Some aspects of calcium metabolism in birds and mammals

Abstract

This thesis is divided into two distinct sections, both of which are concerned with aspects of calcium  metabolism. In the first section, the role of various reproductive steroids on the metabolism of 25-hydroxychole-calciferol (25-HCC) in Japanese Quail was studied. Metabolism of 25-HCC was
followed by incubating ³ [H]25-HCC with renal homogenates, and then separating the metabolites (consisting mainly of 1,25- and 24,25-dihydroxy-cholecalciferol) by a unique TLC system.
Quail raised on a short-day (SD) lighting regime of eight hours light, 16 hours dark (8L:16D) were extensively used in this work: these birds were somatically mature but sexually immature. Their reproductive steroid levels have been well monitored and are known to be minimal, and these birds
proved to be an excellent model for studying the action of exogenous reproductive steroids on vitamin D metabolism. Both 17β-oestradiol and oestrone were shown to stimulate the renal 1-hydroxylase potently in the SD birds, while 17α-oestradiol and oestriol had weak stimulatory actions. This
was the first time that oestrogens other than 17β-oestradiol had been implicated in regulating the 1-hydroxylase and their physiological importance is discussed. Plasma 17β-oestradiol and oestrone levels measured by RIA correlated well with in vivo renal hydroxylase activities.
Oestrogens were shown to stimulate the 1-hydroxylase within 3 hours. Progesterone augmented and testosterone inhibited the oestrogenic stimulation of the 1-hydroxylase. Little is known of the mechanism of action of oestrogen on the 1-hydroxylase, but some evidence is presented suggesting
that de novo protein synthesis is a requirement. Furthermore much circumstantial and some experimental evidence which does not necessarily contradict previous observations, suggests that prolactin may mediate the oestrogen induced stimulation of the 1-hydroxylase. In most experiments a reciprocal relationship was observed between the production of 1,25- and 24,25 dihydroxycholecalciferol, showing that hormonal treatments that stimulated the 1-hydroxylase inhibited the 24-hydroxylase and vice versa.
The second section studied the factors influencing phytate hydrolysis in the gastro-intestinal tract, and the great capacity for hydrolising phytate of the hamster compared to the rat. Phytate is a valuable but often wasted source of phosphorus, therefore this project has economic significance. Alkaline phosphatase and phytase activities were measured in the rat and hamster small intestines. The rat had higher alkaline phosphatase activity than the hamster, and considerable phytase activity, whereas the hamster had none. The disappearance of phytate was monitored during the passage of food through the rat and hamster gut, and it was shown that most of the phytate was hydrolysed in the stomach in both the rat and the hamster. As phytase activity could not be found in the stomach tissue of either species, the phytase had to be of either cereal or microbial origin. Cereal phytase activity was largely ruled out in the hamster by the use of a cereal diet, free of phytase activity, but this source of phytase 

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