Molecular genetic and physiologic background of the growth hormone–IGF-I axis in relation to breeding for growth rate and leanness in pigs

The GH–IGF-I axis is of major importance for the regulation of body growth and composition, and cellular proliferation and differentiation processes. Selective breeding aiming to improve growth rate and/or body composition is accompanied by changes of the GH–IGF-I axis. Research aiming to elucidate the genetic and physiologic mechanism(s) underlying these changes may best use single-trait selection lines. Two such pig selection lines, one for growth rate and one for high lean content, were used in experiments to investigate the mechanisms of the GH–IGF-I axis change during selection. This contribution reviews the selection-related changes in the GH–IGF-I axis as the consequences of selection for whole body growth rate or body composition and effects on local tissue growth rate. A model explaining the observed effects and consequences for the pressure on the physiology is presented. In short, selection related demand for GH induces GH synthesis until a limit is reached. After that the pulsatile GH plasma profile changes, which may also affect expression profiles of genes regulating body composition.