Differences in sympathetic neuroeffector transmission to rat mesenteric arteries and veins as probed by in vitro continuous amperometry and video imaging

Abstract

As arteries are resistance blood vessels while veins perform a capacitance function, it might be expected that sympathetic neural control of arteries and veins would differ. The function of sympathetic nerves supplying mesenteric arteries (MA) and veins (MV) in rats was investigated using in vitro continuous amperometry with a carbon fibre microelectrode and video imaging. We simultaneously measured noradrenaline (NA) overflow at the blood vessel adventitial surface and vasoconstriction evoked by electrical stimulation of perivascular sympathetic nerves. Sympathetic nerve arrangement was studied using glyoxylic acid-induced fluorescence of NA. We found that: (i) there were significant differences between MA and MV in the arrangement of sympathetic nerves; (ii) frequency-response curves for NA overflow and vasoconstriction for MV were left-shifted compared to MA; (iii) the P2X receptor antagonist, pyridoxal-phosphate-6-azophenyl-2',4'-disulphonic acid (PPADS, 10 microm), reduced constrictions in MA but not in MV while the alpha(1)-adrenergic receptor antagonist, prazosin (0.1 microm), blocked constrictions in MV but not in MA; (iv) NA overflow for MA was enhanced by the alpha(2)-adrenergic receptor antagonist, yohimbine (1.0 microm), and attenuated by the alpha(2)-adrenergic receptor agonist, UK 14,304 (1.0 microm), while yohimbine and UK 14,304 had little effect in MV; (v) cocaine (10 microm) produced larger increases in NA overflow in MA than in MV; (vi) UK 14,304 constricted MV but not MA while yohimbine reduced constrictions in MV but not MA. We conclude that there are fundamental differences in sympathetic neuroeffector mechanisms in MA and MV, which are likely to contribute to their different haemodynamic functions.

Figure 1

Glyoxylic acid-induced fluorescence of catecholamines in perivascular sympathetic nerves A and B show low magnification (100×) images of sympathetic nerves while C and D show high magnification images (400×). The nerve plexus in arteries has a mesh-like arrangement of nerve fibres that are orientated along both the longitudinal and circular axis of the blood vessel (C). The plexus in veins has a selective circular arrangement with very few fibres orientated along the longitudinal axis of the blood vessel (D).

Figure 2

In vitro continuous amperometric measurement of NA oxidation currents and video imaging of neurogenic constriction in a MV NA oxidation currents and neurogenic constrictions are frequency dependent. Neurogenic constrictions (top) and NA oxidation currents (bottom) caused by 3 and 20 Hz stimulus trains (60 pulses with a 0.3 ms pulse width) in a MA (A) and a MV (B). The bars under the current traces denote the period of nerve stimulation. C and D, frequency–response curves for NA-oxidation current (C) (*significantly different from MA, P < 0.05) and constriction in MA and MV (D). Data are means ±s.e.m.

Figure 3

Contribution of α₁-adrenergic and P2X purinergic receptors to neurogenic constriction of MA and MV A, effects of PPADS (10 μm) on neurogenic constriction (top) and NA-oxidation current caused by a 20 Hz stimulus train (60 pulses with a 0.3 ms pulse width) in a MA. PPADS blocked the constriction but not the NA oxidation current. B, PPADS did not alter the neurogenic constriction or NA oxidation current in a MV. C, apparent concentration–constriction curves for neurally released NA in MA in the absence and presence of PPADS and prazosin (0.1 μm). Peak NA oxidation currents were measured during stimulus trains (0.2–20 Hz) by converting oxidation currents to apparent NA concentrations using calibrated electrodes. The curve obtained in the presence of PPADS (square symbols) reveals the sensitivity of MA to neurally relased NA. D, experiments similar to ‘C’ except these studies were done in MV. The curve obtained in the presence of PPADS (▪) reveals the sensitivity of MV to neurally relased NA. Prazosin blocked the constriction caused by nerve stimulation in MV but it did not alter the NA oxidation current (not shown). Data are means ±s.e.m.

Figure 5

Comparison of the time course of NA oxidation currents and constrictions in MA and MV A, rate of rise of the NA oxidation current is significantly faster in veins (n = 20) compared to arteries (n = 10). B, the rate of constriction was faster in veins compared to arteries. C, half-decay time of the NA oxidation current was not different in arteries and veins. Oxidation currents were evoked by 3 and 20 Hz trains of stimuli. D, the time to decay of constriction in veins was significantly longer than that in arteries after both 3 and 20 Hz trains of stimulation. *Significantly different from value in arteries (P < 0.05).

Figure 4

Kinetics of NA oxidation current and neurogenic vasoconstriction in MA and MV Responses were evoked using a 20 Hz stimulus train (60 pulses with a 0.3 ms pulse width). The dotted line shows the onset of nerve stimulation.

Figure 6

Effect of yohimbine on NA oxidation currents and neurogenic constriction of MA and MV A, recordings of oxidation current and constriction in a MA in the absence and presence of yohimbine (1 μm). B, oxidation current and constriction in a MV with and without yohimine. Yohimbine increased the current but not constriction in MA. Yohimbine had little effect on the oxidation current in MV but it reduced the constriction. Bars under the current traces indicate the period of nerve stimulation (60 pulses, 0.3 ms pulse duration.) C, percentage increase in NA oxidation currents in MA and MV in the presence of yohimbine. D, frequency–response curves for vasoconstriction in MA and MV in the absence (control) and in the presence of yohimbine. #Significantly different from control (P < 0.05). Data are means ±s.e.m.

Figure 7

Effect of UK 14,304 on NA oxidation currents and neurogenic constriction of MA and MV A, recordings of constriction (upper traces) and oxidation currents (lower traces) in a MA in the absence and presence of UK 14,304 (1 μm). UK 14,304 reduced the current and constriction. B, similar recordings in a MV. UK 14,304 caused a constriction of the MV but had little effect on the oxidation current. The bars under the current traces in A and B indicate the period of nerve stimulation (60 pulses with a 0.3 ms pulse width). C, frequency-dependent inhibition of NA oxidation currents in MA and MV by UK 14,304. Data are expressed as a percentage of the response obtained before UK 14,304. *Significantly different from control. #Significantly different from MA. D, frequency–response curves (1.0–20 Hz) for constriction in MA and MV in the absence (control) and presence of UK 14,304. *Significantly different from control for both MA and MV (P < 0.05). Data are means ±s.e.m.

Figure 8

Effect of cocaine on NA oxidation current and neurogenic constriction in of MA and MV A, representative recordings of constriction (upper traces) and oxidation currents (lower traces) in a MA in the absence and presence of cocaine (10 μm). Cocaine increased the current but had little effect on the constriction. B, similar recordings in a MV. Cocaine did not change the NA oxidation current or constriction in MV. The bars under the current traces in A and B indicate the period of nerve stimulation (60 pulses with a 0.3 ms pulse width). C, frequency-dependent increase in the NA oxidation current in MA but not MV by cocaine. Data are of expressed as a percentage of the response obtained before cocaine treatment in each tissue. #Significantly different from control levels. D, frequency–response curves (1.0–20 Hz) for vasoconstriction in MA and MV in the absence (control) and in the presence of cocaine. #Significantly different from control levels for both MA (P < 0.05). Cocaine did not change constriction in MV at any stimulation frequency. Data are means ±s.e.m.

Figure 9

Effects of combined inhibition of NET and α₂ autoreceptors on NA oxidation currents and constrictions of MA and MV A, representative recordings of constriction (upper traces) and current (lower traces) caused by a 3 Hz train of stimulation of a MA in the absence and presence of cocaine (10 μm) and yohimbine (1 μm). B, representative recordings of constriction (upper traces) and current (lower traces) caused by a 3 Hz train of stimulation in a MV in the absence and presence of cocaine (10 μm) and yohimbine (1 μm). Combined drug application increased the current response in MA and MV but the drugs increased constriction only in MA. C, frequency–response curves (1–20 Hz, 60 pulses) for cocaine/yohimbine induced increases in NA oxidation currents in MA and MV. Combined drug application increased NA oxidation currents in MA at low frequencies (< 7 Hz) but did not change significantly the oxidation currents in MV. Data are expressed as a percentage increase in oxidation currents over values obtained in the same blood vessels before drug application. #Significantly different from control values (P < 0.05). D, frequency–response curves for neurogenic constriction of MA and MV in the absence and presence of combined application of cocaine and yohimbine. Combined drug application increased MA constrictions at 1 and 3 Hz. #Significantly different from control values (P < 0.05). Constrictions of MV were not significantly affected by combined drug application. Data are means ±s.e.m.