The research aimed to elucidate hypercalcemic regulation in juvenile sea bream confronted with an experimentally induced limited access to environmental calcium. The principle to restrict sea bream in its access to calcium in water and diet was to activate hypercalcemic regulatory factors such as PTHrP and study the role of PTHrP in regulation of the calcium balance. Correlations between plasma ionic calcium and plasma PTHrP and between plasma PTHrP and pituitary gland pthrp mRNA expression show that PTHrP, as an endocrine factor, is important in maintenance of fish plasma calcium balance. Variation in the need for calcium (bone mineralisation, vitellogenesis) or the availability of environmental calcium (limited concentrations in water or diet) urge the endocrine system to respond rapidly to regulate the ionic calcium level. PTHrP-involvement in skeletal and scale physiology has been established in bone mineralisation (during growth and development) and in calcium reabsorption from the scales, when processes as vitellogenesis require extra calcium. The very strict control of calcium homeostasis (by a plethora of endocrines: PTHrP, PTH, PRL, stanniocalcin, calcitonin, E2, calcitriol, cortisol, etc.) and plasma ionic calcium levels through swift endocrine adjustments make this a complicated field of research. However, a new array of paradigms with a key role for fish shows involvement of PTHrP in vitellogenesis, cortisol production, Mrp2 activity and melatonin synthesis. Such studies will rapidly expand our knowledge on this pleiotropic hormone. They show that the calciotropic actions of PTHrP concern organismal as well as cellular physiological phenomena. Studies on fish give insight in original functions of PTHrP-regulated processes and, once again, show the power of comparative endocrinology.

Contains fulltext : 30013.pdf (Publisher’s version ) (Open Access)

Radboud Unversity Nijmegen, Animal Physiology, 21 juni 2007

Promotor : Flik, G.

152 p.