Session VIII - Polytrauma

*The Effects of Chronic Nicotine Exposure on Bone Blood Flow

Jeremy Feitelson, MS; Galia Soukhova O'Hare, PhD; X. Li, PhD; Craig S. Roberts, MD; John Fleming, PhD, University of Louisville, School of Medicine, Louisville, KY (all authors ­ a-Fisher-Owen Orthopaedic Fund/EBI)

Purpose: Nicotine is the major vasoactive ingredient in tobacco products and has been implicated in delayed fracture union, impeded bone metabolism, and decreased mineral content of bone. We investigated whether exposure to nicotine increases the constrictor reactivity of the bone vasculature to norepinephrine (NE), a physiologic regulator of bone blood flow.

Methods: A three-phase study was designed. The acute and chronic effects of nicotine on bone vascular reactivity were assessed by obtaining a dose-response curve to norepinephrine. The response curves were compared before and after experimental treatment. Immunohistochemical analysis was performed on bone marrow (with blood vessels) removed from the femur of male Sprague-Dawley rats. Anesthetized male Sprague-Dawley rats (356-393 g) were infused with nicotine dissolved in phosphate-buffered saline solution (PBS) at a rate of 150_g/kg/hr. The PBS was infused at the same rate in control animals. Throughout the nicotine or PBS infusion, bone blood flow and blood pressure values were measured. Afterward, the norepinephrine dose-response curve was repeated while nicotine or the phospate was continuously infused. For the chronic nicotine treatment, either nicotine or PBS was administered to the animals throughout a two-week treatment period; nicotine was released at a rate of 2.4mg/kg/day.

Results: The inner lining of the walls of the microscopic arterioles exposed to nicotinic receptor antibody were darkly stained as compared to the light staining of negative control tissue, suggesting the presence of nicotinic receptors. Before nicotine or PBS infusion, the initial NE dose-response curve produced a maximal increase in blood pressure of 14.7 ± 1.0%, a decrease in bone blood flow of 42.7 ± 9.1%, and an increase in bone vascular resistance of 134.0 ± 44.4%. After infusion of nicotine, the NE maximally increased blood pressure by 11.4 ± 3.5%, decreased bone blood flow by 54.6 ± 8.4%, and increased bone vascular resistance by 170.3 ± 68.3%. By comparison, after infusion of PBS, NE maximally increased blood pressure by 12.3 ± 2.0%, decreased bone blood flow by 23.8 ± 4.3%, and increased bone vascular resistance by 47.9 ± 5.8%. Thus, infusion of nicotine did not significantly affect the bone vascular reactivity to NE. Two weeks of nicotine treatment did not significantly change the baseline values for blood pressure, bone blood flow, or bone vascular resistance. However, bone vascular constriction to NE was significantly enhanced by chronic nicotine treatment. NE (1.9 _g/kg) increased blood pressure in the nicotine-treated animals by 13.6 ± 3.4% as compared to 6.4 ± 0.9% in PBS controls. Bone blood flow decreased by 41.5 ± 3.0% in the nicotine-treated group as compared to 27.4 ± 2.9% in PBS controls. Bone vascular resistance increased 97.7 ± 15.1% in the nicotine group as compared to 47.6 ± 6.6% in PBS controls.

Conclusion: These data indicate that: (1) nicotine receptors are present on the microscopic arterioles in the bone canal, (2) acute (1-hour) infusion of nicotine does not significantly affect basal bone hemodynamics or bone vascular reactivity to norepinephrine, and (3) chronic, 2-week administration of nicotine significantly increases the reactivity of the bone vasculature to exogenous norepinephrine. Changes in bone blood flow and bone vascular function through norepinephrine-mediated mechanisms may provide the key to understanding the orthopaedic consequences of chronic nicotine exposure.