Daily injection of the β2 adrenergic agonist clenbuterol improved poor muscle growth and body composition in lambs following heat stress-induced intrauterine growth restriction

Abstract

Background: Intrauterine growth restriction (IUGR) is associated with reduced β2 adrenergic sensitivity, which contributes to poor postnatal muscle growth. The objective of this study was to determine if stimulating β2 adrenergic activity postnatal would rescue deficits in muscle growth, body composition, and indicators of metabolic homeostasis in IUGR offspring.Methods: Time-mated ewes were housed at 40°C from day 40 to 95 of gestation to produce IUGR lambs. From birth, IUGR lambs received daily IM injections of 0.8 μg/kg clenbuterol HCl (IUGR+CLEN; n = 11) or saline placebo (IUGR; n = 12). Placebo-injected controls (n = 13) were born to pair-fed thermoneutral ewes. Biometrics were assessed weekly and body composition was estimated by ultrasound and bioelectrical impedance analysis (BIA). Lambs were necropsied at 60 days of age.Results: Bodyweights were lighter (p ≤ 0.05) for IUGR and IUGR+CLEN lambs than for controls at birth, day 30, and day 60. Average daily gain was less (p ≤ 0.05) for IUGR lambs than controls and was intermediate for IUGR+CLEN lambs. At day 58, BIA-estimated whole-body fat-free mass and ultrasound-estimated loin eye area were less (p ≤ 0.05) for IUGR but not IUGR+CLEN lambs than for controls. At necropsy, loin eye area and flexor digitorum superficialis muscles were smaller (p ≤ 0.05) for IUGR but not IUGR+CLEN lambs than for controls. Longissimus dorsi protein content was less (p ≤ 0.05) and fat-to-protein ratio was greater (p ≤ 0.05) for IUGR but not IUGR+CLEN lambs than for controls. Semitendinosus from IUGR lambs had less (p ≤ 0.05) β2 adrenoreceptor content, fewer (p ≤ 0.05) proliferating myoblasts, tended to have fewer (p = 0.08) differentiated myoblasts, and had smaller (p ≤ 0.05) muscle fibers than controls. Proliferating myoblasts and fiber size were recovered (p ≤ 0.05) in IUGR+CLEN lambs compared to IUGR lambs, but β2 adrenoreceptor content and differentiated myoblasts were not recovered. Semitendinosus lipid droplets were smaller (p ≤ 0.05) in size for IUGR lambs than for controls and were further reduced (p ≤ 0.05) in size for IUGR+CLEN lambs.Conclusion: These findings show that clenbuterol improved IUGR deficits in muscle growth and some metabolic parameters even without recovering the deficit in β2 adrenoreceptor content. We conclude that IUGR muscle remained responsive to β2 adrenergic stimulation postnatal, which may be a strategic target for improving muscle growth and body composition in IUGR-born offspring.

Daily Injection of the β2 Adrenergic Agonist Clenbuterol Improved Muscle Glucose Metabolism, Glucose-Stimulated Insulin Secretion, and Hyperlipidemia in Juvenile Lambs Following Heat-Stress-Induced Intrauterine Growth Restriction

“…This robust impairment of oxidative pathways manifests in less ATP production by the muscle tissues [ 3 , 26 , 39 , 40 ]. Programmed deficits in β2 adrenergic regulation of IUGR fetal muscle persist well after birth [ 14 , 17 ], and our present findings demonstrate their clear role in the hallmark metabolic dysfunction of IUGR offspring. In fact, 50% of the reductions in muscle-specific and consequent whole-body oxidative metabolic rates were recovered when IUGR lambs were treated daily with the β2 agonist.…”

“…Growth and body composition metrics for these lambs were published previously by Gibbs et al [ 17 ]. Briefly, the bodyweights of IUGR lambs were reduced by 24% at birth, 18% on day 30, and 16% on day 60.…”

“…In this study, we found that postnatal stimulation of β2 adrenergic activity reduced systemic inflammation and improved metabolic homeostasis in IUGR-born juvenile lambs. The loss of skeletal muscle β2 adrenoceptors and their downstream regulatory effects [ 13 , 14 , 17 ] following IUGR contributed to poor glucose oxidative capacity and greater glycogen content in muscle, as both were improved by postnatal clenbuterol injections. Elevated circulating NEFA concentrations in IUGR lambs were also ameliorated by the β2 agonist, consistent with its long-term inhibitory effects on lipolysis [ 33 ].…”

“…The experiments described in this manuscript were approved by the Institutional Animal Care and Use Committee at the University of Nebraska Lincoln (protocols 2381 and 2155), which is accredited by AAALAC International. The lambs used for this study were previously described in detail [ 17 ]. To generate IUGR lambs, Polypay-crossbred ewes were timed-mated to a single ram, fed ad libitum, and housed under heat-stress conditions (35 °C × 12 h/day, 40 °C × 12 h/day, 35% relative humidity, temperature–humidity index = 84 to 86) from the 40th to the 95th day of gestation and under thermoneutral conditions for the remainder of gestation.…”

“…In skeletal muscle and myoblasts (i.e., muscle stem cells), β2 adrenoceptors comprise about 90% of the adrenergic receptor populations [ 15 , 16 ], and their reduction is the primary mechanism of desensitization. Reduced skeletal muscle β2 adrenoreceptor content persists in IUGR-born offspring and coincides with diminished muscle mass and oxidative metabolism [ 2 , 14 , 17 ]. Abhorrent adrenergic regulation of IUGR pancreatic islets also disrupts insulin production and secretion, although this is primarily due to α adrenergic changes [ 13 , 18 , 19 , 20 ].…”