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Definition of terms in this essay
Calcium balance deals with the regulation of a body’s calcium levels in order to maintain homeostasis. A disruption of this process results in either hypo- or hypercalcemia.
Teleost fishes are fishes belonging to teleostei. These bony fishes have movable jaws. This makes them able to extend their mouth while preying. They also have symmetrical or homocercal tails.
Amphibians are vertebrate animals that are ectothermic, non amniotic and that respire through their skin. They are usually aquatic during their larval stage but become terrestrial during adulthood.
The endocrine system is a system theat deals with the use of hormones produced by glands in the regulation of bodily functions and processes.
There are several hormones that take part in the endocrine regulation of calcium balance. However, due to morphological differences, the endocrine process of regulating calcium is different between amphibians and teleost fishes.
Hypercalcaemic hormones are hormones that increase the calcium levels in the plasma by liberating those in the bone.
Parathyroid glands can be found in most amphibians; especially with the anurans. The parathyroid hormone found in amphibians is usually hypercalcaemic, Studies have shown that parathyroid hormone liberates Calcium ions from the bone, increasing the calcium levels in the blood. (YOSHIDA & R. V.TALMAGE, 1962) The removal of the parathyroid gland (Parathyroidectomy) results in a significant decrease of blood calcium levels. Parathyroid hormone works by increasing the production of osteoclases, which are the cause of mobilization bone calcium. (STIFFLER, 1993)
Teleost fish lack parathyroid glands. It has been found out however that the pituitary glands of fishes make up for the loss of the parathyroid hormone by excreting other hypercalcaemic hormones like Prolactin (PRL) and Somalactin (SL). (KANEKO & HIRANO, 1993)The removal of the pituitary gland induces hypocacaemia in fishes. (PANG, K.SCHREIBMAN, M. P., & YOGURO, 1975)
The use of mammalin Prolactin increased the calcium level of calcium deficient killifish whose pituitary has been removed. (WENDELAAR BONGA & PANG, 1989)Studies in eels have determined that prolactin secretion does not depend on ambient calium levels but on the osmolality or sodium levels of the environment(SUZUKI, KANEKO, & HIRANO, 1991).Prolactin also has the same effect in aquatic amphibians. In fact, aquatic amphibians may rely more on porlactin than o the parathyroid hormone as evidenced by the lack of the parathyroid gland in urodeles.
Somalactin is another pituitary hormone that closely resembles the structure of the growth hormobne as well as prolactin. The glycosylation status of SLs reflects the presence or absence of periodic acid–Schiff (PAS)-positive cells of the pars intermedia (PIPAS cells) in the pituitary. This means that PIPAS cells canm be used as indicators in measuring the effect of somalactin. However, studies that have measured the hyperglycaemic properties of PIPAS cells are inconclusive as the relation with somalactin is indirect. (KANEKO & HIRANO, 1993)
Hypocalcamic hormones are the inverse of hypercalcaemic ones, they lower calcium levels by inducing storage into the bones.
The main hypoglycaemic hormone in amphibians is Calcitonin. It is produced in the ultimobranchial glands of the amphibian. It is the main hypoglycaemic hormone in mammals and other vertebrates. In amphibians, studies involving the removal of the ultimobranchial branch show that it leads to an increase in calcium levels while the subsequent application of calcitonin extracts significantly lowers the blood calcium content. (STIFFLER, 1993)
Calcitonin is also secreted by fishes from their own ultimobranchial body. However, incestigations unto the hypocalcaemic properties of calcitonin in fishes have failed to produce tangible and unequivocal results. The known hypocalcaemic hormone in fishes is those produced by the corpuscles of Stannius. They are unique to teleostean and holostean fishes. Studies have shown that their removal induces hypercalcaemia. (FONTAINE, 1964)The main substance produced by the corpuscles, stanniocalcin, is regarded as the primary hypocalcaemic hormone in fishes. (WENDELAAR BONGA & PANG, 1991)
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