Constituents in Skipjack Muscle and Blood — Sather and Rogers 
409 
The data contained in Tables 2 and 3 were 
analyzed statistically to determine whether the 
concentrations differed with muscle type; these 
results are presented in Table 4. The white 
muscle contains a greater amount of water, po- 
tassium, and magnesium which possibly indi- 
cates that this tissue has a larger intracellular 
space. Statistical differences in the calcium and 
chloride contents were not found, which may 
have been due to the TCA used in the extrac- 
tion. However, blanks were carried throughout 
the analysis. 
Figure 1 illustrates that the extracellular vol- 
ume of the red muscles is greater than that of 
the white. The calculated inulin spaces for the 
red and white muscles are 23.79 and 18.97%, 
respectively. The higher sodium content of the 
red muscle verifies the larger extracellular space. 
It was not possible to determine accurately 
the intracellular sodium and chloride contents 
because of their high content in the intravas- 
cular compartment. The other intracellular cal- 
culated concentrations (mEq/liter cell water) 
of the red and white muscles, respectively, 
were: potassium = 160.88, 189.28; magne- 
sium = 33.76, 43.68; calcium = 0.12, 6.30. 
It is well known that the amount of intra- 
cellular potassium determines the threshold 
value for any tissue. Thus, the potential pro- 
duced by this ion for the red muscle was calcu- 
lated to be 83.77 mv and that of the white 
muscle was 88.08 mv — a potential difference 
of 4.31 mv. Therefore, the red muscle would 
have a lower threshold value, indicating that 
possibly this muscle would be utilized more 
than the white. 
Vernick (1964) reported that the red muscle 
of four pelagic species had a higher content 
of thiamine, riboflavin, pantothenic acid, vita- 
min B 12 , myoglobin, and cytochrome C. This 
tissue also had a higher degree of vasculariza- 
tion and larger mitochondria in the sarcoplasm. 
These findings led to the suggestion that the 
red muscle provided energy for the white. 
Hamamoto and Hohl (personal communica- 
tion) discovered that the mitochondrial density 
in the red muscle sarcoplasm of K. pelamis 
was approximately one magnitude greater than 
that in the white. Because the mitochondria 
are the cell’s energy producers, there is a strong 
correlation between the degree of activity of 
the muscle and the number and shape of the 
mitochondria within the muscle cells (Davson, 
1964). In addition, if one considers the color 
of the two muscle types and applies the analogy 
of the breast muscles of chickens versus those 
of the pigeon, it becomes apparent that the 
red muscle of K. pelamis with its abundance 
of mitochondria is possibly used for swimming 
and not as an energy producer for the white 
muscle. The red muscle is, indeed, able to con- 
tract and is probably used for normal swimming 
activity (Rayner, personal communication). 
The white muscle may be used secondarily, 
e.g., for accelerated and rapid movements seen 
during avoidance and feeding reactions. 
Table 5 lists some of the plasma constituents 
of various fishes. It is well known that the 
marine cyclostomes are approximately isosmotic 
to the medium and that the marine cartilagin- 
ous fishes are hyperosmotic to the environment. 
However, the sea water-inhabiting teleosts are 
hyposmotic to their medium. Thus, these ani- 
mals are threatened by desiccation. To prevent 
dehydration the animals must drink water and 
selectively excrete ions. The latter process is 
generally accomplished extrarenally via the 
gills. 
Of the teleosts listed in Tables 5 and 6 only 
the barracuda and herring can be comparable 
to the skipjack, and the eels would be inter- 
mediate in comparison; the other species would 
be least comparable due to their phylogenetic 
placement and their relative inactivity as com- 
pared with the scombroid fishes. The mackerel 
is a scombroid fish, but it inhabits more inshore 
waters than does the skipjack. 
As expected, the electrolyte composition of 
the skipjack plasma (Table 5) is less than that 
of the cyclostomes. However, it approximates 
those of the chondrichthyes. The greater os- 
molality of the latter is due to a higher urea 
content of the plasma. Concentrations of 300- 
400 mM of urea and trimethylamine oxide/ 
liter are essential for elasmobranch osmoregu- 
lation (Urist, 1962). The plasma calcium and 
magnesium in the skipjack are much less than 
those in the chondrichthyes. This can be attrib- 
uted to the apatite, which allows the teleost to 
maintain ionic concentrations independent of 
the external medium, and to the greater effi- 
ciency of the kidney and possibly the gills. 
