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 α,β‐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.     

The role of ATP in non-adrenergic sympathetic vascular control of the nasal mucosa in anaesthetized cats and dogs.

1. In anaesthetized cats and dogs, local intra-arterial injection of noradrenaline and alpha, beta-methylene adenosine 5'-triphosphate (mATP) reduced both nasal arterial blood flow and nasal mucosal volume (a measure of capacitance vessel function). The responses to mATP were not modified by pretreatment with the adrenoceptor antagonists phentolamine and propranolol or the purinoceptor antagonist suramin. The vascular effects of noradrenaline were not altered by suramin, but were virtually abolished by adrenoceptor antagonists. 2. After adrenoceptor blockade, frequency-dependent reductions in nasal arterial blood flow with sympathetic nerve stimulation were reduced by 25 and 39% in cats and dogs, respectively; whereas the volume response was reduced by 56% in cats and 54% in dogs. The remaining non-adrenergic sympathetic nerve-evoked vascular responses were not influenced by suramin. 3. During desensitization to mATP induced by local intra-arterial infusion for 5 min, the remaining non-adrenergic nasal blood flow and volume responses to sympathetic nerve stimulation were reduced in the dog but not in the cat. 4. It is suggested that both adrenergic and non-adrenergic mechanisms are involved in the sympathetic control of the nasal mucosa vascular bed of both species. Since desensitization to mATP markedly reduces the remaining non-adrenergic nasal vasoconstriction evoked by sympathetic nerve stimulation in the dog, ATP is a possible sympathetic mediator in the nasal vascular bed in this species.     

Influence of impulse pattern on noradrenaline release from sympathetic nerves in cerebral and some peripheral vessels

Hardebo , J. E. 1992. Influence of impulse pattern on noradrenaline release from sympathetic nerves in cerebral and some peripheral vessels. Acta Physiol Scand 144 , 333–339. Received 18 June, accepted 7 November 1991. ISSN 0001–6772. Departments of Medical Cell research and Neurology, University of Lund, Lund, Sweden. Earlier findings suggested that the physiological firing rate in sympathetic nerves did not exceed 10 Hz. Later recordings have revealed that this is a net value: impulses are usually not continuous but occur in bursts of high frequencies separated by quiet periods. The effects of continuous and burst-like neurogenic activation in various isolated blood vessels were compared. In the first part of the study changes in vascular tone were registered. Electrical field stimulatory parameters were chosen to give tetrodotoxin-blockable, neurogenic responses. From dose-response experiments in rat caudal artery a net frequency of 6 Hz was chosen, for bursts usually designed as 30 Hz during 0.2 s followed by a quiet period of 0.8 s. In the rabbit ear artery the neurogenic contraction was enhanced by a mean of near 50% during stimulation with bursts. Now appeared a minor phentolamine-resistant portion, which was nor due to release of NPY, 5–HT, histamine or ATP. Also in the rat caudal artery and monkey pial artery a significant enhancement of contraction was seen during burst stimulation, whereas in the rabbit facial vein no significant difference in dilatation through β-receptor activation was obtained. In vessels that do not normally respond with purely neurogenic contractions/dilatations during continuous stimulation, like pial arteries from the rat and rabbit, not even bursts revealed a neurogenic response. In a second series of experiments the influence of continuous and burst-like nerve activation on the release of [¹⁴C]noradrenaline was studied in monkey, rabbit and rat pial arteries, rat caudal artery and rabbit central ear artery and facial vein. In all vessels except rabbit pial artery a significantly higher release was obtained with burst-like compared to continuous stimulation.     

Endothelium-independent potentiation by neuropeptide Y of vasoconstrictor responses in isolated arteries from rat and rabbit

An endothelium-dependent action of neuropeptide Y (NPY) has been implicated in studies on various vascular beds. In the present study, the requirement of an intact endothelium for NPY-evoked potentiation of the response to sympathetic nerve stimulation was determined in the small mesenteric arteries of the rat and in the central ear artery of the rabbit. Further, NPY-mediated inhibition of relaxing influences was determined in small mesenteric arteries of the rat. Vascular segments were mounted in a double myograph, where one of the two suspended vessels was denuded of endothelium by gently rubbing the intimal surface. Removal of endothelium was verified by en-face silver staining. In both species, the response to bursts of transmural field stimulation eliciting 10% of maximal contraction was potentiated 2-4 times in the presence of 10 nM NPY, whether the endothelium was present or not. In small mesenteric arteries precontracted with noradrenaline, addition of acetylcholine (I microM) caused relaxation only in vessels with an intact endothelium. Subsequent addition of 10 nM NPY enhanced vasoconstriction in both intact and endothelium-denuded vessels. The endothelium-independent beta-adrenergic agonist isoprenaline (I microM) relaxed both intact and denuded small mesenteric arteries, and in both further addition of 10 nM NPY increased the contraction to about the same extent. The results demonstrate that NPY potentiates the responses to sympathetic field stimulation in small mesenteric arteries from the rat and in central ear artery from rabbit whether the endothelium is present or not. NPY inhibits both endothelium-dependent and -independent relaxations in small mesenteric arteries from rat.     

Influence of impulse pattern on noradrenaline release from sympathetic nerves in cerebral and some peripheral vessels.

Earlier findings suggested that the physiological firing rate in sympathetic nerves did not exceed 10 Hz. Later recordings have revealed that this is a net value: impulses are usually not continuous but occur in bursts of high frequencies separated by quiet periods. The effects of continuous and burst-like neurogenic activation in various isolated blood vessels were compared. In the first part of the study changes in vascular tone were registered. Electrical field stimulatory parameters were chosen to give tetrodotoxin-blockable, neurogenic responses. From dose-response experiments in rat caudal artery a net frequency of 6 Hz was chosen, for bursts usually designed as 30 Hz during 0.2 s followed by a quiet period of 0.8 s. In the rabbit ear artery the neurogenic contraction was enhanced by a mean of near 50% during stimulation with bursts. Now appeared a minor phentolamine-resistant portion, which was not due to release of NPY, 5-HT, histamine or ATP. Also in the rat caudal artery and monkey pial artery a significant enhancement of contraction was seen during burst simulation, whereas in the rabbit facial vein no significant difference in dilatation through beta-receptor activation was obtained. In vessels that do not normally respond with purely neurogenic contractions/dilatations during continuous stimulation, like pial arteries from the rat and rabbit, not even bursts revealed a neurogenic response. In a second series of experiments the influence of continuous and burst-like nerve activation on the release of [14C]noradrenaline was studied in monkey, rabbit and rat pial arteries, rat caudal artery and rabbit central ear artery and facial vein.(ABSTRACT TRUNCATED AT 250 WORDS)     

Neurotransmitters and pre- and post-junctional receptors involved in the vasoconstrictor response to sympathetic nerve stimulation in rat tail artery

The study was prompted by the need to re-evaluate, in view of the complexity of the evidence in the literature, the relative roles of different sympathetic transmitters and receptors in the contractile response of the tail artery of adult normotensive rats to electrical field stimulation. By the pharmacological approach employed, noradrenaline and adenosine 5'-triphosphate appeared to contribute to this response; the possible roles of other putative transmitters such as neuropeptide Y could not be examined due to lack of specific antagonists. Noradrenaline, clearly the main mediator, exerted both excitatory and inhibitory effects, acting in part via different receptors depending on the stimulus parameters. Thus, yohimbine and prazosin (alpha 2- and alpha 1-adrenoceptor antagonists, respectively) were about equally effective as inhibitors of the noradrenaline-mediated contractile response to stimulation with short trains and/or at low frequency, but the response caused by long trains and/or high-frequency stimulation was much more strongly inhibited by prazosin than by yohimbine. As expected, yohimbine enhanced the [3H]noradrenaline overflow response to long but not to short stimulus trains, presumably because in the latter case the noradrenaline concentration in the relevant biophase was too low to activate the pre-junctional alpha 2-adrenoceptors. Finally, propranolol, an unselective beta-adrenoceptor antagonist, enhanced the neurogenic contraction, indicating that noradrenaline restricts this response by effects via post-junctional beta-adrenoceptors. Adenosine triphosphate appeared to exert dual, excitatory as well as inhibitory, post-junctional effects. Thus, the P2x-purinoceptor desensitizing agent, alpha, beta-methylene adenosine triphosphate, abolished the initial phase, but enhanced the amplitude of the neurogenic contraction, without affecting the nerve stimulation-induced overflow of [3H]noradrenaline. The results indicate that noradrenaline and adenosine triphosphate, the main mediators of the neurogenic contraction of this preparation, act in a more complex fashion than earlier thought; they argue against a significant direct contribution by other putative transmitters but do not exclude that such agents may act indirectly as modulators of this response.     

Transmitter characteristics of small mesenteric arteries from the rat

We have studied the neurogenic response of small mesenteric arteries from the rat to evaluate the involvement of possible co-transmitters under various modes of stimulation. Segments of small branches of the mesenteric artery were mounted in a myograph and the intramural nerves were activated with transmural electrical stimulation. A single stimulation of the nerves caused a contraction that was reduced by only 20% in the presence of adrenergic blocking agents (prazosin or phenoxybenzamine), whereas the steady-state response to continuous nerve stimulation of high frequency was reduced by 90-95%. In contrast, all responses to applied noradrenaline in doses up to at least 1 mM were eliminated by phenoxybenzamine treatment. The stable ATP analogue, alpha,beta-methylene ATP, reduced the response to a single nerve stimulation by 70%, but reduced the contraction caused by continuous high-frequency nerve stimulation by only 10%. None of these agents affected the response to applied neuropeptide Y (NPY). The response of relaxed vessels to nerve stimulation was totally blocked by the combination of an adrenoceptor-blocking agent and alpha,beta-methylene ATP, although even in this situation a further neurogenic response could be revealed in vessels precontracted with vasopressin. Responses to either single stimuli or brief burst stimulations were potentiated after high-frequency stimulation. Both the adrenergic and non-adrenergic components were enhanced to roughly the same extent. Also the potentiated response was eliminated by the combined application of prazosin and alpha,beta-methylene ATP. The non-adrenergic transmitter in the sympathetic nerves of small arteries thus appears to be the dominant transmitter during low-frequency nerve stimulation, causing rapid but phasic activation. Noradrenaline is the most important transmitter for higher frequencies, exerting slower but sustained contractions. The post-stimulatory potentiation affects both the adrenergic and the non-adrenergic part of the neurogenic response.     

The use of fluorescent nuclear dyes and laser scanning confocal microscopy to study the cellular aspects of arterial remodelling in human subjects with critical limb ischaemia

Resistance arteries isolated from patients with critical limb ischaemia (CLI), a hypotensive/hypoperfusion state of the lower leg, have been shown to undergo morphological changes opposite to those observed in hypertension, that is, decreased wall thickness, reduced cross-sectional area and a decreased wall : lumen ratio. The aim of this present study was to use laser scanning confocal microscopy (LSCM) to study intact resistance arteries isolated from patients with CLI, specifically to identify the cellular aspects of the morphological changes identified in ischaemic subcutaneous and skeletal muscle resistance vessels. Using LSCM, a significant reduction in adventitial and medial thickness, cross-sectional area and wall : lumen ratio was confirmed in resistance arteries from both distal ischaemic subcutaneous and skeletal muscle vascular beds when compared with corresponding arteries from the proximal non-ischaemic sites. The cellular composition of the adventitial, medial and intimal layers of these distal ischaemic arteries was significantly different compared with proximal non-ischaemic arteries. These differences in the distal arteries were characterised by hypoplasia in the adventitial and medial layers of the arterial wall and hypertrophy in the intimal layer. The differences observed in both distal ischaemic vascular beds (subcutaneous and skeletal muscle) were similar.     

Loci of Neurogenic and Metabolic Effects on Precapillary Vessels of Skeletal Muscle

Folkow , B., R. R. Sonnenschein , and D. L. Wright , Loci of neurogenic and metabolic effects on precapillary vessels of skeletal muscle. Acta physiol. scand. 1971. 81. 459–471. By cannulation of a branch of the proximally clamped sural artery of the anesthetized cat, distal arterial pressure (DAP) in the gastrocnemius muscle was recorded. Measurement of blood flow, femoral arterial pressure and DAP allowed calculation of total resistance (R_t) and its partition into a distal component (R_d) which included precapillary sphincters and the smaller arterioles, and a proximal component (R_P) which included the larger arteries. With sympathetic vasoconstriction, the initial increase in R_t was accounted for mainly by constriction of the distal vessels which then tended to relax; progressive constriction of proximal vessels accounted for most of the elevated R_t during the steady state; subsequent reactive hyperemia mainly involved distal vessels. R_t was less affected by sympathetic stimulation during exercise than when the muscle was at rest; constriction of distal vessels was more markedly reduced than that of proximal vessels. Ascending dilatation was evident during exercise. Sympathetic cholinergic vasodilatation mainly involved vessels more proximal to those which were dilated early in exercise. The findings are compatible with the concept that capillary flow distribution, as a function of terminal arterioles and precapillary sphincters, is adjusted by local factors towards an optimum for the prevailing metabolic level of the tissue.     

Adrenergic innervation and neurogenic response in large and small arteries and veins from the rat

A combined morphological and physiological analysis of the properties of the adrenergic vasoconstrictor innervation was carried out in the splanchnic vasculature in the rat. Three generations of vessels were studied: (1) the abdominal aorta and the caval vein; (2) the superior mesenteric artery and vein; (3) 200 microns branches of the superior mesenteric artery and their parallelling veins. The adrenergic innervation was visualized by the Hillarp-Falck fluorescence technique, and by the immunohistochemical localization of immunoreactivity to tyrosine hydroxylase and dopamine-beta-hydroxylase. Determination of responses to applied noradrenaline, to transmural nerve stimulation and to direct activation of the muscle was made on ring preparations mounted in a myograph. All vessels were found to be innervated at the adventitio-medial border by noradrenergic nerves with varying density; the small arteries had the highest nerve density, the abdominal aorta was least innervated. When related to the maximal isometric response to applied noradrenaline, the maximal neurogenic response parallelled the density of the adrenergic innervation. Variations in frequency for half-maximal response among the vessels could not, however, be ascribed to innervation properties only. The constriction under isotonic conditions amounted to 20% of the initial circumference in the aorta, and to 30% in the small arteries and veins. The relation between response to applied noradrenaline and to nerve stimulation was similar for isometric and isotonic responses. In the smaller veins, a beta-receptor-mediated decline in the response to applied noradrenaline was seen. This decline was much less pronounced in neurogenic responses. The results indicate a gradation from proximal to peripheral arteries towards denser innervation and greater neurogenic responses. On the venous side only minor differences were found.     

Different nerve responses in consecutive sections of the arterial system

Isometric responses to nerve stimulation and to applied noradrenaline were compared in isolated ring preparations of three consecutive types of rat arteries, viz. the aorta, the superior mesenteric artery and 200-micron branches of the latter. Intramural nerves were activated by graded electrical field stimulation; obtained responses were blocked by tetrodotoxin, phentolamine or prazosin. Also direct muscle activation could be accomplished, using impulses of long duration. In the aorta responses to nerve stimulation were sluggish, could reach only 30-40% of the maximal noradrenaline response and single impulses were ineffective. In the small resistance arteries, neurogenic responses matched the maximal noradrenaline responses and distinct, rapid contractions occurred to single nerve impulses. The superior mesenteric artery was intermediate in these respects. By contrast, on direct muscle activation all vessels responded to single impulses with rapid contractions that differed little in velocity. Pharmacological inhibition of transmitter reuptake increased noradrenaline sensitivity most in the small arteries, but nerve responses were most enhanced in the superior mesenteric artery. These regional differences, probably reflecting decreasing neuromuscular distance with decreasing vessel size, imply that generalizations concerning the relation between responses to nerve stimulation and to exogenous noradrenaline cannot be made from one vascular model only.     

Kinetics of ATP- and noradrenaline-mediated sympathetic neuromuscular transmission in rat tail artery

Electrophysiological, electrochemical and mechanical recordings were employed to study the kinetics of the release and clearance of adenosine 5′-triphosphate (ATP) and noradrenaline (NA) as sympathetic co-transmitters and of the neurogenic and non-neurogenic contractions in rat isolated tail artery. The life-time of ATP and NA released by a single pulse or 10 pulses at 50 Hz was brief (< 100 ms, or < 3 s, respectively); the neurogenic contractile responses occurred largely after the transmitters had been removed from the extracellular space. The ATP-induced neurogenic contractile responses to a single pulse or 10 pulses at 50 Hz were similar in time-course to the responses to direct muscle stimulation at low voltage; both seemed to be caused by activation of nifedipine-sensitive voltage-gated L-type Ca²⁺ channels. The a,- and a₂-adrenoceptor-mediated components of the NA-induced neurogenic contractile response to 10 pulses at 50 Hz were more delayed and prolonged and determined by properties of the post-receptor mechanisms. The per pulse release of both ATP and NA faded rapidly during long high-frequency trains. So did the ATP level at the receptors and the ATP-induced neurogenic contraction. The NA levels and the contractile responses induced via a,- and ^-adrenoceptors were much better maintained during ongoing stimulation at 20 Hz but relaxed rapidly afterwards, suggesting that nerve activity suppressed, and ce'ssation of nerve activity reactivated NA clearance.     

An examination of nerve-mediated, hyoscine-resistant excitation of the guinea-pig colon

1. The mechanical and electrical activity of the smooth muscle of the distal colon of the guinea-pig has been recorded in experiments designed to determine the nature of the nerve-mediated excitation of the muscle.2. The spontaneous contractions of the colon, normally observed in vitro, were similarly antagonized by hyoscine or tetrodotoxin. However, neither drug caused complete cessation of the spontaneous activity.3. The contractile responses of the colon to repetitive stimulation of intramural nerve fibres were of two types, a primary contraction which occurred within 1 sec of the beginning of stimulation and a secondary contraction which occurred after stimulation. The primary contraction was blocked by hyoscine, usually revealing an inhibitory response to stimulation, but the secondary contraction persisted. Similar primary contractions were observed in response to stimulation of the pelvic and sometimes of the periarterial nerves.4. An initial relaxation during, and a secondary contraction after, stimulation were sometimes obtained when the periarterial nerves were stimulated. Propranolol completely blocked both the initial relaxation and the secondary contraction in response to sympathetic stimulation but did not affect either the relaxation or the secondary contraction in response to transmural stimulation.5. The direct effects of noradrenaline and adenosine triphosphate on the mechanical activity of the colon were studied. Both drugs caused a relaxation of the colon. Washout of either drug after a short exposure was followed by a period of increased activity. Especially in preparations of low tone, the initial relaxation and the secondary contraction in response to transmural stimulation could be mimicked by the application and washout of either noradrenaline or adenosine triphosphate.6. Two types of potential change were evoked in the muscle cells of the colon, either separately or in combination, when the intramural nerve fibres were stimulated: excitatory junction potentials (EJPs), which were blocked by hyoscine, and inhibitory junction potentials (IJPs). When the membrane potential recovered following an IJP, it was common to observe action potentials in muscle cells that were initially quiescent. A similar secondary firing of action potentials was initiated by hyperpolarizing the muscle cells with anodal current pulses in the presence of tetrodotoxin in sufficient concentration to block nerve-mediated responses.7. The results reported in this paper lead to the conclusion that the muscle cells of the distal colon of the guinea-pig are influenced by three sets of nerves: cholinergic excitatory, adrenergic inhibitory and intrinsic inhibitory fibres releasing a non-adrenergic transmitter substance. The secondary non-cholinergic excitation arises from a non-specific reaction of the muscle cells to a preceding inhibition.     

Sympathetic vascular control of the pig nasal mucosa (III): co-release of noradrenaline and neuropeptide Y

The overflows of noradrenaline (NA) and neuropeptide Y like immunoreactivity (NPYLI) and vascular responses upon sympathetic nerve stimulation were analysed in the nasal mucosa of pentobarbital anaesthetized pigs. In controls, a frequency-dependent increase in NA overflow was observed whereas detectable release of NPY-LI occurred only at 6.9 Hz. Parallel decreases in blood flow in the sphenopalatine artery and vein and in nasal mucosa volume (reflecting blood volume in the venous sinusoids) were observed. The laser Doppler flowmeter signal (reflecting superficial blood flow) increased upon low and decreased upon high frequency stimulation. Twenty-four hours after reserpine pretreatment and preganglionic decentralization, the NA overflow was abolished while a frequency-dependent release of NPY-LI occurred. Forty, 60 and 80% of the vasoconstrictor responses then remained upon stimulation with a single impulse, 0.59 and 6.9 Hz, respectively. Both the vasoconstriction and NPY-LI overflow, however, were subjected to fatigue upon repeated stimulation. In reserpinized animals release of NPY-LI and vasoconstrictor responses were larger upon stimulation with irregular bursts at 0.59 Hz compared to effects seen at stimulation with continuous impulses. Pre-treatment with the a-adrenoceptor antagonist phenoxybenzamine or the monoamine reuptake inhibitor, desipramine, enhanced NA overflow by 2–3 and 1.5 times at 0.59 and 6.9 Hz, respectively. Phenoxybenzamine significantly reduced the nerve-evoked vascular responses while the release of NPY-LI at 6.9 Hz was increased. Desipramine increased the functional responses but reduced the NPY-LI overflow. During tachyphylaxis to the vasoconstrictor effects of the stable adenosine 5′-triphosphate (ATP) analogue α-β-methylene ATP (mATP) in controls, the vasoconstrictor responses as well as the NA and NPY-LI overflow to nerve stimulation were unmodified. In reserpinized animals, however, the vascular responses and the overflow of NPY-LI were reduced after mATP tachyphylaxis. These data show that both NA and NPY are released upon sympathetic nerve stimulation in the nasal mucosa in vivo and this release seems to be regulated via prejunctional a-adrenoceptors. The lack of effect of mATP tachyphylaxis under control conditions makes it less likely that ATP serves as a major mediator of the large nonadrenergic vasoconstrictor component.     

Sympathetic vascular control of the pig nasal mucosa (2): Reserpine-resistant, non-adrenergic nervous responses in relation to neuropeptide Y and ATP

The possible occurrence of non-adrenergic mechanisms in the sympathetic vascular control of the nasal mucosa was studied in vivo using reserpine-treated pigs (1 mg kg-1, i.v., 24 h earlier) in combination with pharmacological blockade of alpha-adrenoceptors by local phenoxybenzamine (1 mg kg-1, i.a.) infusion. The nasal mucosal depletion (99%) of the content of noradrenaline (NA) in reserpinized animals was not influenced by preganglionic denervation while the depletion (44%) of neuropeptide Y (NPY) was prevented. Upon stimulation with single shocks, 25% of the arterial blood flow reduction and 47% of the nasal mucosal volume reduction (reflecting contraction of venous sinusoids) were still present after reserpine as compared with controls. In reserpinized animals, the vascular responses were slow developing and long-lasting, and about 60% remained at 0.59 Hz and more than 80% at 6.9 Hz. The vascular effects after reserpine were, however, subjected to fatigue, which may explain why phenoxybenzamine treatment still reduced the functional effects in the absence of NA. Local intra-arterial injections of NA, NPY and the metabolically stable adenosine-5'-triphosphate analogue alpha, beta-methylene ATP (mATP) caused reduction in both arterial blood flow and nasal mucosal volume. The C-terminal fragment of NPY (NPY 13-36) also induced nasal vasoconstriction although with a fivefold lower potency than NPY 1-36. Adenosine-5'-triphosphate caused a biphasic vascular effect with vasodilatatory actions at low doses and a short-lasting vasoconstriction followed by vasodilatation at very high doses (100-fold higher than the threshold response to mATP). In contrast to the response to NA, the long-lasting vascular effects of NPY and mATP were resistant to phenoxybenzamine treatment. In conclusion, although NA is likely to mediate most of the sympathetic vascular responses to low-frequency stimulation in the pig nasal mucosa, a large resistance and capacitance vessel component upon high-frequency stimulation seems to be non-adrenergic and mimicked by NPY rather than ATP.     

Integration of blood flow control to skeletal muscle: key role of feed arteries

Blood flow control reflects dynamic, integrated changes in the diameter of vessels that comprise resistance networks. Vasoconstriction and vasodilation can travel rapidly along the vessel wall via the conduction of electrical signals between endothelial and/or smooth muscle cells through gap junctions. Within the hamster cheek pouch, these conducted responses reflect complementary mechanisms for co-ordinating both increases and decreases in arteriolar diameter. In the hamster retractor muscle, vasodilation also conducts along arterioles and into feed arteries, yet vasoconstriction appears constrained to the site(s) of smooth muscle activation. Thus, mechanisms for co-ordinating vasomotor control in resistance networks can vary between tissues that differ in structure and function. The resistance vessels of the retractor (and other skeletal) muscle are richly innervated by sympathetic nerves, which are absent from the cheek pouch. Propagation along sympathetic nerves rapidly co-ordinates smooth muscle cell contraction throughout the resistance network by releasing noradrenaline along the innervation pathway. Passive extension of the retractor muscle activates periarteriolar sympathetic nerves. This activity propagates antidromically into feed arteries and may complement the central (autonomic) vasoconstrictor response to exercise. In a reciprocal manner, muscle contraction evokes arteriolar dilation that is conducted (i.e. 'ascends') into feed arteries and may thereby counteract sympathetic vasoconstriction. With feed arteries anatomically positioned to control blood flow into skeletal muscle, the integration of dilator and constrictor stimuli in these vessels is a key determinant of muscle blood flow during exercise.     

Vasomotor nerve control of isolated arteries and veins.

In order to compare neuro-effector function in different blood vessels, frequency-response relationships were determined for the following preparations: 1)Isolated rings of the proximal saphenous, distal saphenous and ear arteries, the parietal branch of the internal iliac vein and the small saphenous vein from the rabbit, 2) spiral strips of the rabbit pulmonary artery and 3) longitudinal preparations of the rat portal vein. In each rabbit tissue only one low (less than or equal to 4 Hz) and one high (larger than or equal to 8 Hz) transmural nerve stimulation frequency was applied until steady state responses were obtained and these were expressed as a percentage of a maximum response to exogenous noradrenaline (NA) applied in each experiment. The general shape of the frequency-response curves was similar, but differences in steepness and amplitude of the maximum neurogenic response relative to exogenous NA were found. The steepness of the frequency-response relationships of the veins tended to be greater than those of the arteries. It appears that factors such as close neuro-muscular contacts, presence of terminal nerve fibres within the media and the operation of mechanisms for myogenic propagation of activity contribute to the effectiveness of neurogenic vascular control as revealed by frequency-response curves. In vivo, geometrical factors can greatly augment the hemodynamic significance of the observed differences.     

Neuropeptide Y may mediate effects of sympathetic nerve stimulations on colonic motility and blood flow in the cat

The present study investigated sympathetic mechanisms involved in the regulation of colonic motility and blood flow in the cat. Infusion of neuropeptide Y (NPY) close i.a. produced an inhibition of colonic motility and a vasoconstriction of long duration but no post-infusion vasodilatation. In contrast to NPY, porcine pancreatic polypeptide did not evoke any vascular or motility response. On a molar basis, NPY was 25 times more potent than noradrenaline in producing 50% reduction of the colonic blood flow. These vascular and motility effects of NPY were resistant to guanethidine, phentolamine, phenoxybenzamine and propranolol. Thus, the action of NPY on vascular and colonic smooth muscle did not seem to be mediated via adrenergic receptors. Noradrenaline administered close i.a. produced inhibition of colonic motility, and vasoconstriction followed by a rapid vasodilatation. These effects were completely blocked by combined alpha- and beta-adrenoceptor blockade. Electrical stimulation of the splanchnic and lumbar colonic nerves produced an overall inhibition of colonic motility, and vasoconstriction of the proximal and distal colon, respectively, with a rapid post-stimulatory vasodilatation. After combined alpha- and beta-adrenoceptor blockade the inhibitory effect of the nerve stimulations on colonic motility partly remained together with a marked vasoconstriction, which was most pronounced upon lumbar colonic nerve stimulation. All vascular effects of sympathetic nerve stimulation were eradicated by guanethidine, which also abolished the inhibitory motility response to splanchnic nerve stimulation. However, lumbar colonic nerve stimulation elicited a colonic contraction, possibly due to stimulation of afferent C-fibres. The present findings indicate the existence of a sympathetic nonadrenergic neuronal mechanism mediating vasoconstriction and inhibition of colonic motility in the cat. Thus, NPY may be released from noradrenergic neurons to act on colonic smooth muscle and vessels.     

Vasomotor Nerve Control of Isolated Arteries and Veins

In order to compare neuro-effector function in different blood vessels, frequency-response relationships were determined for the following preparations: 1) Isolated rings of the proximal saphenous, distal saphenous and ear arteries, the parietal branch of the internal iliac vein and the small saphenous vein from the rabbit, 2) spiral strips of the rabbit pulmonary artery and 3) longitudinal preparations of the rat portal vein. In each rabbit tissue only one low (≤4 Hz) and one high (≥8 Hz) transmural nerve stimulation frequency was applied until steady state responses were obtained and these were expressed as a percentage of a maximum response to exogenous noradrenaline (NA) applied in each experiment. The general shape of the frequency-response curves was similar, but differences in steepness and amplitude of the maximum neurogenic response relative to exogenous NA were found. The steepness of the frequency-response relationships of the veins tended to be greater than those of the arteries. It appears that factors such as close neuro-muscular contacts, presence of terminal nerve fibres within the media and the operation of mechanisms for myogenic propagation of activity contribute to the effectiveness of neurogenic vascular control as revealed by frequency-response curves. In vivo, geometrical factors can greatly augment the hemodynamic significance of the observed differences.     

Pancreatic polypeptide family (APP,BPP, NPY and PYY) in relation to sympathetic vasoconstriction resistant to α-adrenoceptor blockade

Electrical stimulation of the cat cervical sympathetic trunk caused submandibular salivary secretion and vasoconstriction simultaneously with a contraction of the nictitating membrane. Following α- and β-adrenoceptor blockade by phentolamine or phenoxybenzamine combined with propranolol, the salivary response and the nictitating membrane contraction upon sympathetic stimulation were almost abolished. A considerable vasoconstrictor response (up to 40% of control) however still remained in the submandibular gland. This yasoconstriction, which persisted after α-adrenoceptor blockade, was rather slow in onset and had a long duration without any poststimulatory hyperemia. Local intra-arterial infusions of noradrenaline caused submandibular vasoconstriction, salivary secretion and nictitating membrane contraction. The blood flow response to exogenous noradrenaline did, however, not mimic the effects of sympathetic nerve stimulation with regard to vascular escape. Whereas the vascular escape after nerve stimulation was followed by a prolonged vasoconstriction with a gradual decline, the escape after noradrenaline infusions was accompanied by a normalization of blood flow. Local intra-arterial infusions of pancreatic polypeptide (PP)-related peptides caused a slowly developing vasoconstriction with a long duration in the submandibular gland, but no salivary secretion or contraction of the nictitating membrane. The relative molar potencies as vasoconstrictory agents were about PYY: 1, neuropeptide Y (NPY): 5, avian and bovine pancreatic polypeptid 100. The vasoconstrictor effects of PP-related peptides were resistant to α-adrenoceptor blockade and present also in sympathectomized animals, suggesting a direct action on vascular smooth muscle. Combined local infusions of noradrenaline and NPY caused a vascular response in the submandibular salivary gland which was similar to that seen upon sympathetic nerve stimulation. PYY and NPY caused increase in systemic arterial blood pressure upon systemic administration which indicates general vasoconstrictor actions. This effect was accompanied by a transient bradycardia which was due to inhibition of sympathetic tone, since it was absent in animals treated with propranolol. In conclusion, the present findings illustrate the differential sensitivity to α-adrenoceptor antagonists of the submandibular vasoconstriction and salivation as well as smooth muscle contraction of the nictitating membrane induced by sympathetic nerve stimulation. This remaining vasoconstriction may be explained by release of a nonadrenergic, PP-related transmitter such as NPY which may be present together with noradrenaline in the vascular nerves. Release of an additional vasoconstrictory factor may also account for the finding that infusions of noradrenaline do not mimic the vascular effects of sympathetic nerve stimulation in vivo.