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

Figures

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  Fig. 1. Sympathetic nerves innervating an artery appear to connect to the neuromuscular junction (NMJ) in rat gastrocnemius muscle. A, B: Two representative confocal images showing sympathetic nerves (green, stained with anti-TH antibodies), arteries (magenta, visualized with α-elastin), and NMJs with motor nerves (blue, stained with anti-neurofilament and anti-SV2 antibodies) innervating muscle endplates (red, labeled with fluorescent α-bungarotoxin binding to acetylcholine receptors). a: Three-dimensional (3D) reconstructions spanning depths of 39–86 μm were generated from z-stack images acquired at 1-μm intervals and collapsed into maximum-intensity projections. Channel intensity and contrast were linearly adjusted to prepare RGB images. b, c: Enlarged view of the white boxed region in (a). The gray scale images in (c) present the TH signal. The right panel shows an orthogonal section along the vertical line indicated in the left panel.

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  Fig. 2. Effects of phenylephrine injection on MAP and tetanic force of the gastrocnemius and soleus muscles in rats with (A) and without (B) aortic nerve innervation. Left: Representative recordings of mean arterial pressure (MAP) and tetanic force monitored simultaneously before (control) and after intravenous phenylephrine injection. The arrow indicates the start of injection. The lower trace shows the averaged tetanic force (30 trials) before (control) and during phenylephrine injection. Zero (▲) on the X-axis represents the onset of tetanic tibial nerve stimulation (0.2 ms, 2 × motor threshold, 10 pulses at 100 Hz). Right: Summary graphs showing MAP and tetanic force values before and after injection in four rats. Bars indicate mean values; each symbol with its connecting line represents data from an individual rat.

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  Fig. 3. Percent changes in MAP and tetanic force of the gastrocnemius and soleus muscles induced by phenylephrine. A: Percent changes in tetanic force and MAP induced by saline or phenylephrine (Phe) in rats with intact (open columns) or transected (gray columns) aortic nerves. A value of 100 % represents the control (before injection). Bars and vertical lines indicate mean ± SD (n = 4 rats). Each symbol represents data from an individual rat. B: Relationship between the magnitude of changes in MAP and tetanic force during phenylephrine or saline injection in rats with intact aortic nerves.

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  Fig. 4. Effects of phenylephrine injection on excitatory musculo–lumbar sympathetic reflex discharges in rats with (A) and without (B) aortic nerve innervation. Left: Representative recordings of MAP and musculo–lumbar sympathetic reflex (averaged 30 times; mean of 4 trials in 4 rats) before (control) and during intravenous phenylephrine injection in rats with intact aortic nerves. Zero (▲) on the X-axis indicates the onset of tetanic tibial nerve stimulation (0.2 ms, 2 × motor threshold, 10 pulses at 100 Hz). Right: Summary graphs showing MAP and musculo–lumbar sympathetic reflex before and after injection in four rats. Bars indicate mean values; each symbol with its connecting line represents data from an individual rat.

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  Fig. 5. Percent changes in MAP and excitatory musculo–lumbar sympathetic reflex discharges induced by phenylephrine. A: Percent changes in musculo–lumbar sympathetic reflex and MAP induced by saline or phenylephrine (Phe) in rats with intact (open columns) or transected (gray columns) aortic nerves. A value of 100 % represents the amplitude before injection (control). Bars and vertical lines indicate mean ± SD (n = 4 rats). Each symbol represents data from an individual rat. B: Relationship between MAP and reflex magnitude during drug injection in rats with intact aortic nerves.

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  Fig. 6. Effects of phenylephrine injection on lumbar sympathetic nerve tonic activity. A, B: Representative original recordings of lumbar sympathetic nerve activity before (control) and after intravenous phenylephrine injection in the same rat with (A) and without (B) aortic nerve innervation. C: Summary graphs of sympathetic activity before and after injection in four rats with (left) and without (right) aortic nerve innervation. D: Percent changes in tonic lumbar sympathetic nerve activity induced by saline or phenylephrine (Phe) in rats with intact (open column) or transected (gray column) aortic nerves. A value of 100 % represents the mean activity for 1 min before drug injection. Bars and vertical lines indicate mean ± SD (n = 4 rats). Each symbol represents data from an individual rat.

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  Fig. 7. Schematic diagram summarizing the present findings. The tetanic contraction of hindlimb muscles induced by motor nerve stimulation (Stim.) produces an excitatory somato–lumbar sympathetic reflex. The excitatory reflex is inhibited by baroreceptor input from the aortic nerve, thereby modifying the degree of sympathetic support for tetanic force generation (bottom left). The histological finding that some sympathetic nerve axons connect from the artery to both presynaptic motor nerve terminals and postsynaptic acetylcholine receptor clusters in the neuromuscular junction (bottom right) supports this explanation. Created using BioRender.com.