Acromegaly in the dog and cat

Growth hormone (GH), somatotropin, or somatotrope hormone is a single chain polypeptide with a molecular weight of about 22 kDa, whose amino acid sequence varies considerably between different species. GH is secreted in a pulsatile manner from the acidophilic somatotroph cells in the anterior lobe of the pituitary gland. This pulsatile secretion reflects that of two hypothalamic factors with opposing effects: growth hormone-releasing hormone (GHRH) or somatocrinin, which stimulates the secretion of GH, and somatostatin and somatotropin release-inhibiting factors (SRIF), which inhibit its secretion. Besides these hypothalamic hormones, a peptide, called ghrelin, which stimulates the release of GH and participates in the regulation of the production of pituitary GH, has been identified (Fig. 1).  

Various factors regulate the secretion of GH: (i) negative feedback systems that involve GH itself and somatomedins; (ii) nerve transmitters such as adrenaline; and (iii) metabolic stimuli such as hypoglycaemia and increased plasma concentrations of some amino acids.

The effects of GH can be divided into two main groups: fast or catabolic effects and slow or anabolic effects. The former occur through the direct action of GH on target cells and include an increase in lipolysis and less transport of glucose into cells because of the antagonism of insulin. The slow anabolic effects, on the other hand, are mediated by a growth factor called insulin-like growth factor-1 (IGF-I). This growth factor, synthesised predominantly in the liver, has a molecular structure with about 50% homology to that of insulin and circulates in the plasma bound to IGF-binding proteins (IGF-BP). Importantly, in the dog GH is secreted not only by the pituitary gland, but also by from foci of hyperplastic ductal epithelium in the mammary glands.

The release of GH from breast tissue is a physiological process during the luteal phase of oestrus which, in some female dogs, can cause the development of acromegaly. Furthermore, administration of exogenous progestins can produce clinical signs of acromegaly. The GH induced by progesterone is not secreted in a pulsatile manner, does not respond to stimulation with GHRH and is not inhibited by the administration of somatostatin.

DEFINITION

Hypersecretion of GH in adult subjects causes a syndrome characterized by excessive growth of connective tissue, bone and viscera. In 1886 Pierre Marie, a French neurologist, was the first to describe the hypophyseal origin of this disorder in humans, giving the condition the name of acromegaly from the combination of two Greek words akron (extremity) and megas (large), because of the characteristic enlargement of the hands and feet.

In the cat and dog, as in humans, acromegaly can be caused by a adenoma of the somatotroph cells in the pituitary gland. The discovery that acromegaly may also be caused by exogenous administration of progestins, in that circulating GH in the dog also has a mammary origin, goes back to the 1970s and 1980s.

Acromegaly in the dog

Acromegaly in the cat

Suggested readings

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