256 METABOLIC HORMONES 



and a low level the glucagon, in such species as have this hormone 

 (Saka, 1952). Between them, the two hormones keep the sugar 

 level relatively constant ; but their action is rather synergistic than 

 antagonistic, as the terms diabetogenic and antidiabetogenic 

 suggest. Glucagon mobilizes glucose from store and releases it into 

 the blood ; the resulting increase stimulates the secretion of insulin 

 which lowers the sugar level in the blood, by facilitating its passage 

 into the tissues where it can be utilized. This lowering in blood- 

 sugar level again stimulates glucagon secretion and the see-saw 

 continues. 



This seems to be the only form of control that there is over the 

 secretion of insulin. The case of glucagon is different, in that its 

 secretion can also be stimulated by the endocrinokinetic hormone, 

 STH (§§4.222 and 5.523). This has led to the erroneous impression 

 that insulin secretion is also stimulated by STH ; but it seems that 

 the action is indirect, through the effect of glucagon on the level of 

 sugar in the blood. The action of STH in stimulating growth is 

 due to its increasing the secretion of glucagon, and thereby 

 enlisting the synergic action of insulin to provide the tissues with 

 more glucose. If the supply of insulin falls short, further injection 

 of STH fails to induce any further growth (Owen and Engel, 1957). 



The only other case of direct control of a metabolic hormone is 

 that of the parathyroid glands, which are stimulated to secrete 

 parathormone by a high concentration of phosphates or a low level 

 of calcium in the blood. The parathormone then reduces the 

 phosphate and raises the Ca levels until they pass beyond the 

 threshold values for stimulation of the hormone (§ 5.422). If low 

 calcium and high phosphates are maintained in the blood of mam- 

 mals for some time, they cause hypertrophy of the parathyroid 

 glands. 



As in the case of insulin, earlier reports suggest that the para- 

 thyroid glands of mammals are stimulated by an endocrinokinetic 

 hormone, either thyroxine from the thyroid or TSH from the 

 hypophysis. The actions of these hormones are now interpreted as 

 indirect effects, mediated by the increase in blood phosphates which 

 they induce. In fish, the homologous ultimobranchial body of 

 Astyanax has been shown to hypertrophy, in a similar way to the 

 parathyroid, in response to stimuli leading to a rise of phosphates 



