motic component of the body, as it makes up less than 2% of the total solute in
the bod y fluids. Parathormone and calcitonin, thus, are onl y indirectly concerned
with osmoregulation, in that the structural integrity of the membranes of the body
and the release and actions of hormones, such as those from the neurohypophysis,
are partly dependent on calcium. In mammals, elevated plasma calcium levels, as occur in hyperparathyroidism, result in increased renal water loss (FOURMAN, 1963).
This seems to be due to inhibition of the effects of vasopressin (ADH) on water
reabsorption from the renal tubule.
5. 'Islets of Langerhans' of the Pancreas
The endocrine secretions of the pancreas are formed in groups of tissue called the
' Islets of Langerhans'. At least two histologically distinct types of cells can be identified in these tissues; the a-cells which secrete glucagon and the P-cells insulin.
a- cells have not been identified in the cyclostomes or urodeles. The hormones
are polypeptides that affect the metabolism of glucose and also that of fat and
protein.
Insulin consists of two peptide chains one of which, the A-chain, contains 21
amino acids while the other, the B-chain, has 30 amino acids . These chains are connected by two disulphide bridges. The amino acid sequence has been completely
or partly determined for several mammals, the chicken and several teleost fishes
(L. SMITH, 1966). Of the 59 amino acid positions in the molecule, substitutions
have so far been found at 29 different places in the insulins of different species.
Mammalian insulins usually differ from each other by substitutions at 2 or 3 positions, though guinea pig insulin has 17 changes as compared to the pig or human
hormone. Chicken insulin has 6 amino acid substitutions, compared to the latter,
while teleost fishes have as man y as 17 such alterations. The various am ino acid
changes result in different immunological behaviour, but apparently have surprisingly little effect on biological activity, even when the insulins are tested in contrasting species (L. SMITH, 1966). Ph ysiologically, the most obvious action of insulin is to lower th e blood-glucose concent ration. This effect is seen throughout
the vertebrates from the cyclostome fishes to the mammals (see BENTLEY and FOLLETT, 1965), though the reptiles and birds are less responsive than other species .
The effect of insulin on glucose principally reflects its action in facilitating conversion of glucose to glycogen and fat. It also has important effects in increasing
protein synthesis, and reducing the mobilization of fat from adipose tissue, at least
in some species.
Glucagon has been identified in species of mammals, birds and bony fishes (BERTHET, 1963). Mammalian (pig) glucagon contains 29 amino acids. This hormone
elevates the blood glucose concentration in representative species from most vertebrate groups, but the urodele Amphibia and the cyclostomes (BENTLEY and FOLLETT, 1965) are insensitive to the mammalian hormone. Glucagon increases the
blood-glucose levels by promoting liver glycogenolysis .
64
the bod y fluids. Parathormone and calcitonin, thus, are onl y indirectly concerned
with osmoregulation, in that the structural integrity of the membranes of the body
and the release and actions of hormones, such as those from the neurohypophysis,
are partly dependent on calcium. In mammals, elevated plasma calcium levels, as occur in hyperparathyroidism, result in increased renal water loss (FOURMAN, 1963).
This seems to be due to inhibition of the effects of vasopressin (ADH) on water
reabsorption from the renal tubule.
5. 'Islets of Langerhans' of the Pancreas
The endocrine secretions of the pancreas are formed in groups of tissue called the
' Islets of Langerhans'. At least two histologically distinct types of cells can be identified in these tissues; the a-cells which secrete glucagon and the P-cells insulin.
a- cells have not been identified in the cyclostomes or urodeles. The hormones
are polypeptides that affect the metabolism of glucose and also that of fat and
protein.
Insulin consists of two peptide chains one of which, the A-chain, contains 21
amino acids while the other, the B-chain, has 30 amino acids . These chains are connected by two disulphide bridges. The amino acid sequence has been completely
or partly determined for several mammals, the chicken and several teleost fishes
(L. SMITH, 1966). Of the 59 amino acid positions in the molecule, substitutions
have so far been found at 29 different places in the insulins of different species.
Mammalian insulins usually differ from each other by substitutions at 2 or 3 positions, though guinea pig insulin has 17 changes as compared to the pig or human
hormone. Chicken insulin has 6 amino acid substitutions, compared to the latter,
while teleost fishes have as man y as 17 such alterations. The various am ino acid
changes result in different immunological behaviour, but apparently have surprisingly little effect on biological activity, even when the insulins are tested in contrasting species (L. SMITH, 1966). Ph ysiologically, the most obvious action of insulin is to lower th e blood-glucose concent ration. This effect is seen throughout
the vertebrates from the cyclostome fishes to the mammals (see BENTLEY and FOLLETT, 1965), though the reptiles and birds are less responsive than other species .
The effect of insulin on glucose principally reflects its action in facilitating conversion of glucose to glycogen and fat. It also has important effects in increasing
protein synthesis, and reducing the mobilization of fat from adipose tissue, at least
in some species.
Glucagon has been identified in species of mammals, birds and bony fishes (BERTHET, 1963). Mammalian (pig) glucagon contains 29 amino acids. This hormone
elevates the blood glucose concentration in representative species from most vertebrate groups, but the urodele Amphibia and the cyclostomes (BENTLEY and FOLLETT, 1965) are insensitive to the mammalian hormone. Glucagon increases the
blood-glucose levels by promoting liver glycogenolysis .
64