Abstract and Introduction
Abstract
Introduction: Recent data indicate a direct relationship between the sympathetic nervous system and bone metabolism. The purpose of this study was to evaluate the effects of a beta-1 adrenergic (Adrb1) agonist, dobutamine (DOB), on disuse-induced changes in bone integrity during 28 d of hindlimb unloading (HU).
Methods: Male Sprague–Dawley rats, age 6 months, were assigned to either a normal cage activity (CC) or HU ( n = 24/group). Animals were given one daily bolus dose (4 mg·kg body weight a day) of DOB ( n = 12) or an equal volume of saline (VEH, n = 12).
Results: In vivo peripheral quantitative computed tomography scans revealed a 9% loss in proximal tibia metaphysis (PTM) volumetric bone mineral density (vBMD) over 28 d of disuse. DOB administration during HU significantly attenuated reductions in PTM vBMD and inhibited reductions in mid-diaphysis tibia cross-sectional moment of inertia. A significant decline in PTM bone formation rate in the HU–VEH group (−56% vs CC–VEH) was completely abolished in the HU–DOB group. Significant reductions in strength of the femoral shaft and neck in the HU–VEH group (14% and 15%, respectively) were prevented with DOB treatment.
Conclusion: In conclusion, DOB administration during HU effectively attenuates significant declines in total vBMD at PTM by mitigating associated decrements in bone formation rate. Positive effects of DOB were observed only in unloaded animals, with no effects observed in normal weight-bearing rats. These data provide evidence for the importance of Adrb1 signaling in maintaining osteoblast function during periods of mechanical unloading.
Introduction
Those humans exposed to long-duration sojourns in microgravity experience an accelerated loss of bone mass (approximately 1%–2% per month), which can result in osteopenia. For those crew members experiencing the greatest bone loss after 6 months on the International Space Station, reductions in proximal femur strength estimated by finite element modeling approach the estimated lifetime loss in stance strength for Caucasian women. Various clinical populations demonstrate similar deleterious effects when the usual daily mechanical loading of weight-bearing bones ceases. Prolonged bed rest can result in significant reductions in bone mass at the femoral neck and lumbar spine, with decreased bone volume fraction (BV/TV) and trabecular thickness. In addition, spinal cord injury patients experience particularly severe and rapid reductions in bone mass, particularly in cancellous bone sites (approximately 35% reduction).
The rodent hindlimb unloading (HU) model is a well-established ground-based model for investigating disuse effects on bone and muscle. HU results in significant reductions in disuse-sensitive cancellous bone mass, architecture, and material properties due to early increases in bone resorption followed by prolonged depression of bone formation rate (BFR).
β-adrenergic receptor agonists, activated by the sympathetic nervous system, may affect bone metabolism via three different β-adrenergic receptors (Adrb1, 2, 3) and can elicit opposing effects on bone mass. Activated Adrb2 receptors, present on osteoblasts and osteoclasts, result in apoptosis and diminished bone mass. Adrb1 receptors, on myocardial cells, increase cardiac contractility when activated and are also present on osteoblasts and osteoclasts. Activation of Adrb3 receptors, the primary adrenoreceptor on adipocytes, up-regulates lipolysis.
Adrb1 and Adrb2 receptors are both present on osteoblasts, but Adrb2 receptors are the predominant subtype. Adrb2 receptor agonist administration leads to increased bone resorption, resulting in reduced cancellous bone mass and negative changes to microarchitecture. Stimulation of osteoblast Adrb2 receptors enhances osteoclast differentiation and activity. In addition, Adrb2 receptor knock-out (KO) and Adrb1 receptor KO mice demonstrate a high and low bone mass phenotype, respectively, whereas Adrb1/2 receptor double KO mice exhibit a marked reduction in cancellous BFR and low bone mass phenotype versus wild-type littermates. Adrb1 and Adrb1/2 receptor-deficient mice do not respond to mechanical loading, whereas Adrb2 receptor-deficient mice and wild-type littermates respond normally. Taken together, these data suggest that the high bone mass phenotype in Adrb2 receptor KO mice may be caused by enhanced β−1 adrenergic receptor activity stimulating bone formation in the absence of the inhibitory effects of β−2 adrenergic receptors on osteoblasts. However, the role that Adrb1 receptor stimulation has on bone during reduced mechanical loading is not yet defined and may be important to further our understanding of the mechanistic role of β-adrenergic signaling responsible for bone loss during periods of disuse.
Dobutamine (DOB) is a nonspecific Adrb agonist with dominant β−1 receptor activity and a small amount of Adrb2 receptor activity. Its ability to primarily activate Adrb1 receptors makes it an attractive synthetic catecholamine to study in coordination with mechanical unloading. DOB administration during rodent HU significantly blunts disuse-induced reductions in femoral midshaft bone area and cross-sectional moment of inertia (CSMI) by mitigating the decline in periosteal bone mineral apposition rate (MAR).
The aim of this experiment was to elucidate the effects of an Adrb1 agonist on bone mass and strength during HU in skeletally mature rats. We hypothesized that Adrb1 receptor stimulation during HU would diminish disuse-associated reductions in bone mass and strength, whereas DOB given to normal weight-bearing animals would cause absolute increases in mass and strength.