Removal of half the sympathetic innervation does not reduce vasoconstrictor responses in rat tail artery

Abstract

Following reinnervation of denervated rat tail arteries, nerve-evoked contractions are at least as large as those evoked in normally innervated arteries despite a much lower nerve terminal density. Here nerve-evoked contractions have been investigated after transection of half the sympathetic innervation of normal tail arteries. After 1 week, the noradrenergic plexus 50-70 mm along the tail was about half as dense as control. Excitatory junction potentials recorded in smooth muscle cells of arterial segments isolated in vitro were half their normal amplitude. Surprisingly, nerve-evoked contractions of isometrically mounted segments were not reduced in amplitude, as was also the case after only 3 days. After 1 week, enhancement of nerve-evoked contractions by blocking either neuronal re-uptake of noradrenaline with desmethylimipramine or prejunctional α2-adrenoceptors with idazoxan was similar to control, suggesting that these mechanisms are matched to the number of innervating axons. The relative contribution of postjunctional α2-adrenoceptors to contractions evoked by long trains of stimuli was enhanced but that of α1-adrenoceptors was unchanged. Transiently, sensitivity to the α1-adrenoceptor agonist phenylephrine was slightly increased. After 7 weeks, amplitudes of nerve-evoked contractions remained similar to control, and sensitivity to phenylephrine had recovered but that to the α2-adrenoceptor agonist clonidine was slightly raised. The normal amplitude of nerve-evoked contractions after partial denervation is only partly explained by the greater contribution of α2-adrenoceptors. While the post-receptor mechanisms activated by nerve-released transmitter may be modified to amplify the contractions after partial denervation, our findings suggest that these mechanisms are normally saturated, at least in this artery.

Figure 1. Density of noradrenergic axons on the rat tail artery was reduced 5–7 days after transection of one ventral collector nerve (VCN)

A, micrographs showing tyrosine hydroxylase (TH)-immunolabelled axons in longitudinal sections (20 μm thick) through the perivascular plexus of the tail artery at two levels along the tail of a control rat (above) and of a rat in which both VCNs had been transected 7 days previously (below). After cutting both VCNs, the plexus 10–20 mm along the tail was not affected but had disappeared at 40–50 mm along the tail. The 100 μm scale bar applies throughout. B, micrographs showing catecholamine fluorescence in whole mount preparations of FAGLU-treated tail arteries from ∼60 mm along the tail of two control rats (above) and two rats in which only the left VCN had been transected 5 days previously (below). After transecting one VCN, the configuration of the plexus remained similar to control but its brightness was reduced. With both techniques, each pair of vessels (above and below) was prepared and processed at the same time. The 50 μm scale bar applies throughout.

Figure 2. Amplitude of intracellularly recorded excitatory junction potentials (EJPs) was reduced in tail artery segments 60 mm along the tail at 5–7 days after partial denervation (PD)

Electrical responses evoked by (A) single supramaximal stimuli and (B) trains of five impulses at 1 Hz averaged for six control arteries (above) and six PD arteries (below). EJP amplitude was reduced by ∼50% in PD arteries. The slow noradrenaline-mediated depolarisation (NAD) evoked by trains of stimuli tended to be smaller in PD arteries but this was not significant.

Figure 3. Partial denervation (PD) of tail arteries for 1 week did not reduce the amplitude of contractions evoked by short and long trains of perivascular stimuli

A, averaged traces showing increases in wall tension produced by 25 impulses at low frequencies (0.1, 0.3, 0.5 and 1 Hz) in segments from control (upper trace, n= 7) and 1 week PD (lower trace, n= 7) arteries. B, averaged traces showing increases in wall tension produced by 10 impulses at 1 and 10 Hz and 100 impulses at 1 Hz in segments from control (left, n= 7) and 1 week PD (right, n= 7) arteries. Note that the decay phase of the contractions was prolonged in PD arteries. C, peak increases in wall tension produced by 25 impulses at different frequencies, as in A. D, peak increases in wall tension produced by short and long trains of stimuli, as in B. Data for control (open bars, n= 7) and PD arteries (closed bars, n= 7) are presented as means and SEMs. Asterisks indicate significant differences between control and PD arteries (Tukey tests, *P < 0.05).

Figure 4. Partial denervation (PD) of tail arteries for 1 week did not significantly affect the enhancement of contraction amplitude by blocking neuronal re-uptake of noradrenaline

A, overlaid averaged traces showing increases in wall tension produced by 10 impulses at 1 and 10 Hz and 100 impulses at 1 Hz in segments of control (left, n= 7) and PD (right, n= 7) arteries in the absence and in the presence of desmethylimipramine (DMI; 30 nm, thicker traces as indicated by arrows). Note that the decay phase of contractions was prolonged after PD, both in the absence and in the presence of DMI. B, peak increases in wall tension evoked by these trains of stimuli in the presence of DMI. C, relative change in peak amplitude of contractions produced by DMI (ratio of amplitude in DMI to amplitude in control solution). Data for control (open bars, n= 7) and PD arteries (closed bars, n= 7) are presented as (B) means and SEMs or (C) medians and interquartile ranges. In PD arteries, contractions tended to be larger than control in the presence of DMI and the facilitatory effect of DMI on contractions to the long trains at 1 Hz to be reduced, but these effects were not significant.

Figure 5. Partial denervation (PD) of tail arteries for 1 week did not change the facilitatory effect of idazoxan (0.1 μm) on the amplitude of contractions to short high-frequency trains, but increased the acceleration of their decay produced by idazoxan

A, averaged traces showing contractions evoked by 10 impulses at 1 Hz (left) and 10 Hz (right) in control (above, n= 7) and PD (below, n= 7) arteries before (thin traces) and during (thicker traces) exposure to idazoxan (0.1 μm). B, relative changes in amplitude of contractions to 10 impulses at 1 and 10 Hz produced by idazoxan in control (open bars, n= 7) and 1 week PD arteries (closed bars, n= 7). Data are presented as means and SEMs. The dashed line indicates no change in contraction amplitude. Contractions to 1 Hz in both control and 1 week PD arteries were reduced in amplitude by idazoxan whereas those to 10 Hz were increased. Both responses were abbreviated in idazoxan.

Figure 6. Blockade of contractions evoked by prolonged trains at 1 Hz by the α₂-adrenoceptor antagonist idazoxan was enhanced 1 week after partial denervation (PD) of tail arteries, but blockade by the α₁-adrenoceptor antagonist prazosin was not affected

Percentage blockade of contractions evoked by (A) 10 and (B) 100 impulses at 1 Hz produced by prazosin (10 nm), idazoxan (0.1 μm) or both agents in control (open bars, n= 7) and PD arteries (closed bars, n= 7). Data are presented as means and SEMs. Asterisks indicate significant differences between control and PD arteries (Tukey tests: *P < 0.05, **P < 0.01).

Figure 7. Sensitivity of the tail artery to the α₁-adrenoceptor agonist phenylephrine was transiently increased after partial denervation (PD)

Data for control and PD arteries (A, C) after 1 week (n= 7 for both groups) and (B, D) 7 weeks (n= 8 for both groups) are presented as percentage of the maximum contraction to PE. A and B, in the absence of DMI, PE concentration–response curves for PD arteries lay to the left of those for control arteries. C and D, in the presence of desmethylimipramine (DMI, 30 nm), all concentration–response curves were shifted to the left. In DMI, the curve for 1 week PD arteries still lay to the left of that for control arteries but the curve for the two groups of 7 week arteries were overlaid. Data are presented as means and SEMs (some SEM bars are smaller than the symbols). P values indicate significant differences between the curves (ANOVA between-group comparisons).