Frequency-dependence of 3H-noradrenaline secretion from human vasoconstrictor nerves: modification by factors interfering with alpha-or beta-adrenoceptor or prostaglandin E2 mediated control

Neural Control of Vascular Function in Skeletal Muscle

The sympathetic nervous system represents a fundamental homeostatic system that exerts considerable control over blood pressure and the distribution of blood flow. This process has been referred to as neurovascular control. Overall, the concept of neurovascular control includes the following elements: efferent postganglionic sympathetic nerve activity, neurotransmitter release, and the end organ response. Each of these elements reflects multiple levels of control that, in turn, affect complex patterns of change in vascular contractile state. Primarily, this review discusses several of these control layers that combine to produce the integrative physiology of reflex vascular control observed in skeletal muscle. Beginning with three reflexes that provide somewhat dissimilar vascular patterns of response despite similar changes in efferent sympathetic nerve activity, namely, the baroreflex, chemoreflex, and muscle metaboreflex, the article discusses the anatomical and physiological bases of postganglionic sympathetic discharge patterns and recruitment, neurotransmitter release and management, and details of regional variations of receptor density and responses within the microvascular bed. Challenges are addressed regarding the fundamentals of measurement and how conclusions from one response or vascular segment should not be used as an indication of neurovascular control as a generalized physiological dogma. Whereas the bulk of the article focuses on the vasoconstrictor function of sympathetic neurovascular integration, attention is also given to the issues of sympathetic vasodilation as well as the impact of chronic changes in sympathetic activation and innervation on vascular health. © 2016 American Physiological Society.

Characterization of tritiated noradrenaline release from the rat preoptic area with microdialysis in vivo

Present techniques are unable to provide a sensitive and accurate index of noradrenergic activity in the rat preoptic area. In this study, we have examined the brainstem A1 noradrenergic input to the preoptic area using a new technique whereby [3H]noradrenaline is preloaded into the preoptic area and release of radioactivity from this region is measured subsequently using microdialysis in vivo. Electrical stimulation of the ipsilateral A1 area for 20 min at 5, 10, and 15 Hz evoked significant increases in dialysate radioactivity that were repeatable and frequency-dependent. After removal of calcium from the perfusion medium, basal release of radioactivity was markedly reduced and the effect of A1 stimulation abolished. Changing to a 100 mM K+ medium evoked an increase in the release of radioactivity that was sixfold greater than that seen after A1 stimulation. Separation of the dialysate with HPLC showed that 33% of the increase in measured radioactivity after A1 stimulation was directly attributable to [3H]noradrenaline and the remainder to the metabolites vanillylmandelic acid, 3,4-dihydroxymandelic acid, and 3,4-dihydroxyphenylglycol. In contrast, the increase in radioactivity after K+ depolarization was due almost completely to [3H]noradrenaline. Addition of 10 microM clonidine to the perfusion medium markedly reduced basal release of radioactivity, but had no effect on evoked release following A1 stimulation. Conversely, perfusion with 10 microM yohimbine had no effect on basal release, but significantly increased evoked release after A1 stimulation. These results now provide a characterization of noradrenergic activity in the preoptic area and indicate the importance of the A1 noradrenergic input to this region. The technique of measuring radioactivity with microdialysis after preloading with [3H]noradrenaline provides a relatively simple, sensitive index of noradrenergic activity in vivo with good temporal resolution.

Release of [3H]-noradrenaline from the sympathetic nerves to bovine mesenteric lymphatic vessels and its modification by alpha-agonists and antagonists

1. Isolated segments of bovine mesenteric lymphatic vessels were loaded with [3H]-noradrenaline and its efflux in response to field stimulation examined. Vessels were attached to an isometric force transducer for the simultaneous recording of mechanical activity. 2. Field stimulation at 1, 4 and 8 Hz (0.3 ms pulses, 1 min train) increased spontaneous contraction rate and evoked 3H release up to a maximum of 4.5% of total tissue 3H at 8 Hz. Output per pulse was maximal at 4 Hz. 3. Tetrodotoxin (3 x 10(-6) M) blocked the release of 3H in response to field stimulation although the drug did not attenuate release evoked by high K+ (65 mM) solution. Field-evoked release of 3H was also absent in Ca2+ -free solution containing EGTA (1 mM). 4. When vessels were preincubated with labelled transmitter plus cocaine (5 x 10(-5) M) evoked release of 3H was absent. After preloading with [3H]-noradrenaline, cocaine (10(-6) M) potentiated both the mechanical response to field stimulation and evoked 3H release. 5. The relatively non selective alpha-adrenoceptor antagonist phentolamine (3 x 10(-6) M) and the alpha 2-antagonists yohimbine (10(-8) M) and rauwolscine (10(-6) M) significantly increased evoked 3H release at both of the frequencies examined (1 and 4 Hz). In contrast, the selective alpha 1-antagonist prazosin (10(-6) M) failed to alter 3H release to 4 Hz stimulation although release at 1 Hz was potentiated in the presence of the drug. 6. The postsynaptic excitatory response to field stimulation remained in the presence of prazosin (10(-6) M), but was converted to an inhibitory effect in the presence of phentolamine (3 x 10(-6) M), yohimbine (10(-6) M) or rauwolscine (10(-6) M). 7. Evoked 3H efflux was significantly reduced by clonidine (10(-6) M), xylazine (10(-6) M) and exogenous noradrenaline (5 x 10(-7) M), although phenylephrine (10(-6) M) reduced release only at the lower of the two frequencies tested (1 Hz). 8. These findings suggest that release of 3H by field stimulation reflects endogenous transmitter release and that this is subject to autoinhibition via feedback onto inhibitory prejunctional alpha 2-adrenoceptors. The postjunctional excitatory response is mediated via postjunctional alpha 2-adrenoceptors.

Interaction of nonsteroidal anti-inflammatory drugs with antihypertensive and diuretic agents. Control of vascular reactivity by endogenous prostanoids

Indomethacin and some other nonsteroidal anti-inflammatory drugs partially antagonize the blood pressure lowering effect of drugs used to treat hypertension. They can also produce a mild elevation of blood pressure in normotensive individuals. The elevated arterial pressure caused by these agents is associated with increases in the vascular resistance of mainly the renal and splanchnic beds. This may be due to direct inhibition of the synthesis of vasodilator prostanoids, or it may be due to indirect potentiation of the action of the sympathetic nervous system or of angiotensin II. Nonsteroidal anti-inflammatory drugs also cause renal retention of sodium and this probably contributes to their hypertensive effects. In humans, the sodium retention may involve increased reabsorption in the proximal tubule. Although a direct tubular action is possible, these drugs may change proximal sodium reabsorption by their vascular effects. However, the exact mechanism is not understood. These interactions are clinically significant and may complicate the treatment of common diseases.

Interaction between prejunctional alpha 2-receptors and neuronal transmitter reuptake in small mesenteric arteries from the rat

The role of the prejunctional alpha 2-receptors for the response to vaso-constrictor nerve stimulation has been examined before and after inhibition of neuronal transmitter reuptake in mesenteric resistance vessels from the rat. Small arteries (diameter about 200 micron) feeding the jejunum were mounted in a myograph for recording of their isometric wall tension during transmural field stimulation of the intramural nerves. Blockade of prejunctional alpha 2-receptors with 0.01 microM idazoxan (RX 781094) caused a marginal potentiation of the neurogenic response when neuronal reuptake was left intact. Also, inhibition of reuptake alone with 3 microM cocaine had little effect on the response. However, when both alpha 2-receptors and reuptake had been inhibited, a strong enhancement of the neurogenic vasoconstriction was observed. Similar findings were made when yohimbine and LU 3-010 instead were used for alpha 2-blockade and reuptake inhibition, respectively. The results thus indicate that in these resistance arteries the effector response is normally influenced by the combined activity of alpha 2-receptors and uptake, and that failure of one mechanism increases the activity of the other, so as to maintain a largely constant effector response.

Relationship between the overflow of endogenous and radiolabelled noradrenaline from canine blood perfused gracilis muscle

The effects of sympathetic nerve stimulation (SNS) on the overflow of endogenous noradrenaline (NA) and on vasoconstrictor responses were studied in blood perfused canine gracilis muscle in situ. A conventional tracer technique with 3H-labelled NA (3H-NA) was used in parallel. At rest there was a net extraction of endogenous NA and adrenaline across the tissue. The SNS evoked overflow of endogenous NA was frequency-dependent and logarithmically correlated to the vasoconstrictor responses. The neuronal uptake inhibitor desipramine doubled the SNS induced overflow of endogenous NA without enhancing the vasoconstrictor responses. A further fourfold increase in NA overflow was caused by a dose of the alpha-blocker phenoxybenzamine which reduced the vasoconstrictor responses by 50-75%. Less than 10% of the spontaneous 3H efflux was recovered as unmetabolized 3H-NA, whereas virtually all 3H overflow evoked by SNS was 3H-NA. The fractional release of NA or 3H-NA per nerve impulse increased with increasing frequencies of SNS under all conditions studied. Although there was a preferential release of the newly stored radiolabelled transmitter, results concerning endogenous NA and 3H-NA overflow were qualitatively similar, also under conditions with marked changes in transmitter overflow. Endogenous NA gave a more reproducible index of transmitter overflow than did 3H-NA and, in particular, total 3H. The overflow of endogenous NA closely reflects SNS evoked neuronal release of NA in blood perfused skeletal muscle and seems more suitable than conventional radiotracer techniques for studies of NA release under in vivo conditions.

Effect of epinephrine on norepinephrine release from rat kidney during sympathetic nerve stimulation

Experiments were performed to study presynaptic beta-adrenoceptor facilitation of sympathetic neurotransmitter release in the isolated perfused rat kidney and evaluate the effect of epinephrine on norepinephrine release during sympathetic nerve stimulation. The right kidney was isolated and perfused with Krebs-Ringer solution. Norepinephrine storage sites were labelled with [3H]norepinephrine. Increasing concentrations of isoproterenol and salbutamol when perfused through the kidney, caused an enhancement of the stimulus-induced release of [3H]norepinephrine at 0.5 and 2 Hz, with the maximum facilitatory effect being observed at 0.5 Hz. The effect of salbutamol on [3H]norepinephrine release was concentration-dependent and more pronounced than that of isoproterenol. While propranolol (10(-9)-10(-5)M) by itself did not cause any significant changes in the stimulus-induced release of [3H]norepinephrine, it antagonized the facilitatory action of salbutamol on [3H]norepinephrine release during periarterial nerve stimulation. When epinephrine (10(-10)-10(-7)M), was perfused through the kidney in the presence of cocaine, it caused a concentration-dependent inhibition of the stimulus-induced release of [3H]norepinephrine release elicited during periarterial nerve stimulation. However, when epinephrine was perfused in the presence of cocaine, phentolamine and corticosterone it caused a slight but significant increase in the stimulus-induced release of [3H]norepinephrine; the highest concentration (10(-7)M) still caused a decrease in the [3H]norepinephrine release. These results, while providing evidence for the existence of presynaptic facilitatory beta-adrenoceptors on renal sympathetic nerves, fail to support the hypothesis that these receptors have a physiological role in the regulation of sympathetic neurotransmitter release.

Modulation of the concentration of noradrenaline at the neuro-effector junction in human saphenous vein

We studied the importance of neuronal and extraneuronal uptake and of the pre-junctional alpha-adrenergic feed-back mechanism for the junctional noradrenaline concentration in the human saphenous vein. All major metabolites of the enzymatic breakdown of noradrenaline were detected in the overflow of superfused veins loaded with [3H]-noradrenaline. The efflux of 3,4-dihydroxyphenylglycol (DOPEG) was drastically reduced in preparations labelled after neuronal uptake blockade indicating its neuronal origin; the other metabolites are formed extraneuronally since they behaved distinctly differently from DOPEG under several experimental conditions. Extraneuronal uptake followed by enzymatic breakdown removes the same amount of noradrenaline from the biophase during nerve activity as that diffusing intact out of the tissue, whereas neuronal uptake appears only half as effective since the overflow of intact noradrenaline increases by only 48% in the presence of desmethylimipramine (DMI). However, in preparations mounted for isometric tension recording, neuronal uptake blockade potentiated contractions to alpha-adrenergic activation, emphasizing the functional importance of the neuronal disposition mechanism. By contrast, no evidence was found for a hydrocortisone-sensitive extraneuronal uptake compartment, suggesting that extraneuronal removal may have little, if any, functional importance. During nerve stimulation, yohimbine increased the amount of labelled noradrenaline present in the superfusate, while exogenously added noradrenaline decreased it in the presence of cocaine. Thus, prejunctional alpha-adrenoceptors can modulate the junctional concentration of neurotransmitter in the human saphenous vein.

Does presynaptic regulation of sympathetic transmission occur within a limited range of neuronal activity?

Recent reports indicate that the presynaptic negative feedback mechanism involved in regulating sympathetic neurotransmission is only functional within a narrow range of neuronal activity. The specific aim of this report is to examine the release of sympathetic transmitter at varying frequencies of stimulation, and the effects of alpha-adrenoceptor antagonists on the overflow. The experiments were carried out in the 3H-noradrenalin-labeled heart of the guinea pig. The overflow of tritium (per pulse) increased from a stimulation frequency of 0.125 to 10 Hz and declined at 30 Hz. Phentolamine or yohimbine produced facilitation of the overflow from 0.125 to 10 Hz; the effect was more pronounced at lower frequencies of stimulation. We conclude that the presynaptic alpha-adrenoceptor-mediated negative feedback mechanism operates over a wide range of cardiac sympathetic nerve activity.

Radioenzymatic assay measurement of yohimbine's influence on adrenergic neurotransmission of rat vas deferens: unique consequence of using normetanephrine as an uptake2-blocking agent

The effect of the alpha 2-receptor antagonist, yohimbine on norepinephrine overflow was studied in the transmural-stimulated isolated rat vas deferens. A radioenzymatic assay was used to measure the endogenous norepinephrine overflow. In initial studies conducted in the presence of uptake1 blocker desipramine (10(-6) M) and uptake 2 blocker, normetanephrine (10(-5) M) there was an apparent uncoupling of the influence of yohimbine on nerve-stimulated contractile response from norepinephrine overflow. These results were found to be due to the electrolytic O-demethylation of normetanephrine with the resultant generation of large quantities of norepinephrine obscuring the influence of yohimbine on nerve-stimulated norepinephrine overflow from the vas deferens. These findings serve as a warming to the use of normetanephrine as an uptake1 blocker when radioenzymatic assay is used to measure norepinephrine overflow from transmural-stimulated isolated tissue preparations. Yohimbine (10(-6) M), in the absence of uptake blockade, causes a 3-fold enhancement of nerve-stimulated norepinephrine overflow at 1 Hz and a 2-fold enhancement at 10 Hz. This report demonstrates utilization of a radioenzymatic assay to study endogenous norepinephrine overflow from rat vas deferens. Results for yohimbine are complementary to others using measurement of 3H-label overflow from [3H]norepinephrine prelabeled isolated tissue.

Facilitation, and muscarinic and alpha-adrenergic inhibition of the secretion of 3H-acetylcholine and 3H-noradrenaline from guinea-pig ileum myenteric nerve terminals

The transmitter stores of cholinergic and noradrenergic neurons in guinea-pig ileum myenteric plexus were labelled by preincubation with 3H-choline or 3H-noradrenaline (3H-NA), respectively. Secretion of transmitter was evoked by electrical field stimulation. In the presence (but not in the absence) of eserine (10 microM) the secretion of 3H-acetylcholine (3H-ACh) per shock increased with the frequency of stimulation. Half maximal secretion was obtained at 0.7 Hz (apparent Km Freq). Variation in number of shocks per train did not influence the secretory response per shock (at 6 Hz). The secretion of 3H-NA per shock also increased with the frequency of stimulation (Km Freq = 1.5 Hz). Comparison of the inhibitory effects of oxotremorine and of exogenous NA, on the secretory responses to stimulation at frequencies close to Km Freq, showed that the secretory mechanisms of both neurons are 13-17-fold more sensitive to the inhibitory effect of their 'own', than to that of 'foreign', agonist. In both types of neuron the inhibitory effects of oxotremorine and of NA were competitively antagonized by atropine and by yohimbine, respectively. Dissociation constants for the respective antagonist were essentially the same, irrespectively of the type of neuron, indicating that the properties of 'presynaptic' muscarinic or alpha-receptors are independent of whether they occur on cholinergic or on noradrenergic terminals.

Site(s) and ionic basis of alpha-autoinhibition and facilitation of "3H'noradrenaline secretion in guinea-pig vas deferens

1. Mechanisms controlling the secretion of [(3)H]noradrenaline from the noradrenergic nerves of guinea-pig isolated vas deferens, prelabelled by incubation with [(3)H]noradrenaline, were studied using (a) different modes of (extramural or transmural) electrical nerve stimulation (a total of 300 shocks of varying strength, and a duration of 2 msec) at 1-30 Hz, or (b) depolarizing concentrations of K(+) (60-110 mm).2. The fractional rise in efflux of (3)H-labelled material (Deltat) was used to measure the secretion of [(3)H]noradrenaline.3. The dependence of [(3)H]noradrenaline secretion on the external Ca(2+) concentration (1-8 mm) was essentially hyperbolic. Double reciprocal plot analysis (1/Deltat vs. 1/Ca(2+)) of the data yields that blockade of alpha-autoinhibition (phentolamine 1 mum) does not increase the maximal secretory velocity, but does enhance the apparent affinity of the secretory mechanism for external Ca(2+). Exogenous noradrenaline has (qualitatively) opposite effects. The interaction between alpha-autoinhibition and external Ca(2+) thus shows a ;competitive' pattern, indicating that restriction of the utilization of external Ca(2+) is a major mechanism in alpha-autoinhibition of noradrenaline secretion, in this system.4. Phenoxybenzamine (10 mum) and phentolamine (1 mum) increased the secretion of [(3)H]noradrenaline evoked by depolarization with K(+) much less than that caused by electrical nerve stimulation (frequencies up to 10 Hz). Exogenous noradrenaline (1-5 mum) depressed the secretion evoked by both modes of stimulation. The results indicate that alpha-autoinhibition of [(3)H]noradrenaline secretion is mainly operative when the secretory stimulus requires conduction of nerve impulses between varicosities.5. The frequency dependence of [(3)H]noradrenaline secretion was hyperbolic, both in the presence and in the absence of alpha-autoinhibition; at each frequency the secretion (Deltat per shock) increased with the Ca(2+) concentration in the medium (0.6-8 mm). Double reciprocal plot analysis (1/Deltat vs. 1/frequency) of the data yields that the pattern of interaction between external Ca(2+) and facilitation depends on the presence or absence of alpha-autoinhibition (phentolamine 1 mum); in the former case it is ;non-competitive', in the latter ;competitive'. Similar analysis of the effect of facilitation by increasing the length of stimulus trains (from 5 to 300 pulses) at a constant frequency (5 Hz), on the Ca(2+) dependence of Deltat (1/Deltat vs. 1/Ca(2+)) in the absence of alpha-autoinhibition also yields that facilitation promotes utilization of external Ca(2+). These results apparently imply that a rise in external Ca(2+), in the presence of alpha-autoinhibition, augments the secretory response to electrical nerve stimulation mainly by promoting recruitment of active units (varicosities?), without markedly altering their ;affinity' for facilitation. In the absence of autoinhibition (when all units are already recruited?), the results seem to imply that facilitation promotes depolarization-secretion coupling in each, by more efficient utilization of external Ca(2+).6. The pattern of interaction between alpha-autoinhibition and facilitation depends on the Ca(2+)concentration in the medium. At or below the physiological level of Ca(2+) in extracellular fluid (1.2 mm) it is ;non-competitive', indicating that alpha-autoinhibition and facilitation act, at least in part, at separate targets under these conditions. At high (5.4 mm) external Ca(2+) the pattern becomes almost purely ;competitive', indicating that facilitation can, under suitable conditions, overcome all manifestations of alpha-autoinhibition.7. The secretion evoked by electrical nerve stimulation (Deltat per shock, at 1 or 10 Hz) increased with the strength of applied shocks, both when applied extra- or transmurally, in the presence or absence of alpha-autoinhibition. In the former case the rise in (Deltat per shock) vs. (current strength) was hyperbolic, in the latter it followed a biphasic pattern. Double reciprocal plot analysis (1/Deltat vs. 1/current) of the data yields a ;non-competitive' pattern of interaction between facilitation or alpha-autoinhibition, and exogenous current, when stimulation was extramural. When it was transmural the pattern is ;competitive'. The results seem to imply that hyperpolarization, or depolarization, of nerve terminals are major mechanisms whereby alpha-autoinhibition and facilitation, respectively, exert their effects on the secretory response to electrical nerve stimulation.8. Neither activation of Na(+), K(+)-ATPase, nor promotion of G(Cl) appear to be critically involved in alpha-autoinhibition. Experiments with known blockers of G(K) (tetraethylammonium, 4-aminopyridine and Rb(+)) did not give support to the notion that promotion of K(+) efflux is a mechanism whereby prejunctional alpha-adrenoceptors cause (hyperpolarization of nerve terminals and) autoinhibition of secretion. If alpha-autoinhibition does involve K(+) channels in the nerve terminal membrane, then these must be different from the (voltage-sensitive) K(+) channels blocked by the above mentioned inhibitors of K(+) efflux.9. The results are discussed in the context of a model that assumes that local control of noradrenaline secretion from noradrenergic nerves may be exerted both by control of invasion of terminals, and by control of depolarization-secretion coupling in each invaded varicosity. Under suitable conditions facilitation and alpha-autoinhibition may interact at both levels. It proposed that utilization of external Ca(2+) plays a pivotal role for both, and that restriction of invasion of nerve terminal varicosities is the main effect of alpha-autoinhibition, while promotion of depolarization-secretion coupling is the main effect of facilitation, at physiological concentrations of Ca(2+) in the medium. For the nerve the role of this dual control system is proposed to be to ensure ;rotational' activation of varicosities, and for the effector cell of noradrenergic junctions, to increase the signal/noise ratio.

Possible role of nerve impulse induced sodium ion flux in a proposed multivesicular fractional release of adrenaline and noradrenaline from the chromaffin cell

Based on own observations concerning a two-compartment storage of CA in the adrenal medulla and a cation exchange dependent release of CA from perfused chromaffin granules in vitro, and encouraged by recent reports from other laboratories about the importance of sodium ions for the CA release from the adrenal gland, we propose a modification of the current quantal theory of CA secretion. Instead of secretion of quanta, each quantum corresponding to the content of one vesicle, we envisage a concomitant fractional release of CA from multiple vesicles adjacent to the chromaffin cell membrane. The CA secretion should be the result of a cation exchange across the contact area between the plasma membrane and the granule membrane during the period of depolarization caused by the nerve impulse. The size of the released quanta should be determined by the nerve impulse induced sodium ion flux and the number of such ions which reach the CA binding ionic sites in the cation exchanger pool (the release pool) of the granules.