6. ADRENAL STEROIDS AND ACTH
349
(leukopenia) is not pituitary-dependent (Slicher et al., 1962; Ball and
Slicher, 1962). ACTH causes leukopenia at low doses but leukocytosis at
high doses 2 hr after injection in intact F. heteroclitus, while cortisol
causes leukocytosis after 2 hr in intact or hypophysectomized fish (Slicher,
1961). In intact P. latipinnu a high dose of ACTH caused leukopenia at
2 hr and leukocytosis at 4 hr (Slicher and Ball, 1962). Thus, ACTH and
cortisol appear to mediate the leukocytosis of stress in these cyprinodonts.
There is also some evidence that gonadal steroids influence the WBC
response to cortisol in F. heteroclitus ( Slicher, 1961).
IV. THE PITUITARY-ADRENOCORTICAL AXIS
A. General Considerations
In higher vertebrates, the activity of the adrenal cortex is, for a large
part, under the control of the hypothalamus and ACTH secreted from
the anterior lobe of the pituitary gland, Thus, when certain adrenocorticosteroid concentrations in the plasma fall below a particular level
or as a result of other stimuli, the pituitary gland increases its output
of ACTH. There is then hypertrophy and hyperplasia of the adrenal cortex with subsequent increases in steroid secretory rates. The converse is
also true in that high adrenal steroid concentrations in the plasma lead
to a diminution of ACTH output by the pituitary. This negative feedback
relationship between two endocrine organs ensures a relatively constant
level of either hormone in the blood. Such a mechanism may be operative in teleosts since steps in the feedback loop have been outlined
though the role of the hypothalamus is yet to be established. However,
the evidence in cyclostomes and elasmobranchs is, to date, fragmentary
and sometimes contradictory. This is particularly true in the cyclostomes,
where the adrenocortical tissue is not identified with certainty.
B. Biochemistry of ACTH
Our present knowledge of the biochemistry of the ACTH molecule is
derived from studies on mammalian species. Since mammalian ACTH has
been found to be active in representatives of every vertebrate class
studied, there is good reason to believe that the basic structure of the
molecule does not vary greatly from class to class. Mammalian ACTH is
a polypeptide with an established sequence of 39 amino acids, of which
the first 24 peptide chains, counting from the NH, terminal, are essential
349
(leukopenia) is not pituitary-dependent (Slicher et al., 1962; Ball and
Slicher, 1962). ACTH causes leukopenia at low doses but leukocytosis at
high doses 2 hr after injection in intact F. heteroclitus, while cortisol
causes leukocytosis after 2 hr in intact or hypophysectomized fish (Slicher,
1961). In intact P. latipinnu a high dose of ACTH caused leukopenia at
2 hr and leukocytosis at 4 hr (Slicher and Ball, 1962). Thus, ACTH and
cortisol appear to mediate the leukocytosis of stress in these cyprinodonts.
There is also some evidence that gonadal steroids influence the WBC
response to cortisol in F. heteroclitus ( Slicher, 1961).
IV. THE PITUITARY-ADRENOCORTICAL AXIS
A. General Considerations
In higher vertebrates, the activity of the adrenal cortex is, for a large
part, under the control of the hypothalamus and ACTH secreted from
the anterior lobe of the pituitary gland, Thus, when certain adrenocorticosteroid concentrations in the plasma fall below a particular level
or as a result of other stimuli, the pituitary gland increases its output
of ACTH. There is then hypertrophy and hyperplasia of the adrenal cortex with subsequent increases in steroid secretory rates. The converse is
also true in that high adrenal steroid concentrations in the plasma lead
to a diminution of ACTH output by the pituitary. This negative feedback
relationship between two endocrine organs ensures a relatively constant
level of either hormone in the blood. Such a mechanism may be operative in teleosts since steps in the feedback loop have been outlined
though the role of the hypothalamus is yet to be established. However,
the evidence in cyclostomes and elasmobranchs is, to date, fragmentary
and sometimes contradictory. This is particularly true in the cyclostomes,
where the adrenocortical tissue is not identified with certainty.
B. Biochemistry of ACTH
Our present knowledge of the biochemistry of the ACTH molecule is
derived from studies on mammalian species. Since mammalian ACTH has
been found to be active in representatives of every vertebrate class
studied, there is good reason to believe that the basic structure of the
molecule does not vary greatly from class to class. Mammalian ACTH is
a polypeptide with an established sequence of 39 amino acids, of which
the first 24 peptide chains, counting from the NH, terminal, are essential