Possible role of insulin-like growth factor-II C-peptide on catecholamine release and ultrastructural aspects of chromaffin cells in the adrenal gland of the frog

The present study was undertaken to demonstrate that insulin-like growth factor-II C-peptide (IGF-II C-peptide) affects the function of the adrenal gland of Rana ridibunda (Anura, Amphibia) by stimulating chromaffin cells. Previous studies have shown that insulin-like growth factors affect adrenal gland function in mammals. On the basis of these findings, frogs were injected with IGF-II C-peptide (2.5 μg/0.2 ml), whereas control animals were injected with Ringer solution (0.2 ml). The adrenal glands were removed at 12 and 48 h after injection and fixed, embedded in paraffin wax and Epon, and examined by immunohistochemistry and transmission electron microscopy to investigate whether there were structural changes and activation of chromaffin cells in the frog adrenal gland. Sections were stained with hematoxylin and eosin for overall tissue analysis and, in parallel, serotonin was localized using the streptavidin–biotin complex technique while dopamine β-hydroxylase was shown by the peroxidase–antiperoxidase-3, 3′-diaminobenzidine tetrachloride method. After injection of IGF-II C-peptide, chromaffin cells released serotonin and synthesized dopamine β-hydroxylase. The most pronounced effect of IGF-II C-peptide on the chromaffin cells was observed at 12 h after injection. Our results indicate that there is a possible role of IGF-II C-peptide on chromaffin cell activity enhancing catecholamine release in the adrenal gland of the frog.