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274
J. Physiol.^ (1944) I03, (^274 289) 546.262.I:6I2.
THE ACID-LABILE CO2 IN MAMMALIAN MUSCLE
AND THE (^) pH OF THE MUSCLE FIBRE
BY E. J. CONWAY AND P. J. FEARON
The Departnent of Biochemistry, University College, Dublin
(Received 7 December 1943)
Wallace & Hastings [1942] and Wallace & (^) Lowry [1942] have obtained rela-
tively high values for the C02 content of resting mammalian muscle, which at
first sight appear to be inconsistent with the theory concerning the distribution
of ions in frog muscle, as presented from this laboratory [Conway & Boyle,
1939; Boyle & Conway, 1941]. With their figure of about 11 mM./kg. of total
C02 in resting mammalian muscle, and (^) allowing the free (^) C02 to be in approxi- mately equal (^) concentration within and without the fibre, and the (^) remainder of the total acid-labile 002 to be (^) HCO, it (^) follows that the ratio of HCO- concentrations across the membrane would be far higher than that (^) expected from a Donnan relation with (^) the K (^) concentrations. Therefore, either the mammalian muscle fibre has a very different (^) electrolyte distribution and a different membrane (^) permeability from that of frog muscle, or the (^) fraction of the (^) total 002 assigned by Wallace & (^) Hastings to HCO is much too high. It yvas decided (^) therefore to investigate the nature of the total available (^) C02 in mammalian muscle. The solution to this problem has the further significance that it (^) allows the pH value inside the muscle fibre to be (^) determined; for whatever fraction is shown to be in (^) truth HCO within the fibre, this, in con- junction (^) with the free C02, determines the pH, at (^) least within the accuracy of the pK' figure. The (^) following symbols are used in the (^) calculations:
[W]s=g. water^ per^ kg.^ serum,
[W]m=g. water per kg. muscle,
[HCO3]m, [HCO3]8, (^) [HCO3]e, [HCOS]b=^ mM.^ HC0^ per kg.^ muscle, (^) serum, extracellular fluid and whole blood respectively,
[HC03], =^ mM.^ HCO per kg. fibre water,
[HC03],nv =^ mM.^ HCO^ per^ kg.^ extracellular^ water,
[HCO3]. =^ mM.^ HCO^ per^ kg.^ serum water.
ACID-LABILE C02 275
Similarly (^) with the other ions, Na, Cl, K and H.
[C02]m, [C02]8, (^) [CO2OJ. and^ [C02]ew are^ likewise mM. free^ C02 per kg. muscle, serum, (^) fibre water and extracellular water,
[C02, total]m=the total acid-labile C02 per kg. muscle,
[CO2, Ba sol.]m=the total Ba-soluble C02 in muscle.
The true H2C03 is considered as included in the free C02 concentration.
METHODS
Total acid-labile CO2 in muscle. The animal was anaesthetized with (^) ether, and then a (^) portion of the (^) abdominal muscle quickly excised, and introduced (^) immediately into N/5 (^) C02-free KOH in a, weighed round-bottom centrifuge tube, the cork being removed (^) momentarily for the purpose. 25 ml. N/5 KOH (^) was uAed as a routine in a 50 ml. tube with about (^8) g. muscle. Unless (^) the quantity of abdominal (^) muscle was small it was cut as rapidly as possible into (^) approximately 2 g. portions. The whole (^) process of removal and introduction occupied no more than 10-15 sec. (^) from the time of sectioning the muscle. With the leg muscle of (^) rabbit or cat, about 10 g. were quickly excised and held over the (^) KOH tube; quantities a few mm. thick were (^) rapidly sectioned by sharp scissors and dropped into the alkali. The stopper was replaced, the tube weighed, (^) and the contents well mixed. The whole was then placed in the refrigerator for an hour, being shaken several times throughout this period. The tube was then spun for a few minutes and 0-5 ml. (^) volumes pipetted quickly into the outer chambers (^) of Conway units (no. 2 size), already prepared with 0-2 ml. N/25 Ba(OH)2 containing 5 % B.D.H. (^) universal indicator, in the central chamber, and 0-2 ml. 2N (^) H2S04 in the outer chamber [Conway, (^) 1939; O'Malley, Conway & FitzGerald, 1943]. The titrations were (^) carried out after an hour, with N/40 HCI from a (^) Conway micro-burette [Conway, 1939], to a (^) green end-point. The determinations were made (^) in triplicate. The large standard units (Conway unit no. (^) 1) were also occasionally used with 2 ml. extract, 1-3 ml. (^) N/40, Ba(OH)2 in the central and 0-5 ml. 2N H2S04 in the (^) outer chamber, with subsequent titrations of 1 ml. vol. (^) removed from the central chambers into (^) small tubes, using N/10 HCI. Bla7dk determinations (^) were carried out in a similar manner, a volume of C02-free (^) water corre- sponding to the water content of the muscle being pipetted into the alkali, and carried through the whole procedure as for muscle. Calculation of mM. C02/kg. mu8cle. In (^) the calculation it is assumed that the muscle contributes its (^) water to the total fluid volume, the (^) membranes being under the conditions freely permeable to all (^) electrolytes other than protein which also escapes in a (^) certain measure. The volume corre- sponding to (^1) g. of muscle is thus 0 77w^ +^ 25, where 25 ml. (^) KOH solution is used. w For the procedure desoribed with the no. 2 units the following formula applies:
mM. (^) C02/kg. muscle =0-0625x -(3 + °°), (^) (1) where x =large divisions on (^) burette corresponding to the CO2 absorbed, taking the blank (^) readiDt as zero absorption (each large division on the (^) burette =0-01 c.c.). For the (^) procedure with the no. 1, or standard units, the (^) formula is
mM. C02/kg. muscle=0-0812x (^) (3 + 100). (^) (2) The (^) Ba-soluble fraction. This was determined in a (^) similar way, but prior to the CO2 absorptions in the micr6-diffusicn (^) units 5 ml. vol. of the alkali extract were pipetted into capped 15 ml. centri- fuge tubes (tapered) and 1 ml. (^) saturated BaCI2 pipetted, added and mixed. These tubes were then centrifuged at^ about 3000 r.p.m. for at least (^90) min. For (^) abdominal muscle of rats, very young
S.D. of 1-2 and for the leg muscle of two cats it was 10-1 and 10-8 (^) mM./kg.
Comparing these values with those. of Wallace & Hastings [1942] using a
different method [Danielson & Hastings, 1939], the mean value for the leg
muscle of (^) the cat (taking their 14 control series) was 11-0 with S.D. of (^) 1-3, agreeing very well, (^) therefore, with the (^) above results.
10
E6 /
(^0 20 40 60 80 100 120 140 160 ) Extraction time (min.)
Fig. 1. Time curve of CO2 extraction from abdominal muscles of^ young rabbits (^) (about 0-5 (^) kg.) by N/5 KOH. Each point in general from^ one^ rabbit; two (^) points obtained with some rabbits.
The Ba-soluble fraction
The average values for the C02 remaining on addition of 1 vol. saturated
BaCl2 to 5 vol. of extract and spinning for 90 min. (occasionally much
longer) at approximately 3000 r.p.m. were for the abdominal muscle of the
rat, rabbit and guinea-pig, 8-0, 5-2, and 5-8 mM./kg. respectively (14-8,
11-4 and 10-3 mM./kg. being the corresponding values for the total C02), and
for the leg muscle of rabbit and cat the value was 7-0 mM./kg. (compared
with 10-6 mM./kg. for the total C02). Thus under the conditions of the experi-
ment more than half the total C02 is not precipitated by BaCl2, which is pre-
sumptive evidence that this fraction is not present in muscle as free C02 or
HCO3, or in the alkaline extract as C03 ion. This (^) conclusion has been proved by two independent (^) procedures.
(1) Effect of addition of KHCO3 to the muscle extracts. To 5 ml. portions of
alkalinle extract of the muscle small (^) volumes of KHCO3 solution (0d1-0-2 ml.)
were added and mixed before adding the saturated BaCl2, corresponding
volumes of C02-free water being added to control tubes. The results are shown
in Table 2. If the (^) C0% were not (^) entirely precipitated it might be expected that
higher values would be found for the analyses after centrifuging. In fact,
ACID-LABILE (^) C02 277
E. J. CONWAY AND P. J. FEAROA
Animal
(^2) rats 2 rats 1 rat 3 rats -3 rats
Young rabbit
Guinea-pig
TABLE 1.
Total C02 B mM./kg. Abdominal muscle 13- 15- 14- 15- Mean 14- 10- 10- 11- 12- 12- 11- 11- 10-
Mean 11-4±0- (8.5) (11-1) (9-4) (10-3) (9-5) (11-1) (12.2) Mean 10-3 ± 0- Leg muscle
Total a-soluble CO
7- 11-
6- 6- 8- 3- 6- 6- 6- 4- 4- 3-
6- 5-2±0* 5- 6-
6- 6- 5- 5-8±0-
Turbidity of^ Ba extract after centrifuging, absolute units
0-
0- 0- 0- 0- 0- 0- 0-
0- 0-
Young rabbit 8-9 7- 11-2 9- 12-3 6-6 0- 10-7 6-4 (^) 0- 9-3 4-7 (^) 0- Grown rabbit 11-8 (^) Cloudy 9-0 7- 11-3 7- Mean 10-6±0-4 7-9±0- Cat (^) 10-8 9-0 Cloudy 10-1 7- Mean 10-4 (^) 8- The (^) brackets for total CO2 for guinea-pig abdominal muscle indicate (^) that different animals were used for (^) determining this quantity and for the Ba-soluble fraction. The (^) turbidities listed give absolute values (^) over the readings for the centrifuged control without BaCl2 (^) which usually gave results not (^) differing from water. (^) The ± figures after the means give the S.D. of the mean (^) values.
TABLE 2.
Muscle Abdominal
Leg
Ba-soluble (^) CO2 in extract , & A~~ Before adding After adding KHCO3 mM./kg. KHCO3 mM./kg. muscle (^) muscle 7-4 7- 11-3 10- 7-2 6- 6-6 (^) 5- 4-3 5- 4-7 3-
Amount of KHCO3 added as (^) mM./kg. muscle 10- 8- 4- 13- 6- 5-
278
Animal 2 rats
Rabbit
280 E. J. CONWAY AND P. J. FEARON
(^2) mM.l/kg. The muscle was then extracted in the usual manner and the (^) KHCO3 added in (^) small measured volumes (^) (0-1-02 ml.) of standard solutions to 5 ml. vol. of the extract (^) (obtained (^) from about (^) 17 g. muscle in 50 ml. N/5 KOH). After (^) mixing, 1 ml. saturated (^) BaCI2 was added and (^) again rapidly mixed. It (^) may be noted that different (^) turbidity conditions are obtained if the (^) KHCO additions are (^) made after the BaCl2 addition. There is then (^) present a fine flocculent (^) precipitate, whereas (^) with the above procedure no flocculent (^) precipitate is produced but rather a fine (^) cloud.
0-()26/
0-22 -
E- 0-14 -
0-10 'S
0*
0-02- x ( x^ X
0-2 0-6 (^10) 1-4 1-8 2-2 (^) 2- mMI. (^) C02/1. extract Fig. (^) 2. Curves A and B represent turbidities of alkali extracts of (^) evacuated abdominal muscle from (^) young rabbits as (^) described in text, after additions of KHCO3 (^) and then saturated.BaCl2, 1 vol. to 5 vol. of extract. Curve (^) A1 represents the A extracts (^) after 5 min. centrifuging. Curve B1 (^) represents the B extracts (^) after 90 min. centrifuging. Curve C is for clear human serum freed of (^) CO2 and rendered alkaline to (^) simulate the muscle extracts, KHCO3 (^) added and then BaCl2 as for (^) muscle. The dilution of the serum in the (^) alkaline fluid before BaCl addition was about 1 in 5.
Fig. 2 shows (^) the results obtained in two (^) such experiments (curves A and B). The (^) remaining small amount of (^) acid-labile CO2 in the muscle (^) is here not con- sidered to be present (^) in the extract as C03, but the small amount of CO3 in
the alkali itself is added on to the KHCO3 additions. The abscissae give the
concentrations of carbonate (^) present and the ordinates the turbidities in absolute values.
It will be seen that the additions cause a uniform rise in turbidity, at least
up to the approximate level (dotted vertical line) of the Ba-soluble fraction of
the total (^) acid-labile C02 in the extracts of fresh muscle. The relation of increase
in turbidity to increase in added CO2 is nearly but not quite linear. The tur-
bidity changes 0-10 in absolute units for a CO2 addition corregponding to the
level of the Ba-soluble fraction of fresh muscle. As already noted the turbidity
of the centrifuged extracts (unevacuated muscle) after BaCl2 addition is often
inappreciable, averaging 0-008 for abdominal muscle, which was the muscle
most examined. Curve A1, in Fig. 2, shows the effect on the extracts used for curve A (^) (with
BaCl2 addition) of centrifuging for 5 min. The turbidity is still relatively high,
but (^) there is now a slight fall with increasing (^) KHCO3. Curve B1 shows the (^) effect
on the B series of centrifuging for 90 min. Very little turbidity is left and
there is no difference for the increasing KHCO3 additions.
Curve C is for clear human serum diluted 1 in 5 with N/5 (^) KOH, and which had (^) previously been freed of CO2 by (^) slight acidification and exposure for an hour in micro-diffusion units. 5 ml. (^) vol. were taken (^) and additions of KHCO3 made as above with subsequent (^) BaCl2 addition. It will (^) be seen that the effect on the turbidity is very similar to that with extract of abdominal (^) muscle.
The results show clearly that the comparatively large Ba-soluble fraction
of the total 002 in the alkaline (^) extracts cannot be present as CO".
The nature of the Ba-soluble fraction
A large fraction of the total acid-labile C02 in muscle is Ba-soluble in alkaline
media, and this at once suggests (^) [Henriques, 1928, 1929, 1935; Faurholt, 1924, 1925; Meldrum & (^) Roughton, 1932, 1933; Roughton, 1935] that it (^) may be carbamino C02. Now such compounds (^) possess the characteristic property that around a pH of 7 0, when the C02 tension falls, they are rapidly split, yielding free CO2. A series of (^) observations on the effect of exposure of (^) strips of the abdominal muscle of guinea-pigs (numbers of (^) which were available at the time) in vacuo were therefore (^) carried out. The results of these (^) experiments are sum- marized in Fig. 3 (each point being the mean of 3-6 determinations). It (^) will be seen that both (^) the total and Ba-soluble fraction show a (^) rapid initial fall of 2-3 mM./kg. (^) after which the Ba-soluble fraction falls only very slowly, and is little more than halved after full (^) evacuation for 1 hr. at room temperature. At and after 45 (^) min. there is no appreciable difference between (^) the curves of
the (^) total and the Ba-soluble C02, (^) so that all (^) the free C02 and (^) HCO3 has then
disappeared from the muscle. This (^) would seem to indicate that the greater part
of the Ba-soluble fraction may not be carbamino C02. The mean value of the
sum (^) of the free CO2 and HCO in the abdominal (^) muscle of the guinea-pig is
ACID-LABILE (^) C02 281
ACID-LABILE CO
TABLE 4
Tissue' Plasma
Whole blood
Muscle (abdominal)
Muscle (leg)
Symbol W, (g./kg.) [CO2, total] ClUS [K], pH Wb (g./kg.) [K]b [Cl]b Wm (g./kg.) [CO2, (^) total]I [CO2, Ba-sol.]m [Cl]m Blood (g./kg.) Wm (g./kg.) [CO2, total]m [CO2., Ba-sol.]m [Cl]m [K],, Blood (g./kg.)
Value (^) of symbol
Rabbit (^) Guinea-pig 924 (A) - 20-4 (^) (8) - 99-5 (^) (10) 109-0 (^) (8) 4.9 (5) 7- (^817) (A) 45 (A) 82 (A) - (^782) (5) 771 (5) 11-4 (^) (8) 10-3 (7) 5-2 (^) (9) 5-8 (6) 26-6 (7) 22-7 (^) (5) 28 (5) (^29) (5) 767 (5) (^767) (5) 10-6 (8) 7-0 (7) 12-1 (6) - 111 (C) - (^25) (5) 24 (5) All values as mM./kg., unless otherwise stated. The (^) symbol (^) [Cl], means mM. (^) chloride/kg. plasma and (^) [K]m means mM. (^) potassium/kg. muscle and (^) similarly for the other (^) symbols. W8 and (^) Wm mean g. water/kg. plasma and muscle. (^) Figures in brackets (^) give numbers of (^) analyses. (A) refers to (^) Abderhalden's data [1899], and (^) (C) to Constantino's (^) [1911].
The pH within the muscle fibre
The value (^) may be first calculated without (^) implying a Donnan relation across
the membrane.
In the Henderson-Hasselbalch equation (as given in equation (4)) [C02]fW
and [HCOS],fW are the concentrations of free C02 and HCO- within the fibre
(the very small value of (^) H2CO3 may be (^) neglected). A (^) value of (^) pK=6-1 may be (^) assumed, as the ionic strength within the fibre will (^) probably not (^) differ markedly from that of blood (^) plasma.
To calculate [HCO3]fW we need to know the value of the intercellular space
and this must be obtained (^) independently of the chloride data. For the (^) leg muscle of (^) the rabbit we have the inulin ratio (^) [Conway & (^) FitzGerald, 1942]
inulin/kg. plasma =^ 0-07.^ Manery^ & (^) Hastings [1939] found for (^) radioactive
inulin/kg. plasma
sodium (^) (24Na) a (^) ratio of 0-086 with the (^) gastrocnemius muscle of the (^) rabbit; Hahn, Hevesey & Rebbe (^) [1939] obtained a very similar value of (^) 0-085. The interchange with (^) radioactive Na in such (^) experiments is no doubt entirely (^) extracellular, and it is not (^) surprising that the inulin value should be (^) slightly lower, as some Na (^) may be held by fixed (^) anions in the sarcolemma. But, taking (^) the sodium ratio as 0-086 for (^) the extracellular water, this, apart from (^) the blood in muscle, may (^) be calculated as follows. In the (^) calculation the
283
E. J. CONWAY AND P. J. FEARON
mean water content of rabbit leg muscle is considered to be 767 g./kg. muscle,
and the (^) water in serum to be 920 g./kg.
If it be supposed that in (^1) kg. of leg muscle there are x kg. of extracellular
fluid in addition to y kg. of blood, then
[Na] external to the fibres= 0- 99x (^) [Na]ew+ y [Na]b
=0 99x [Na]ew+0 5y [Na]8. (3)
Here it is taken, in agreement with Wallace & Hastings [1942], that 1 kg. of
extracellular fluid contains 0*99 kg. water, and that the sodium per kg. (^) whole
blood-[Na]b-is approximately half that per kg. serum or 05, [NaL.
Now (^) [Na]w may be taken as (^) [Na]w x (0.95/0.92), where 0'95 is the Donnan ratio and 0-92 is the water content of (^) the serum. Using these values as well (^) as
0-025'found by us for the blood in the excised leg muscle of the rabbit, and the
figure 0-086 above, x is found (^) from equation (3) to be 0-075.
The extracellular fluid, apart from blood, is therefore 0x075 kg./kg. muscle.
The water content of this is 0-99 x 0'075 = 0-074, and the water content of
0-025 kg. blood is 0-025 x 0-77 =0O019. The total extracellular water is therefore
0-074 + 0-019 = (^) 0-093; and the intracellular (^) water
0-767 - 0-093 = 0-674.
To determine the (^) HC0 content of the intracellular water it is (^) necessary to
assess the HCO and free C02 in the spaces outside the fibres and the free C
within the (^) fibres. Now
[HC03]sw (mM.^ HCO/kg.^ of the serum^ water)^ =^21 0,
and (^) therefore (^) [HC03]euX (mM. HC0O/kg. of^ extracellular^ water)-21^20 x^ 1 05 =22* 1. (^) Also, (^) [C02],w =1.1, and since the solubility coefficient of CO2 is 0-553 ml./g. seruim water, 0-540 ml./g. extracellular water and 0-592 (^) ml./g. intracellular water [as given by Wallace & (^) Hastings, 1942] then [C02]ew (^) (mM. free (^) C02/kg. extracellular water) = (^) 1-08, and (^) [C02],w (mM. free C02/kg.) = (^) 1-18. Since (^) the total C02 content of the (^) serum is 20-4 mM./kg. (Table 4), that (^) of whole blood may (^) be taken as approximately 17 mM./kg. From (^) the value of the total C02 content of muscle apart (^) from the Ba-soluble fraction (that is, (^) from 10-6 - 7-0 = (^36) mM./kg.) we must subtract then (^) the following to obtain the HCO- (^) content of the fibre water:
0 075 x 1-08 ... free (^) C02 in (^) extracellular fluid other than blood, 0-075 x 22-1 ... (^) HCO in extracellular fluid other than (^) blood, 0-025 x (^17) ... total C02 in the blood in (^) muscle, 0-674 x 1-18 ... free (^) C02 in the fibre water (or extracellular fluid)
E. J. CONWAY AND P. J. FEARON
when (^) the value of (^) [K]f, is inserted from (^) equation (7), and (^) [Cl]f,, (as in (^) equa-
tion (^) (11), similar to (^) [K]f,,) is determined as (^) follows:
[cl],~= [Cl]m-s^ x^ [CI]s^ -^ 0-025^ [Cl]b
[Cl]fw 0-767 -s-0-025 x 0-
12-2-s x 108-0-025 x 82 0-748 -s 101 - 108s (^) (10) 0-748 -s whence, from^ equation (9), 110- 5-3s11-@Sx (^10411) -10s5.3108s x
0'748-s X0-738-s =3 0,(
from which (^) s=0-071,
which is very similar to the 0*075 value obtained above from direct analyses.
Inserting this value for 's' in (^) equation (8)
[H]f,, =^ 10-5^ 9,
and, the pH value= 59, or, if we consider the pH value as the negative
logarithm of the H ion activity (to correspond to equation 5 above) the value
is (^) slightly raised and becomes 6-0.
Hlere again no great exactness can be claimed for the result, but it is pro-
bably within +^ 0.1 pH, and agrees well with the previous calculation, based
directly on the (^) bicarbonate system.
Similar calculations, applied to the data for the abdominal muscle of the
rabbit, give a pH within the fibres of 6-0, which is similar to that of the leg
muscle.
DIsCUSSION
Ba-soluble C02 in muscle
The total CO2 liberated.and escaping after acidifying muscle may be termed
acid-labile C02; as shown (^) here, only the smaller part of this total in nmam-
malian muscle is in the ionized HCO3 or in the free C02 form, the greater part
being Ba-soluble in alkaline (^) media.
The proofs of the reality of the existence of a Ba-soluble fraction of such
magnitude, and that BaCO3 (^) is not merely suspended or (^) protected from pre- cipitation by the (^) proteins, have consisted in the (^) quantitative precipitation of
small amounts of C02 added as KHCO3 to the alkaline extract before the
addition of the BaCl2, as well (^) as by turbidity studies of the (^) centrifuged samples,
and turbidity effects produced by HC03 additions to evacuated muscle
extracts. The proof obtained from the (^) turbidity study alone would appear con- clusive for the abdominal (^) muscles of guinea-pigs, very (^) young rabbits and rats. The alkali (^) extracts of leg muscles of fully grown rabbits and cats, when (^) treated
with BaCl2, show some turbidity after centrifuging, though this is no reason
for supposing that such turbidity is caused by BaCO3, since as with abdominal
muscle this is quantitatively (^) precipitated when small volumes of (^) KHCO
solutions are added before the BaCl2. Evidence has been presented in the
paper that the greater part of this Ba-soluble fraction (^) may be in some (^) form other than carbamino C02.
The pH inside the muscle fibre
It will be seen that the nature of the acid-labile C02 in mammalian muscle
as determined in these experiments altogether invalidates the pH calculation
made by Wallace & Hastings, their calculated value of 6-93 + 0-12 being nearly
one whole unit ofpH too high. The true value appears to be approximately 6-0,
and is thus practically the same as that calculated for frog muscle, i.e. 5-
[Boyle & Conway, 1941].
The calculations of the H-ion concentration from the study of the bicarbonate
system are in (^) agreement with a Donnan relation across the membrane for K+, H+, Cl- and HCO and such a relation for K+ and C1- has (^) been demon-
strated for very wide changes of these ions in frog muscle [Boyle & Conway,
1941]. Supporting (^) evidence for mammalian muscle is given by Wilde (^) [1943] and Darrow [1944]. It is also of interest to (^) note that a value of 6-0 was found by Vles [as quoted by Rous, 1925] for frozen and (^) ground mouse tissues by various (^) physico-chemical methods, and a figure as low as 5-6 by Rous (^) [1925] from intravital staining of (^) voluntary muscle in mice with the minimum disturbance of (^) the living tissues.
The permeability of the muscle fibre to HCO
In (^) previous communications the principle was (^) demonstrated for frog muscle (and it has also been found for gland tissue as will be described later) that the
cell membrane in general is permeable both to cations and anions but there are size
limits for these ions. Na, Mg and Ca are excluded as ions, though they (^) obtain entrance into cells, probably in (^) unionized organic combination. On the other hand the muscle (^) is not permeable to the larger anions, (^) such as those of the
phosphate esters, but is freely permeable to chloride and very probably to
HCO-. The permeability to HCOj throws open to -any particular group of
cells the whole HCO- buffering of the internal medium. The evidence brought
forward by Wallace & (^) Hastings [1942] and Wallace & Lowry [1942] (^) for the
impermeability of the muscle membrane may now be considered.
(a) Working with the leg (^) muscles of cats they state, 'the intracellular
bicarbonate (^) remains relatively unchanged despite wide changes (^) in the extra-
cellular (^) bicarbonate and it is concluded that the (^) muscle cell is normally im-
permeable to the bicarbonate ion'.
19-
ACID-LABILE (^) C02 287
ACID-LABILE C02 289
- Allowing for the small amount of free carbon dioxide in the (^) barium-
insoluble fraction, it is shown (^) that the ratio of bicarbonate concentration in
muscle to that in plasma is the same as for chloride.
- The pH inside the muscle fibre as determined by the (^) Henderson-Hassel-
balch equation applied to the bicarbonate (^) system is 60.
- The pH of the muscle fibre determined from the Donnan (^) principle
appliedto a (^) membrane permeable to K+, H+, C1- and HCO is likewise 60.
Our thanks are due to the Irish Medical Research Council for (^) apparatus purchased by a (^) grant in (^) aid.
REFERENCES
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