Constituents in Skipjack Muscle and Blood — Sather and Rogers 
411 
Fish with high plasma electrolyte values usually 
have tissues with relatively high electrolyte 
values. The grouper, mackerel, and herring 
have greater amounts of muscle sodium than 
does the skipjack. The sodium content of the 
muscles of the eel is intermediate between the 
skipjack’s red and white muscle content. This 
is to be expected because the plasma sodium 
of K. pelamis closely aproximates that of the 
eel. However, it is also apparent that the blood 
sodium of the mackerel and the skipjack are 
present in nearly equal concentrations. At first 
glance the difference in the muscle sodium con- 
tent of the two species is obscure, but it will 
be recalled that the body temperature of tuna 
is 6°-12°C higher than their environment and, 
therefore, the muscles of the skipjack would 
probably be more active metabolically than 
those of the mackerel. If this is truly the case, 
the sodium pump of the skipjack would be 
much more efficient, thus producing a lesser 
intracellular sodium content than that present 
in the mackerel and possibly in the other higher 
teleosts. 
Upon comparing the potassium content of 
the various muscles, it becomes apparent that 
the plasma content does not necessarily influ- 
ence the muscle content. This is obvious on 
examining the values for Muraena and Scom- 
beromorus. The plasma potassium content of 
the former animal is 1.95 mEq/liter in con- 
trast to a muscle content of 165 mEq/kg water. 
The potassium content of the plasma for Scom- 
ber omor us is 10.3 mEq/liter as compared with 
a muscle content of 153 mEq/kg water. Also, 
on examination of the blood and muscle con- 
centration of the teleosts, no obvious order is 
evident, e.g., the blood potassium order is: 
mackerel > grouper > skipjack > eel, and the 
muscle order is: skipjack white > eel> mack- 
erel > skipjack red > grouper. It appears, 
therefore, that the difference in muscle potas- 
sium may be under greater metabolic control 
than is sodium. Thus, the extracellular potas- 
sium may be entirely under the influence of the 
hormonal and genetic composition of the animal. 
The chloride content of a tissue, like the 
sodium content, is greater extracellularly than 
intracellularly. It would then be expected that 
the chloride content of the muscles would par- 
allel that of the plasma. However, the data in 
Tables 5 and 6 do not support this hypothesis. 
The importance of chloride in a tissue is to 
maintain electrochemical neutrality. Thus, the 
chloride content of a tissue is maintained pas- 
sively as a result of the Na+ and K+ distri- 
bution. As noted above, this ionic distribution 
is genetically influenced and thus the Cl - dis- 
tribution would subsequently be controlled but 
in a more subtle manner. Further examination 
of Table 6 reveals that the muscle and blood 
chlorides of the grouper are greater than those 
of the eel, and also that the chloride values 
for the skipjack are greater than those of the 
mackerel. However, the relationship between 
blood and muscle chlorides terminates at this, 
point, because the skipjack muscles have the 
greatest chloride content, but the plasma 
chloride content is intermediate between those 
of the eel and the mackerel. 
It is not possible to make similar compari- 
sons with the herring because the blood values of 
this fish could not be located. Such data would 
be informative because the herring is more 
closely related systematically and ecologically 
to the skipjack than to the eel and grouper. 
The data for muscle calcium and magnesium 
of marine teleosts are very meager. In Table 6 
only one direct comparison can be made, that 
between M. helena and K. pelamis. The values 
for the herring cannot be considered because, 
as was noted by Robertson (I960), the muscle 
samples were contaminated with bone frag- 
ments. The calcium content of both muscle 
types of the skipjack is less than that of the 
eel, although the blood calcium levels of both 
species are approximately the same. 
The results of the comparison of the mag- 
nesium contents are opposite to those of the 
calcium comparison. The eel has about twice 
the amount of blood magnesium that the skip- 
jack does. The differences in the muscle con- 
tent are that the red muscle of K. pelamis has 
about twice the amount, and the white muscle 
has about three times the amount found in 
the eel. This may be due to a greater pre- 
ponderance of myosin and adenosine triphos- 
phate (ATP) in the muscles of the skipjack. 
It is known that magnesium serves as a co- 
factor for bridging ATP and creatine to the 
creatine kinase molecule during transphospho- 
rylation (White et al., 1964). It is quite pos- 
