Roles of noradrenaline and ATP in sympathetic vasoconstriction of the guinea-pig main ear artery

Control of the Cutaneous Circulation by the Central Nervous System

The central nervous system (CNS), via its control of sympathetic outflow, regulates blood flow to the acral cutaneous beds (containing arteriovenous anastomoses) as part of the homeostatic thermoregulatory process, as part of the febrile response, and as part of cognitive-emotional processes associated with purposeful interactions with the external environment, including those initiated by salient or threatening events (we go pale with fright). Inputs to the CNS for the thermoregulatory process include cutaneous sensory neurons, and neurons in the preoptic area sensitive to the temperature of the blood in the internal carotid artery. Inputs for cognitive-emotional control from the exteroceptive sense organs (touch, vision, sound, smell, etc.) are integrated in forebrain centers including the amygdala. Psychoactive drugs have major effects on the acral cutaneous circulation. Interoceptors, chemoreceptors more than baroreceptors, also influence cutaneous sympathetic outflow. A major advance has been the discovery of a lower brainstem control center in the rostral medullary raphé, regulating outflow to both brown adipose tissue (BAT) and to the acral cutaneous beds. Neurons in the medullary raphé, via their descending axonal projections, increase the discharge of spinal sympathetic preganglionic neurons controlling the cutaneous vasculature, utilizing glutamate, and serotonin as neurotransmitters. Present evidence suggests that both thermoregulatory and cognitive-emotional control of the cutaneous beds from preoptic, hypothalamic, and forebrain centers is channeled via the medullary raphé. Future studies will no doubt further unravel the details of neurotransmitter pathways connecting these rostral control centers with the medullary raphé, and those operative within the raphé itself. © 2016 American Physiological Society. Compr Physiol 6:1161-1197, 2016.

Purinergic signaling and blood vessels in health and disease

Purinergic signaling plays important roles in control of vascular tone and remodeling. There is dual control of vascular tone by ATP released as a cotransmitter with noradrenaline from perivascular sympathetic nerves to cause vasoconstriction via P2X1 receptors, whereas ATP released from endothelial cells in response to changes in blood flow (producing shear stress) or hypoxia acts on P2X and P2Y receptors on endothelial cells to produce nitric oxide and endothelium-derived hyperpolarizing factor, which dilates vessels. ATP is also released from sensory-motor nerves during antidromic reflex activity to produce relaxation of some blood vessels. In this review, we stress the differences in neural and endothelial factors in purinergic control of different blood vessels. The long-term (trophic) actions of purine and pyrimidine nucleosides and nucleotides in promoting migration and proliferation of both vascular smooth muscle and endothelial cells via P1 and P2Y receptors during angiogenesis and vessel remodeling during restenosis after angioplasty are described. The pathophysiology of blood vessels and therapeutic potential of purinergic agents in diseases, including hypertension, atherosclerosis, ischemia, thrombosis and stroke, diabetes, and migraine, is discussed.

Water immersion and EMLA cause similar digit skin wrinkling and vasoconstriction

Water immersion skin wrinkling tests limb sympathetic vasoconstrictor function. We have recently shown that water immersion wrinkling is accompanied by digit vasoconstriction and postulated that vasoconstriction is the main underlying mechanism. To test this further, we applied vasoconstrictive cream (EMLA) to the distal digit and compared the degree of skin wrinkling and digit blood flow reduction with those after water immersion. In 25 healthy volunteers (6 male, 19 female; mean age, 35 yr) subjected to EMLA and water immersion, both clinical wrinkling scores and reduction in digit blood flow (mean of 2.01 and 2.29 cm/s, respectively) were nearly identical. Control using aqueous cream resulted in minimal skin wrinkling and nonsignificant reduction in digit artery flow (P = 0.170). These data further support that water immersion skin wrinkling is mediated by vasoconstriction. The EMLA cream patch test may develop into a useful screening test for hand sympathetic vasoconstrictor function.

Transmitter characteristics of cutaneous, renal and skeletal muscle small arteries in the rat

AIM

We studied transmitter characteristics of proximal and distal arteries supplying skin (saphenous artery and its medial tarsal branch), kidneys (terminal branches of renal artery and interlobar arteries) and skeletal muscle (proximal and distal sections of external sural artery).

METHODS

Artery segments were mounted in an isometric myograph and intramural nerves were activated by electrical field stimulation. Adrenergic and purinergic components of the neurogenic response were blocked using phenoxybenzamine and alpha,beta-methylene adenosine triphosphate (mATP), respectively.

RESULTS

Arteries from skin or kidney developed rapid and prominent neurogenic contractile responses, with half-maximal amplitude reached within 5-15 s; responses in proximal vessels were greater than in distal vessels. Arteries from skeletal muscle responded to sympathetic stimulation with a moderate contraction developing over 1 min or more, the response of distal segments was greater than that of proximal segments. In skeletal muscle vessels the sympathetically evoked contraction was completely blocked by phenoxybenzamine, whereas in skin and renal vessels it was the combined effect of noradrenaline and adenosine triphosphate (ATP). Given alone, mATP did not change the magnitude of the response to nerve stimulation, but increased its latency and also potentiated the response to exogenous noradrenaline. In all vascular beds, distal vessels were more sensitive to noradrenaline and mATP.

CONCLUSION

It thus appears that the noradrenaline/ATP ratio of the sympathetic vasoconstrictor response differs between vascular beds in a way that is consistent with known differences in the selective regulation of regional vascular resistance by the sympathetic nervous system.

Pathway specific expression of neuropeptides and autonomic control of the vasculature

In this article, we review the immunohistochemical evidence for the pathway-specific expression of co-existing neuropeptides in autonomic vasomotor neurons, and examine the functional significance of these expression patterns for the autonomic regulation of the vasculature. Most final motor neurons in autonomic vasomotor pathways contain neuropeptides in addition to non-peptide co-transmitters such as catecholamines, acetylcholine and nitric oxide. Neuropeptides also occur in preganglionic vasomotor neurons. The precise combinations of neuropeptides expressed by neurons in vasomotor pathways vary with species, vascular bed, and the level within the vascular bed. This applies to both vasoconstrictor and vasodilator pathways. There is a similar degree of variation in the expression of neuropeptide receptors in the vasculature. Consequently, the contributions of different peptides to autonomic vasomotor control are closely matched to the functional requirements of specific vascular beds. This arrangement allows for a high degree of precision in vascular control in normal conditions and has the potential for considerable plasticity under pathophysiological conditions.

Cotransmission from sympathetic vasoconstrictor neurons to small cutaneous arteries in vivo

This study has characterized constrictions of small cutaneous arteries in the guinea pig ear in response to electrical stimulation of the cervical sympathetic nerve (SNS) in vivo. Video microscopy and on-line image analysis were used to examine diameter changes of ear arteries (80-140 micrometers resting diameter) in anesthetized guinea pigs. Trains of 50-300 impulses, but not single pulses or short trains, produced frequency-dependent (2-20 Hz) constrictions. The purinoceptor antagonist suramin (30 microM) greatly reduced constrictions produced by exogenous ATP but did not affect constrictions produced by SNS at 10 Hz or exogenous norepinephrine. The alpha(2)-adrenoceptor antagonist yohimbine (1 microM) enhanced the peak amplitude of sympathetic constrictions at lower stimulation frequencies (1-5 Hz). The amplitude of constrictions to SNS at 10 Hz was reduced, and the latency of constrictions was increased by the alpha(1)-adrenoceptor antagonist prazosin (1 microM). Constrictions to SNS at 10 Hz remaining after prazosin treatment were reduced in amplitude by dihydroergotamine (2 microM) and were attenuated further by the neuropeptide Y Y(1)-receptor antagonist 1229U91 (0.3 microM). Thus norepinephrine and neuropeptide Y act as cotransmitters to mediate sympathetic constriction of small ear arteries at higher stimulation frequencies (10 Hz), but ATP does not seem to contribute directly to these constrictions.

Regional differences in sympathetic neurotransmission to cutaneous arteries in the guinea-pig isolated ear

The effects of sympathetic nerve stimulation on different cutaneous arteries were examined in arteries isolated from guinea-pig ears, by measuring membrane potential changes in smooth muscle cells in response to electrical field stimulation. Resting membrane potential (RMP) was similar in proximal (main ear artery) and distal (3rd or 4th branch order) cutaneous arteries (mean -71 mV). Single stimuli evoked excitatory junction potentials (EJPs) in all arteries. The EJPs in proximal arteries were twice the amplitude, and the time constant of EJP decay was almost half the value, compared with distal cutaneous arteries. EJP amplitude was reduced by > 90% by suramin (30 microM) or alpha,beta,methylene-ATP (alpha,beta,m-ATP)(1 microM) in all proximal, and most distal arteries. Residual responses in distal arteries were resistant to tetrodotoxin. The N-type calcium channel blocker, omega-conotoxin GVIA (30 nM), reduced EJP amplitude by 70-100% in both proximal and distal arteries. Successive EJPs evoked by trains of stimuli at 1 to 5 Hz were depressed in amplitude in proximal arteries, but showed facilitation in distal arteries. EJP depression in proximal arteries was reversed to facilitation by the alpha2-adrenoceptor antagonist, yohimbine (30 nM). Trains of stimuli delivered at 10-20 Hz produced summation of EJPs and active membrane responses in 30% of proximal arteries. Active responses were never detected in distal arteries. Slow depolarizations following the EJPs were detected in most arteries after trains of stimuli, and were abolished by prazosin (0.3 microM) or omega-conotoxin GVIA (30 nM). The density of the perivascular plexus of axons innervating proximal arteries, demonstrated with catecholamine fluorescence histochemistry, was twice that in distal cutaneous arteries. These regional differences in sympathetic neurotransmission suggest that cutaneous vasoconstriction in response to thermoregulatory stimuli, which occurs predominantly in distal cutaneous segments, is likely to be qualitatively different from cutaneous vasoconstriction of proximal arteries in response to other physiological stimuli.

Evidence that tachykinins are the main NANC excitatory neurotransmitters in the guinea-pig common bile duct

Application of electrical field stimulation (EFS; trains of 10 Hz, 0.25 ms pulse width, supramaximal voltage for 60 s) to the guinea-pig isolated common bile duct pretreated with atropine (1 microM), produced a slowly-developing contraction ('on' response) followed by a quick phasic 'off' contraction ('off peak' response) and a tonic response ('off late' response), averaging 16+/-2, 73+/-3 and 20+/-4% of the maximal contraction to KCl (80 mM), n=20 each, respectively. Tetrodotoxin (1 microM; 15 min before) abolished the overall response to EFS (n 8). Neither in vitro capsaicin pretreatment (10 microM for 15 min), nor guanethidine (3 microM, 60 min before) affected the excitatory response to EFS (n 5 each), showing that neither primary sensory neurons, nor sympathetic nerves were involved. Nomega-nitro-L-arginine (L-NOARG, 100 microM, 60 min before) or naloxone (10 microM, 30 min before) significantly enhanced the 'on' response (294+/-56 and 205+/-25% increase, respectively; n=6-8, P<0.01) to EFS. The combined administration of L-NOARG and naloxone produced additive enhancing effects (655+/-90% increase of the 'on' component, n = 6, P<0.05). The tachykinin NK2 receptor-selective antagonist MEN 11420 (1 microM) almost abolished both the 'on' and 'off late' responses (P<0.01: n=5 each) to EFS, and reduced the 'off-peak' contraction by 55+/-8% (n=5, P<0.01). The subsequent administration of the tachykinin NK1 receptor-selective antagonist GR 82334 (1 microM) and of the tachykinin NK3 receptor-selective antagonist SR 142801 (30 nM), in the presence of MEN 11420 (1 microM), did not produce any further inhibition of the response to EFS (P>0.05; n=5 each). At 3 microM, GR 82334 significantly reduced (by 68+/-9%, P<0.05, n=6) the 'on' response to EFS. The contractile 'off peak' response to EFS observed in the presence of both MEN 11420 and GR 82334 (3 microM each) was abolished (P<0.01; n=6) by the administration of the P2 purinoceptor antagonist pyridoxalphosphate-6-azophenyl-2',4'-disulphonic acid (PPADS, 30 microM). PPADS (30 microM) selectively blocked (75+/-9 and 50+/-7% inhibition, n = 4 each) the contractile responses produced by 100 and 300 microM ATP. Tachykinin-containing nerve fibres were detected by using immunohistochemical techniques in all parts of the bile duct, being distributed to the muscle layer and lamina propria of mucosa. In the terminal part of the duct (ampulla) some labelled ganglion cells were observed. In conclusion, this study shows that in the guinea-pig terminal biliary tract tachykinins, released from intrinsic neuronal elements, are the main NANC excitatory neurotransmitters, which act by stimulating tachykinin NK2 (and possibly NK1) receptors. ATP is also involved as excitatory neurotransmitter. Nitric oxide and opioids act as inhibitory mediators/modulators in this preparation.

Contribution of alpha-adrenoceptors to depolarization and contraction evoked by continuous asynchronous sympathetic nerve activity in rat tail artery

1. The effects of continuous but asynchronous nerve activity induced by ciguatoxin (CTX-1) on the membrane potential and contraction of smooth muscle cells have been investigated in rat proximal tail arteries isolated in vitro. These effects have been compared with those produced by the continuous application of phenylephrine (PE). 2. CTX-1 (0.4 nM) and PE (10 microM) produced a maintained depolarization of the arterial smooth muscle that was almost completely blocked by alpha-adrenoceptor blockade. In both cases, the depolarization was more sensitive to the selective alpha-adrenoceptor antagonist, idazoxan (0.1 microM), than to the selective alpha 1-adrenoceptor antagonist, prazosin (0.01 microM). 3. In contrast, the maintained contraction of the tail artery induced by CTX-1 (0.2 nM) and PE (2 and 10 microM) was more sensitive to prazosin (0.01) microM, than to idazoxan (0.01 microM). In combination, these antagonists almost completely inhibited contraction to both agents. 4. Application of the calcium channel antagonist, nifedipine (1 microM), had no effect on the depolarization induced by either CTX-1 or PE but maximally reduced the force of the maintained contraction to both agents by about 50%. 5. We conclude that the constriction of the tail artery induced by CTX-1, which mimics the natural discharge of postganglionic perivascular axons, is due almost entirely to alpha-adrenoceptor activation. The results indicate that neuronally released noradrenaline activates more than one alpha-adrenoceptor subtype. The depolarization is dependent primarily on alpha 2-adrenoceptor activation whereas the contraction is dependent primarily on alpha 1-adrenoceptor activation. The links between alpha-adrenoceptor activation and the voltage-dependent and voltage-independent mechanisms that deliver Ca2+ to the contractile apparatus appear to be complex.

Effects of heptanol on electrical activity in the guinea-pig vas deferens

1. The effects of the putative intercellular uncoupling agent I-heptanol on electrical activity in the guinea-pig vas deferens were studied by use of intracellular and extracellular recording techniques. 2. At concentrations of 0.5, 1 and 2 mM, heptanol rapidly, monotonically and reversibly attenuated intracellularly recorded excitatory junction potential (e.j.p.) amplitude without affecting its time course, while spontaneous excitatory junction potentials (s.e.j.ps) were left unaffected. 3. Heptanol did not affect either the extracellularly recorded evoked excitatory junction current (e.j.c.), or the nerve terminal impulse that preceded it. These observations indicate that heptanol does not affect nerve impulse conduction, neurotransmitter release, or the postjunctional receptors involved in the production of the e.j.p. 4. E.j.ps appear to be suppressed by heptanol due to its intercellular uncoupling effects. Therefore, functional intercellular coupling may be necessary for the generation of the e.j.p. in smooth muscle.

Electrical activity in rat tail artery during asynchronous activation of postganglionic nerve terminals by ciguatoxin-1

1. The effects of ciguatoxin-1 (CTX-1) on the membrane potential of smooth muscle cells have been examined in rat proximal tail arteries isolated in vitro. 2. CTX-1 (> or = 10 pM) increased the frequency of spontaneous excitatory junction potentials (s.e.j.ps). At 100-400 pM, there was also a marked and maintained depolarization (19.7 +/- 1.4 mV, n = 14, at 400 pM). 3. In 20-400 pM CTX-1, perivascular stimuli evoked excitatory junction potentials (e.j.ps) which were prolonged in time course relative to control. 4. Although threshold and latency of the e.j.p. were not affected by CTX-1 (< or = 400 pM), propagated impulses were blocked at > or = 100 pM. 5. The spontaneous activity and the depolarization produced by CTX-1 were reduced in the presence of Ca2+ (0.1 mM)/Mg2+ (25 mM), omega-conotoxin (0.1 microM) or Cd2+ (50-100 microM). 6. All effects of CTX-1 were abolished by tetrodotoxin (0.3 microM). 7. Raised Ca2+ (6 mM) reduced the depolarization and spontaneous activity produced by CTX-1. 8. In 400 pM CTX-1, the membrane repolarized (17 +/- 3.2 mV, n = 4) following the addition of phentolamine (1 microM). S.e.j.ps and e.j.ps were selectively abolished by suramin (1 mM), and the membrane repolarized by 1.3 +/- 1.6 mV (n = 4). 9. We conclude that CTX-1 releases noradrenaline and ATP by initiating asynchronous discharge of postganglionic perivascular axons. In 100-400 pM CTX-1, the smooth muscle was depolarized to levels resembling those recorded in this artery during ongoing vasoconstrictor discharge in vivo.

Blockade of noradrenaline-induced constrictions by yohimbine and prazosin differs between consecutive segments of cutaneous arteries in guinea-pig ears

1. The study has examined the receptors mediating constriction produced by brief local application of noradrenaline (NA) to cutaneous arteries and arterioles in the ear vasculature of anaesthetized guinea-pigs. The innervation of the corresponding vascular segments has been examined immunohistochemically at the conclusion of the pharmacological experiments. 2. Small arteries of branch order 4 (4 degrees, 40-110 microns internal diameter) were more sensitive to the vasoconstrictor action of NA than were more proximal arteries of branch order 3 (3 degrees, 60-150 microns internal diameter), or more distal arteries and arterioles of branch orders 5 to 7 (5 degrees-7 degrees, 18-85 microns internal diameter). This higher sensitivity of 4 degrees arteries was maintained after blockade of neuronal uptake with desipramine (1 microM), and after blockade of beta-adrenoceptors with propranolol (1 microM). 3. NA-induced vasoconstrictions of distal arterioles (5 degrees-7 degrees) were abolished or greatly reduced by yohimbine (1 microM). The blockade by yohimbine decreased progressively with increasing vessel diameter of proximal arteries, while the blockade by prazosin (1 microM) increased progressively in arteries > 40 microns diameter. 4. In 3 degrees and 4 degrees arteries, a substantial component (approximately 50%) of NA-induced vasoconstrictions remained after combined treatment with yohimbine and prazosin, in the presence or absence of desipramine. These constrictions were not further reduced by benextramine (1-10 microM), but were abolished by dihydroergotamine (1-10 microM). Constrictions induced by ATP (0.1-1 mM) were not affected by dihydroergotamine. 5. 5-Hydroxytryptamine (3-100 microM) had a variable effect on 3 degree and 4 degree arteries including: concentration dependent constrictions (n = 3); small constrictions at some concentrations, and dilatations or no change in diameter at other concentrations (n = 6); concentration-dependent dilatations only (n = 3). The 5-HT2 receptor antagonist, ketanserin (0.1-0.3 micro M), did not affect NA-induced constrictions.6. In 16 arterial segments ranging from 3 degree arteries to 60 arterioles, there was a significant correlation between the presence of neuropeptide Y-immunoreactive (NPY-IR) sympathetic axons and the degree of blockade of NA-induced constrictions by yohimbine, prazosin and dihydroergotamine.7. These results demonstrate marked differences in the postsynaptic adrenoceptors mediating vasoconstriction to a bolus of NA applied briefly to the adventitial surface of different segments of the cutaneous vasculature of the guinea-pig ear. Furthermore, the presence or absence of adrenoceptors sensitive to blockade by yohimbine or prazosin is related to the proportion of sympathetic axons innervating each vascular segment which contain NPY-IR.