

, %

0

20

40

60

80

100

0 25 50 75 100

pH 7.4PCO2=40 mmHg[2,3-DPG]/[Hb]=0.8

P50

H+

CO22,3-DPG

Hb-O2 equilibrium curve

PO2 , mmHg

SO2

, %

0

20

40

60

80

100

0 25 50 75 100

pH 7.4PCO2=40 mmHg[2,3-DPG]/[Hb]=0.8

P50

H+

CO22,3-DPG

Blood-O2 equilibrium curve

0%

25%

50%

75%

100%

0 20 40 60 80

Hb-

O2

satu

ratio

n, o

r SO

2

PO2 , mmHg

pH 7.4, PCO2 = 40 mmHg[2,3-DPG] / [Hb] = 0.8

pH 7.6

[2,3-DPG] / [Hb] = 1.2

P50

Blood-O2 equilibrium curve

0%

25%

50%

75%

100%

0 20 40 60 80

Hb-

O2

satu

ratio

n, o

r SO

2

PO2 , mmHg

pH 7.4, PCO2 = 40 mmHg[2,3-DPG] / [Hb] = 0.8

pH 7.6

[2,3-DPG] / [Hb] = 1.2

P50

DURING EXERCISE, THE OVERALL BENEFICIAL EFFECT OF ALKALOSIS IS QUESTIONABLE

PROLONGED ALKALOSIS IS NOT COMPATIBLE WITH NORMAL BODY HOMEOSTATSIS

AT REST, THE NEED TO OXYGENATE TISSUES CONFLICTS WITH THE NEED TO MAINTAIN H+ HOMEOSTASIS

(Edwards,1936)Altitude (km)

0 1 2 3 4 5 6 70

1

2

3

4

5

6

7

8

9

10continuousintermittent[Lab]

(mM)

LACTATE PARADOX

Work load (watt)0 50 100 150 200 250 300 350 400

0

2

4

6

8

10

12

14

[La b

] (m

M)

SL1ALT1ALT2ALT3

resting level

0

3

6

9

12

15

0 1 2 3 4 5 6 7 8 9

Altitude (km)

[La b

]max

(mM

)Operation Everest IICaucasians (different sources)Altitude natives (different sources)Caucasians (Marconi, pers.comm.)Sherpas (Marconi, pers.comm.)Caucasians (Marconi et al., 1998)

resting level

0

3

6

9

12

15

0 1 2 3 4 5 6 7 8 9

Altitude (km)

[La b

]max

(mM

)Operation Everest IICaucasians (different sources)Altitude natives (different sources)Caucasians (Marconi, pers.comm.)Sherpas (Marconi, pers.comm.)Caucasians (Marconi et al., 1998)

Operation Everest IICaucasians (different sources)Altitude natives (different sources)Caucasians (Marconi, pers.comm.)Sherpas (Marconi, pers.comm.)Caucasians (Marconi et al., 1998)

(Cerretelli, 1976)

Buffer capacity of the whole body

1) At altitude, the buffer capacity of acclimatized lowlanders is lower than at sea level

2) The highest Δ[H+] is similar at altitude and at sea level, and is independent of [Lab]

1

2

Simultaneous measurements of arterial pH and [La] in a group of acclimatized lowlanders after 9 weeks at 5,260 m did not show any significant difference in total body buffer capacity, compared to sea level, despite the reduction in bicarbonate buffer. (Van Halle et al, 2001; Wagner et al, 2002)

This implies that only non-bicarbonate buffers play a substantial role in the regulation of acid-base balance during anaerobic exercise at altitude

048

121620

SL ALT

[La b

] pea

k(m

M)

Kayser et al, 1993

0

10

20

30

40

ControlBicarbonate

Δ[H

+ ] (n

M)

SL ALT

*

*

0

100

200

300

* *

0

3

6

9

12

15

*** *

Wpe

ak(W

)[L

a b] p

eak

(mM

)

Grassi et al, 1996

SL Alt1

Alt1

-O2

Alt5

Alt5

-O2

·

0

3

6

9

12

15 SL

ALT

Grassi et al, 2001

[La b

] pea

k(m

M)

10 s 30 s Increm. ex

Lactate paradox is at least in part consequence of transient reduced working capacity and/or exercise mode

Until year 2000 there was consensuson the occurrence and persistence of a lactate paradox, even though the origin of the phenomenon was poorly understood.

An interesting feature of lactate paradox was its slow reversal upon restoring normoxia

[Lab]p (mM)

0

3

6

9

12

15

*

* **

*SL

1

ALT

1

ALT

2

ALT

3

SL2

SL3

SL4

SL5

SL6

[Lab]p (mM)

0

3

6

9

12

15

*

* **

*SL

1

ALT

1

ALT

2

ALT

3

SL2

SL3

SL4

SL5

SL6

5,050 m

0

4

8

12

16

0 25 50 75 100

Tib 1

Tib 2

Nep

[La](mM)

% VO2 peak.

Hochachka et al (1991) hypothesized that lactate paradox is permanent in Quechuas transferred to sea level.

24

68

101214

0 2 4 6 8 10Altitude exposure (weeks)

[La b

]max

(mM

)

Lundby et al ., 2000 Grassi et al.,1995 van Hall et al., 2001 Grassi et al.,1996 Grassi et al.,2001Kayser et al.,1993b Kayser et al., 1993a Personal Data Personal Data

Evolution of the “lactate paradox” as a function of exposure duration at altitudes ranging from 5,000 to 5,500 m. There is a fast drop of [La]max within 2-5 weeks from the onset of hypoxia and a progressive recovery thereafter to pre-exposure levels

• It is assumed that a better coupling between ATPsupply and demand may be responsible for lactate paradox.

• The progressive disappearance of lactate paradox during prolonged acclimatization means a loss of an acquired adaptive feature, as a consequence of a progressive impairment of muscle function.

• Lactate paradox may underlie a complex gene-based “reorganization” of muscle metabolism occurring in hypoxia.

Time, weeksHypoxia

Lactate paradox

HIF-1

Enzymes

Deterioration

ROS

Antioxidants

0 5 10 Time, weeksHypoxia

Lactate paradox

HIF-1

Enzymes

Deterioration

ROS

Antioxidants

0 5 10

Proposed time courses of the main events that occur during a period of hypoxia and after return to sea level (from Cerretelli and Samaja, EJAP 2003)

1

2glycolytic

3