§5.111 RESPIRATION I77 



AvES. Judging by their high normal temperature of 39 °C (as 

 compared with about 37 °C in man), the basal metaboHsm and heat 

 output of birds must be the highest of any vertebrates. Yet evidence 

 for the control of heat production and oxygen consumption by the 

 thyroid is scanty. The basal metabolic rate drops by 20 per cent in 

 pigeons within a week of thyroidectomy (Marvin and Smith, 

 1943). Heat output and respiration have also been shown to vary 

 in different races of pigeons, and this is believed to be due to 

 genetic variations in the activity, but not necessarily in the size, of 

 their thyroid glands (Riddle, 1947). 



Mammalia. Hormone control of oxygen consumption is well- 

 established in mammals, marked increase accompanying hyper- 

 function of the THYROID GLAND or injection of thyroxine (Fig. 

 5-3). A small injection of saline gives control evidence that the 

 increase in oxygen uptake by the rat, due to the experimental 

 procedure, is barely significant and of a different order of magnitude 

 from the increase due to thyroxine of whatever origin (Matty, 1954) ; 

 the converse effect, of thyroid removal, is not shown in the figure. 



An important feature of the results is the close similarity in 

 effect produced by injections of roughly similar amounts of thyroid 

 extract from a mammal and from an elasmobranch or a teleost 

 fish (Fig. 5-4). 



There has been much discussion as to the exact role of thyroxine 

 in mammalian metabolism, for measurements of its effect upon 

 oxygen consumption only show the end stage in what may be a 

 long chain of events, any or all of which may be sensitive to the 

 hormone. Although thyroxine has been shown to act upon a very 

 large number of enzyme systems in the body, the only consistent 

 reaction that has been demonstrated in vitro is the effect of 

 thyroxine in increasing the phosphorus turnover in oxidative 

 phosphorylation (§ 5.211 ; Rawson et al.y 1955). 



In vertebrates the thyroid gland is apparently always under 

 endocrinokinetic control by thyrotrophin, TSH, from the 

 adenohypophysis. In goldfish, Cyprinus (Miiller, 1953), as well as 

 in mammals, there is now definite evidence that secretion of the 

 thyroid gland in relation to its metabolic action is stimulated by 

 thyrotrophin, when this is injected as a purified extract (§ 4.221). 

 This will probably be found to occur in all vertebrates. 



