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To determine if decreased vascular responsiveness in the medial collateral ligament (MCL) of anterior cruciate ligament transected (ACL-t) rabbit knees is due to pericyte deficiency associated with angiogenesis. Vascular responses to potassium chloride (KCl), phenylephrine, acetylcholine, and sodium nitroprusside (SNP) were evaluated in ACL-t rabbit knees (n = 6) and control knees (n = 5) using laser speckle perfusion imaging. Ligament degeneration was determined by ultrasound imaging. Vascular and pericyte volume were measured using quantitative immunohistochemical volumetric analysis using CD31 and α-smooth muscle actin antibodies with co-localization analysis. Perfusion was increased in the ACL-t rabbits 2.5-fold. Responsiveness to phenylephrine, SNP, and acetylcholine was significantly decreased in the ACL knee while no change in KCl responses was seen. MCL ultrasound imaging revealed decreased collagen organization, increased ligament thickness, and increased water content in the ACL-t MCL. Vascular Volume was increased fourfold in ACL deficient knees, while pericyte volume to endothelial volume was not changed. No difference in CD31 and α-SMA co-localization was found. Blood vessels in the MCL of ACL-t knees do not lack smooth muscle. The MCL vasculature can undergo constrictive response to KCl, but have impaired receptor mediated responses and impaired nitric oxide signaling., (© 2014 Orthopaedic Research Society. Published by Wiley Periodicals, Inc.)

Background: Opioid receptors are possibly involved in the perception of exertion and the ventilatory response to exercise. We compared incremental cycling exercise in conditions of normoxia and hypoxia (11% O2) after injection of the opioid receptor antagonist naloxone (30 mg i.v.) or placebo. Naloxone was expected to increase sensation of breathing and cycling and to curtail exercise performance more in hypoxia., Methods: Ten healthy subjects (29 ± 2 years, 183 ± 6 cm, 75 ± 7 kg, mean ± SD) cycled in normoxia and hypoxia until voluntary exhaustion, receiving naloxone or placebo in a balanced double-blind crossover design., Results: Hypoxia decreased peak power output by 37%-39% with placebo and naloxone (P < 0.001, no effect of naloxone). Switching to normoxia at exhaustion in hypoxia allowed continuing up to 97%-100% of power developed in normoxia with placebo and naloxone (P < 0.001, no effect of naloxone). Perceived exertion increased in hypoxia, dropped upon switching to normoxia and increased again towards exhaustion, no effect of naloxone. SpO2 (earlobe oximetry) was lower in hypoxia, dropping to 64%-68% with naloxone and placebo. The ventilatory response to exercise in normoxia and hypoxia was not changed by naloxone., Conclusions: It follows that in healthy subjects the ventilatory response and the perception of exertion in hypoxia as compared to normoxia do not involve the endogenous opioid system, and the latter does not play a role in limiting maximum exercise capacity in hypoxia.