Histological changes in insulin-immunoreactive pancreatic β-cells, and suppression of insulin secretion and somatotrope activity in brook trout (Salvelinus fontinalis) maintained on reduced food intake or exposed to acidic environment.

Immature brook trout (Salvelinus fontinalis) were randomly divided into a pH control, a pH and food control and an acid-stressed group. Fish in the first two groups were held at neutral pH and those in the last group were maintained at pH 4.2 for up to two months. The food supply to the pH and food control group was restricted to simulate the reduction in food intake demonstrated for acid-stressed trout. Plasma insulin levels were significantly decreased from 5-20 ng/ml to 1-2 ng/ml and plasma cortisol levels were significantly increased from 5-10 ng/ml to as high as 70 ng/ml in the acid-stressed brook trout. Concomitantly, a significant decrease of 21-39% in the proportion (volume density) of insulin immunoreactive β-cells was observed within the principal pancreatic islets. Somatic growth was stunted and ultrastructural morphometry revealed the suppression of somatotrope secretory activity in the acid-stressed fish. Restriction of food supply induced a smaller but still significant decrease in circulating levels of insulin which was however not accompanied by a reduction in insulin immunoreactive β-cells. The rise in plasma cortisol levels was not significant, and the plasma levels of glucose and protein were unaffected. Nevertheless, somatotrope secretory activity was suppressed and somatic growth was stunted. This study demonstrates for the first time the complexity of the endocrine response to acid stress and that some of the response to acid stress can be attributed to the lowering of food intake.