Activation of aortic baroreceptors depresses the somato–lumbar sympathetic reflex, reducing hindlimb muscle contractile force

We investigated whether the same sympathetic nerves innervate arteries and neuromuscular junctions and aortic baroreceptor stimulation reduces muscle force in rats. Histological analysis with fluorescent labeling revealed nicotinic acetylcholine receptors, motor nerves, adrenergic nerves, and arteries in gastrocnemius muscle tissue. Whole-mount preparations revealed a potential extension of adrenergic nerves around an artery connected to the neuromuscular junction. Physiological analysis under anesthesia was conducted to examine the effects of baroreceptor activation on muscle contractility and contraction-induced lumbar sympathetic reflex discharges following tetanic motor nerve stimulation. Intravenous phenylephrine injection increased mean arterial pressure to ∼150 mmHg and reduced both tetanic force and sympathetic reflex discharges when the aortic depressor nerves were intact. Bilateral aortic nerve denervation nearly abolished these effects. These findings indicate that aortic baroreceptor afferent signaling decreases hindlimb muscle contractility, most likely by inhibiting the contraction-induced sympathetic reflex. Sympathetic nerves distributed to arteries and neuromuscular junctions may underlie this modulation.