Characterization of compensatory growth in hybrid striped bass (Morone chrysops x Morone saxatilis): Hormones and mechanisms

Compensatory growth (CG) is a period of growth acceleration that exceeds normal rates after animals are alleviated of certain growth-stunting conditions. We assessed whether CG responses could be induced in hybrid striped bass (HSB; Morone chrysops X Morone saxatilis) through feeding and/or temperature manipulation, and then examined whether components of the growth hormone (GH)/insulin-like growth factor (IGF) endocrine growth axis could be contributing to the response. We found that CG responses, characterized by elevations in specific growth rate, feed consumption and improved feed conversion, can be elicited following periods of feed restriction alone or in combination with low water temperature. Full catch-up growth was achieved through a combination of cold-banking and feed restriction. The onset of CG during feed manipulation was marked by significant elevations in total hepatic IGF-I mRNA, plasma IGF-I and 40kDa IGF binding protein, indicating that elevated synthesis and secretion of the growth factor along with its binding protein may regulate rapid growth responses. Strong correlations between systemic IGF-I and SGR suggest that circulating levels of IGF-I may serve as a biomarker of growth in HSB and perhaps other fishes. Changes in hepatic IGF-II mRNA paralleled body weight fluctuations, strengthening the hypothesis that IGF-II may be an important regulator of variable growth rates in fishes. We found that feed deprivation also caused reductions in GH receptor (GHR) gene expression which were subsequently restored during realimentation. Hence, the GHR may be a critical mediator of changes in IGF-I and growth rates observed prior to and during CG. A ten-fold increase in skeletal muscle IGF-I mRNA from previously depressed levels was seen during realimentation, suggesting the growth factor may also act on tissues in a paracrine/autocrine fashion to facilitate rapid growth. Taken together, these data show that HSB undergo considerable phases of accelerated growth when preceded by sufficient catabolism, and that an up-regulation of endocrine (IGF-I, -II, 40kDa IGFBP) and paracrine/autocrine (IGF-I) components of the GH/IGF axis likely facilitate the response.