Responses to administration of growth hormone releasing hormone and glucose in steers receiving stair-step and extended restriction on feeding

Insulin secretion and signaling in response to dietary restriction and subsequent re-alimentation in cattle

The objectives of this study were to examine systemic insulin response to a glucose tolerance test (GTT) and transcript abundance of genes of the insulin signaling pathway in skeletal muscle, during both dietary restriction and re-alimentation-induced compensatory growth. Holstein Friesian bulls were blocked to one of two groups: 1) restricted feed allowance for 125 days (period 1) (RES, n = 15) followed by ad libitum feeding for 55 days (period 2) or 2) ad libitum access to feed throughout (periods 1 and 2) (ADLIB, n = 15). On days 90 and 36 of periods 1 and 2, respectively, a GTT was performed. M. longissimus dorsi biopsies were harvested from all bulls on days 120 and 15 of periods 1 and 2, respectively, and RNA-Seq analysis was performed. RES displayed a lower growth rate during period 1 (RES: 0.6 kg/day, ADLIB: 1.9 kg/day; P < 0.001), subsequently gaining more during re-alimentation (RES: 2.5 kg/day, ADLIB: 1.4 kg/day; P < 0.001). Systemic insulin response to glucose administration was lower in RES in period 1 (P < 0.001) with no difference observed during period 2. The insulin signaling pathway in M. longissimus dorsi was enriched (P < 0.05) in response to dietary restriction but not during re-alimentation (P > 0.05). Genes differentially expressed in the insulin signaling pathway suggested a greater sensitivity to insulin in skeletal muscle, with pleiotropic effects of insulin signaling interrupted during dietary restriction. Collectively, these results indicate increased sensitivity to glucose clearance and skeletal muscle insulin signaling during dietary restriction; however, no overall role for insulin was apparent in expressing compensatory growth.

Effect of feed restriction and subsequent re-alimentation on hormones and genes of the somatotropic axis in cattle

The objective of this study was to characterize the effect of feed restriction and compensatory growth during re-alimentation on the functionality of the somatotropic axis. We blocked 60 bulls into one of two groups: 1) restricted feed allowance for 125 days (period 1) (RES, n = 30) followed by ad libitum feeding for 55 days (period 2) or 2) ad libitum access to feed throughout (ADLIB, n = 30). A growth hormone releasing hormone (GHRH) challenge was performed during each period. At the end of each period, 15 animals from each treatment were slaughtered and hepatic tissue collected. Hepatic expression of 13 genes of the somatotropic axis was measured by qRT-PCR. RES displayed a lower growth rate during period 1 (0.6 vs. 1.9 kg/day; P < 0.001), subsequently gaining more than ADLIB animals during period 2 (2.5 vs. 1.4 kg/day; P < 0.001). Growth hormone response to GHRH was not different between treatments at either time-point (P > 0.05); however, resultant plasma IGF-1 was lower in period 1 and greater in period 2 in RES animals (P < 0.05). Expression of IGFBP2 was higher (P < 0.01) and IGF1 (P < 0.001) and GHRIA (P < 0.05) lower in RES compared with ADLIB during period 1, with no difference evident in period 2 (P > 0.05). Collectively, the results of this study are consistent with uncoupling of the somatotropic axis following feed restriction. However, there is no evidence from this study that the somatotropic axis per se is a significant contributor to compensatory growth.