Chronic decentralization potentiates neurovascular transmission in the isolated rat tail artery, mimicking the effects of spinal transection

Abstract

Spinal cord transection produces a marked increase in the response of the isolated rat tail artery to sympathetic nerve stimulation, possibly as a result of a decrease in ongoing sympathetic activity. We have tested the effects of removing ongoing nerve activity on neurovascular transmission by cutting the preganglionic input to postganglionic neurones supplying the tail artery (decentralization). Isometric contractions to nerve stimulation were compared between decentralized arteries and those from age-matched and sham-operated controls. Nerve-evoked responses of decentralized arteries were much larger than those of control arteries at 2 and 7 weeks post operatively. The extent of blockade of nerve-evoked contraction by alpha-adrenoceptor antagonists prazosin (10 nM) or idazoxan (0.1 microM) was reduced. Decentralized arteries were transiently supersensitive to the alpha1-adrenoceptor agonist phenylephrine and the alpha2-adrenoceptor agonist clonidine; the unchanged sensitivity to methoxamine and phenylephrine after 2 weeks indicated no effect on the neuronal noradrenaline uptake transporter. Decentralized arteries were hypersensitive to alpha,beta methylene-ATP, but the P2-purinoceptor antagonist suramin (0.1 mM) did not reduce nerve-evoked contractions. Enlarged responses to 60 mM K+ after both 2 and 7 weeks were correlated with the response of the arteries to nerve stimulation, suggesting that increased postjunctional reactivity contributes to the enhanced contraction. Comparison between data from decentralized arteries and our previous data from spinalized animals showed that the two lesions similarly potentiate nerve-evoked contractions and have similar but not identical postjunctional effects. The enhanced vascular responses following a reduction in tonic nerve activity may contribute to the hypertensive episodes of autonomic dysreflexia in spinally injured patients.

Figure 1. Decentralization enhances contractions to perivascular nerve stimulation

A, contractions of 2 week sham-operated (upper trace) and 2 week decentralized arteries (lower trace) to stimulation of the perivascular nerves with 25 pulses at 0.1, 0.3 and 0.5 Hz. B, contractions in the same tissues evoked by 10 pulses at 1 and 10 Hz, and 100 pulses at 1 Hz. The pressure calibration in A applies also in B. In comparison with the sham-operated artery, the decentralized artery generated greater increases in effective pressure during all trains of stimuli. The contractions of the decentralized artery were also prolonged in time course.

Figure 2. Enhancement of contraction in decentralized arteries is greatest at low frequencies of nerve stimulation

A–F, peak increases in effective pressure evoked by different frequencies of perivascular stimulation in (A, B) 2 day control (n = 6) and decentralized (n = 7) arteries, (C, D) 2 week control (n = 7) and decentralized (n = 7) arteries, and (E, F) 7 week control (n = 7) and decentralized (n = 10) arteries. Open and hatched columns represent data for control and decentralized arteries, respectively. A, C and E, responses to 25 pulses at 0.1, 0.3 and 0.5 Hz, and 100 pulses at 1 Hz. B, D and F, responses to 10 pulses at 1 and 10 Hz. Data are presented as median and interquartile range, and statistical differences are indicated (*P < 0.05 and **P < 0.01). Arteries from decentralized animals had enlarged responses to neural activation at all postoperative (p.o.) times, and the effects were greater at lower frequencies of stimulation.

Figure 3. Blockade of nerve-evoked transmission by α-adrenoceptor antagonists is reduced in decentralized arteries

A–C, blockade of contractions evoked by 100 pulses at 1 Hz produced by the α-adrenoceptor antagonists prazosin (10 nm), idazoxan (0.1 μm) and phentolamine (1 μm) in (A) 2 day control (n = 6) and decentralized (n = 7) arteries, (B) 2 week control (n = 7) and decentralized (n = 7) arteries, and (C) 7 week control (n = 7) and decentralized (n = 10) arteries. Open and hatched columns represent data for control and decentralized arteries, respectively. Prazosin and idazoxan were applied alone and together as indicated. The column labelled ‘Phentolamine’ indicates the percentage blockade after adding this agent in the presence of both prazosin and idazoxan. Data are presented as median and interquartile range, and statistical differences are indicated (*P < 0.05 and **P < 0.01). In comparison with control, the blockade of electrically evoked contractions produced by prazosin and idazoxan was reduced in 2 week and 7 week decentralized arteries. For 2 day decentralized arteries, only the blockade produced by prazosin was reduced.

Figure 4. Decentralized arteries are transiently hypersensitive to clonidine

Concentration–response curves for (A–C) phenylephrine, and (D–F) clonidine, in control arteries (○) and at 2 days (A and D), 2 weeks (B and E), and 7 weeks (C and F) following decentralization (•). Data are presented as median and interquartile range. The curves are the best fits to the Hill equation. The sensitivity to α-adrenoceptor agonists was similar to control at all time points except for a transient hypersensitivity to clonidine at 2 weeks p.o. Note, however, that arteries decentralized for only 10 days were hypersensitive to both clonidine and phenylephrine (see text).

Figure 5. Lack of purinergic nerve-evoked responses in either control or decentralized tail arteries despite hypersensitivity to α,β-methylene ATP

A, effects of suramin (0.1 mm) applied alone or together with phentolamine (1 μm) on contractions of control arteries (open columns, n = 6) and 2 week decentralized arteries (hatched columns, n = 6) to trains of 25 stimuli at 0.5 (a) and 1 (b) Hz. B, the peak amplitude of the responses of control arteries (○) and 2 week decentralized arteries (•) to 0.3, 1, 3 and 10 μmα,β-methylene ATP (α,β-mATP). Data are presented as median and interquartile range, and statistical differences are indicated (*P < 0.05 and **P < 0.01). While suramin did not inhibit nerve-evoked contractions, responses of decentralized arteries to α,β-methylene ATP were increased.

Figure 6. Enhanced responses to 60 mm K⁺ suggest a nonspecific increase in reactivity in decentralized arteries

A, contractions of 2 week sham-operated and 2 week decentralized arteries to 60 mm K⁺ in the presence of α-adrenoceptor antagonists (10 nm prazosin + 0.1 μm idazoxan). B–D, comparison of responses to 60 mm K⁺ in (B) 2 day control (n = 6) and decentralized (n = 7) arteries, (C) 2 week control (n = 7) and decentralized (n = 7) arteries, and (D) 7 week control (n = 7) and decentralized (n = 10) arteries. Data are presented as median and interquartile range, and statistical differences are indicated (**P < 0.01). In comparison with the arteries from control animals, the 2 week and 7 week decentralized arteries had larger responses to 60 mm K⁺. In addition, on washout, the decay of the K⁺-evoked contraction was much slower in 2 week decentralized arteries (see A).

Figure 7. Decentralization and spinal transection produce similar changes in the reactivity of tail arteries

A–E, comparison of data obtained from decentralized arteries (2 weeks ○; 7 weeks □) with those from spinalized arteries (2 weeks • 8 weeks ▪) (Yeoh et al. 2004). Combined data for age-matched and sham-operated control arteries are also shown (2 weeks Δ 7–8 weeks ▴). A, peak amplitudes of contractions to 25 pulses at 0.1 and 0.5 Hz. B, peak amplitude (Amp) and half-width (HW) of contractions to 10 pulses at 10 Hz. C, peak amplitude (Amp) and half-decay time (HD) of contractions to 60 mm K⁺. D, percentage blockade of contractions evoked by 100 pulses at 1 Hz produced by prazosin (10 nm, Praz) and idazoxan (0.1 μm, Idaz) applied alone and together as indicated. The data points labelled ‘Phent’ indicate the percentage blockade after adding phentolamine (1 μm) in the presence of both prazosin and idazoxan. E, EC₅₀ values for phenylephrine (PE) and clonidine (Clon). Data are presented as median and interquartile range and, statistical differences between decentralized and spinalized arteries are indicated (*P < 0.05 and **P < 0.01). Decentralized and spinalized arteries had indistinguishable responses at 2 weeks. After 7–8 weeks, contractions to both nerve stimulation and raised K⁺ were no longer prolonged in decentralized arteries and these were less sensitive to clonidine. In contrast, the combinations of α-adrenoceptor antagonists blocked the contractions of spinalized arteries to a lesser extent.