Potentiative effects of alpha agonistic sympathomimetic amines on vasoconstriction by adrenergic nerve stimulation

The effects of catecholamines and other sympathomimetic amines with alpha adrenergic activity on vasoconstriction were studied. Short ring segments were prepared from five rabbit blood vessels including the mesoduodenal, brachial, central ear and pulmonary lobar arteries and the saphenous vein. Constriction was elicited by electrical field stimulation of adrenergic neurons. This response was markedly potentiated in the mesenteric preparation by low concentrations of norepinephrine, epinephrine, phenylephrine, methoxamine, naphazoline and oxymetazoline. The potentiation occurred with amine concentrations not sufficient to elevate the spontaneous transmitter release or basal smooth muscle tone. Propranolol, cocaine and metanephrine did not prevent the potentiation. Isoproterenol and dopamine potentiated the response only in high concentrations and tyrosine was without an effect. An unidentified extraneuronal action was probably responsible for the potentiation. The mesenteric artery was unique for its marked potentiation as the brachial artery was potentiated only slightly and the other three vessels were inhibited by the alpha adrenergic agents in the constrictor response to stimulation.

Diminished purinergic modulation of the vascular adrenergic neurotransmission in spontaneously hypertensive rats

The inhibitory effects of adenosine and ATP on the pressor response of the perfused mesenteric vascular bed to perivascular adrenergic nerve stimulation were compared between spontaneously hypertensive rats (SHR) and normotensive Wistar-Kyoto rats (WKR). Both adenosine and ATP inhibited the neurogenic vasoconstriction in a dose-dependent manner in WKR, in which the inhibitory effect of adenosine was approximately eight times greater than that of ATP on the basis of 50% inhibitory doses (ID50). In the same preparation isolated from SHR, the inhibitory effects of both adenosine and ATP were significatly smaller than that in WKR. The ID50 values in the SHR for both agonists were approximately six times those found in WKR. These compounds also inhibited the pressor response to norepinephrine (0.3 microgram/ml) infusion, but the degrees of inhibition were markedly less than those with the neurogenic pressor response, and were not significantly different between WKR and SHR. These results suggest that presynaptic inhibition of vascular adrenergic neurotransmission by purine compounds is reduced in SHR.

Effects of adenosine triphosphate and prostaglandins on vascular adrenergic transmission

The effects of prostaglandins (PGs) on the adrenergic neuroeffector transmission and their relationship to the action of ATP were investigated. This was in view of the putative negative feedback mediated by ATP or a related purine compound, and the reported stimulation of PG synthesis by adenine nucleotides. The central ear artery and saphenous vein of the rabbit were isolated and their contractile responses to adrenergic nerve stimulation monitored. These responses were markedly reduced by PGE1 and PGE2 and significantly augmented by indomethacin and aspirin, suggesting the occurrence of the PGE-mediated negative feedback. PGF2a facilitated the response of the vein but was without affect on the artery, while arachidonic acid was facilitatory on the former and inhibitory on the latter. Possibly a PGF2a-like substance is formed in the presence of arachidonic acid and, in the vein, masks the effect of any PGE. ATP depressed the arterial and venous contractile response to adrenergic nerve stimulation. This inhibition was not significantly affected by indomethacin or aspirin. It was enhanced in the artery and diminished in the vein by arachidonic acid, but only to the extent of algebraic sum of the effects of ATP and the acid. It seems possible that the purine- and PGE-mediated feedback mechanisms are independent and parallel pathways.

Neurogenic vasodilation of cat cerebral arteries

Transmural nerve stimulation (TNS) with 0.3-msec pulses between 1 and 25 Hz dilated cat cerebral artery segments in the presence of active muscle tone. Maximum vasodilatation occurred at 8 Hz. The dilator response to exogenous acetylcholine, but not to TNS, was abolished by atropine. Neither physostigmine nor hemicholinium affected the dilator response to TNS, which persisted after administration of guanethidine, phenoxybenzamine, propranolol, reserpine, and chronic sympathectomy. However, it was abolished by tetrodotoxin and cold storage. When examined histochemically, cat and rabbit cerebral arteries exhibited a rich plexiform distribution of acetylcholinesterase which was not affected appreciably by sympathetic denervation. These results suggest that vasodilation is not mediated through modification of sympathetic activity. They also indicate the existence of a nonadrenergic, possibly noncholinergic, vasodilator innervation in cat cerebral arteries. Preliminary studies suggest that the transmitter is not histamine, ATP, prostaglandins, gamma-aminobutyric acid, dopamine, or serotonin. The cat cerebral artery segments contrast with the isolated rabbit cerebral arteries which predominantly constrict in response to TNS and show a small dilator response.

Purinergic inhibition of adrenergic transmission in rabbit blood vessels

The effect of ATP and its congeners on the adrenergic neuroeffector transmission was evaluated in isolated blood vessels of the rabbit. ATP, ADP, AMP and adenosine inhibited the contractile response of the portal vein to adrenergic nerve stimulation, with a threshold concentration of the order of 0.1 muM and ED50 of about 1 microM. These agents, but not papaverine, inhibited the nerve stimulation-induced response in preference to the norepinephrine- or serotonin-induced response in the portal and saphenous veins and pulmonary and ear arteries. In the portal vein labeled with [3H]norepinephrine, ATP diminished the nerve stimulation-induced efflux of tritiated material. This nucleotide also reduced the KCl-induced tritium efflux but not the tyramine induced-efflux in the [3H]norepinephrine-labeled thoracic aorta. ATP had no significant effect on the uptake of [3H]norepinephrine in the portal vein, ear artery and thoracic aorta. Indomethacin and theophylline partially blocked the inhibitory action of ATP on the neurogenic constrictor response in some of the ear artery and saphenous vein preparations. Desipramine, atropine, propanolol, haloperidol and 2,2'-pyridylisatogen, a blocking agent against ATP in the taenia coli, were without such antagonistic effect. The results are consistent with a proposed negative feedback modulator role of ATP or a related purine compound in adrenergic transmission.

Modes of vasoconstrictor and vasodilator neurotransmission

Vasoconstrictor and vasodilator neuroeffector transmission occurs in a variety of modes. Models are presented depicting vascular segments under the direct control of a single or multiple innervation, or an indirect influence via secondary release of a vasoactive substance. The neuroeffector relationship in the rabbit portal vein is described to illustrate the coexistence of several modes of transmission. ATP or its congener is released from the nonadrenergic vasodilator nerves in this vein, possibly as the transmitter. Further, a similar substance is released from the adrenergic vasoconstrictor nerves. It possibly acts directly on smooth muscle cells to relax them, and on the nerve terminals to inhibit the adrenergic transmitter release. Evidence suggests that such purine-mediated feedback mechanism may also occur at many other adrenergic and nonadrenergic neuroeffector synapses.

Uptake of 3H-norepinephrine in rabbit mesenteric blood vessels

The uptake of traited norepinephrine in isolated rabbit mesoduodenal blood vessels was measured. Neuronal uptake was estimated utilizing the inhibitory effect of cocaine, and expressed on the basis of wet and dry tissue weights as well as circumferential area of the vascular wall. This area was presumed to approximate the area of the adrenergic nerve terminal plexus. The wet weight of smaller tissues was apt to be underestimated due to excessive drying during weighing; dry weight was more consistent over a wide range of tissue size. Either on the basis of weight or circumferential are, neuronal uptake in arteries increased as the diameter diminished, but uptake was practically constant in all segments of the veins. Arterial uptake per unit circumferential area was greater than in the accompanying veins, but this was not necessarily the case when uptake was expressed per unit weight. Neuronal norepinephrine may be a useful index of adrenergic nerve density. On this basis it is suggested that the adrenergic neural vasoconstriction increases with decrease in diameter of both arteries and veins in rabbit mesoduodenum.

Development of neuroeffector mechanisms in the carotid artery of the fetal lamb

A survey has been made of mechanisms associated with vascular adrenergic neuroeffector transmission in the lamb fetuses between 53 days and term gestation. The common carotid artery was isolated for studies of enzymic activities, uptake of norepinephrine (NE) and reactivity to vasoactive agents. The extra-neuronal NE uptake, monoamine oxidase and catechol-O-methyltransferase activities were present in the carotid artery of the youngest fetuses. The contractile responses to NE and serotonin and neuronal NE uptake preceded the response to adrenergic nerve stimulation during fetal growth. These results suggest that the mechanisms for adrenergic transmitter inactivation, transmitter action on vascular smooth muscle cells, and neuronal transmitter delivery develop in that sequence.

Neurogenic sympathetic vasoconstriction of the rabbit basilar artery

When examined by fluorescence microscopy the rabbit basilar artery contains a rich adrenergic-like plexus at the adventitiomedial junction. The fluorescence disappears upon chronic reserpinization and bilateral superior cervical ganglionectomy. Transmural stimulation of intramural nerves a results in a response which is predominantly constrictor but also contains a small, inconstant dilator component. The constrictor response is abolished by chronic reserpinization, bilateral superior cervical ganglionectomy, and cold storage of the preparation. The constriction is prevented by the adrenergic neuron blocking agents guanethidine and bretylium but not by such alpha-adrenergic receptor blocking agents as phenoxybenzamine (PBZ), phentolamine, and tolazoline. Our results show that doses of the three latter agents sufficient to abolish contractions to norepinephrine (NE) in concentrations of up to 10(-2) M only potentiate and prolong the contractile response to nerve stimulation. The beta-adrenergic receptor blocking agent, propranolol, and inhibitors of NE neuronal uptake, such as desipramine (desmethylimipramine, DMI) and cocaine, do not influence the size of the neurogenic response. These results suggest that the vasoconstrictor component of the rabbit basilar artery response to transmural nerve stimulation (TNS) is mediated via sympathetic adrenergic-like neurons, but at the same time also raise the question whether the transmission process is typical of classic adrenergic neuroeffector mechanisms.

Changes in the contractile response of arteires and veins from hypertensive rabbits to sympathetic nerve activity: assessment of some postsynaptic influences

Contractile responses to field stimulation of intramural nerves of arteries and veins taken from rabbits made hypertensive by partial constriction of the abdominal aorta have been related to the carotid artery pressure. The increase in contraction of cephalic and short saphenous veins with rise in carotid artery pressure can be accounted for by an increase in sensitivity of the alpha-adrenergic receptor. The neurogenic contraction of the ear artery increased with carotid artery pressure rise. Changes in some of the extraneuronal factors that influence transmitter distribution and disposition in the tunica media were examined. In hypertensive animals, the percentage of released adrenergic transmitter entering the vessel wall might be expected to decrease due to an increase in medial thickness. However, this percentage was not significantly altered in the ear artery probably due, in part, to a concomitant increase in medial permeability to the transmitter. Extraneuronal transmitter disposition factors, i.e. extraneuronal uptake, monoamine oxidase, and catechol-O-methyltransferase activity are directly related to the wet weight of the vessel wall. Thus, their contribution to transmitter disposition would be expected to increase with increase in vessel wall thickness and tend to reduce the response to sympathetic activity. As the contractile response increased in the hypertensive vessels despite such changes, the increase in effector cell mass and density of neuronal terminal plexus, shown previously to increase with hypertension, are more important than these other considerations.

Nonsympathetic dilator innervation of cat cerebral arteries

The results presented provide strong support for the presence of vasodilator innervation in the cat cerebral arteries. The dilator innervation is neither adrenergic nor cholinergic and does not originate in the superior cervical ganglia. The nature of the vasodilator transmitter is unidentified. Such innervation, however, may be involved in the regulation of cerebral blood flow, especially in view of the capability of some cat cerebral vessels to develop intrinsic muscle tone.

Neurogenic release of purine compounds in blood vessels

The isolated thoracic aorta, ear artery and portal vein of the rabbit concentrated tritiated material on exposure to tritiated adenosine, adenine and its nucleotides. 3H-adenosine was transformed and retained predominantly as 3H-adenosine triphosphate in the portal vein and aortic adventitia, but to a lesser extent in the aortic medial or muscle layer. On transmural stimulation, portal vein and aortic adventitial strips pretreated with 3H-adenosine released tritiated material, which was recovered mainly as tritiated adenosine and nucleotides. Guanethidine abolished this release in the aortic adventitia and greatly diminished it in the portal vein. Both the release and vasodilation induced by transmural stimulation in the portal vein were abolished by tetrodotoxin. It is suggested that purine compounds are released locally within vascular walls from both the adrenergic and nonadrenergic nerves. The results are compatible with the view that adenosine triphosphate or a related compound may function as an inhibitory modulator in association with the adrenergic nerves and as a vasodilator transmitter of the non-adrenergic nerves in the portal vein.

Evidence for an increase in adrenergic nerve function in blood vessels from experimental hypertensive rabbits

The possibility of changes in the adrenergic innervation of blood vessels in experimental hypertension was investigated by measuring arterial norepinephrine content, neuronal uptake of norepinephrine, and the neurogenic contractile response in rabbits made hypertensive by partial constriction of the abdominal aorta proximal to the kidneys. Two to 3 weeks after surgery, norepinephrine content was increased in the arteries above the ligature, where arterial blood pressure was increased, but not in the arteries below the ligature, where arterial blood pressure was normal, in the heart, or in the veins. Neuronal norepinephrine uptake per unit length of vessel and the neurogenic contractile response increased with the rise in arterial blood pressure. The neurogenic contractile response can be taken as an indication of an increase in transmitter release. The results taken together suggest an increase in the function and possibly the amount of the adrenergic neuroneal terminal in hypertension. Since the distributions of the changes in the adrenergic innervation and the increases in smooth muscle cell proliferation in hypertension are similar, these two processes may be interrelated.

Analysis of changes in reactivity of rabbit arteries and veins two weeks after induction of hypertension by coarctation of the abdominal aorta

Vessel dimensions and characteristic responses to norepinephrine were measured in various arteries and veins of the rabbit made hypertensive by partial constriction of the upper abdominal aorta. The ear, radial, and basilar arteries taken from the circulation proximal to the ligature (the hypertensive arteries) were thickened in proportion to the rise is arterial blood pressure. The water, sodium, and potassium contents of these and all other vessels were not significantly changed in the hypertensive rabbits. The maximum response to norepinephrine in the ear artery, a representative vessel from the hypertensive part of the rabbit, was increased, whhereas the sensitivity of this vessel to norepinephrine expressed as the ED50 did not alter with changes in the arterial blood pressure. In contrast, the thickness and the maximum response to norepinephrine of the saphenous artery, representative of vessels distal to the ligature (normotensive vessels) and of the saphenous and cephalic veins were unaltered. The sensitivity as indicated by the norepinephrine ED50 of the veins, but not of the saphenous artery, increased with a rise in carotid artery blood pressure. These results suggest that the increased responsiveness to norepinephrine of arteries proximal to the ligature is due to changes in muscle mass and that the increased responsiveness of the veins is due to increased sensitivity to norepinephrine.

Development of autonomic control of fetal circulation

Development of parasympathetic and sympathetic reflexes controlling heart rate, vascular pressures, and blood flows was investigated in fetal lambs weighing 300-5,800 g (65-165 days' gestation). Cardiovascular responses to veratridine injections, atrial stretching, bilateral cervical vagotomy, and cholinergic blockade with atropine were used to test parasympathetic activities. Responses to propranolol and phenoxybenzamine were used to test beta- and alpha-adrenergic activities. Autonomic ganglionic blockade and stimulation provided additional information on both cholinergic and adrenergic systems. Fetal responses to various tests were compared to those of the mother. Results show: a) little parasympathetic tone on resting heart rate and other circulatory functions exists prior to fetal maturity; b) despite the feeble resting tone, the parasympathetic system is capable of exerting significant control when stimulated in both premature and mature fetuses, the capability increases as fetus approaches term; c) alpha- and beta-adrenergic tone in control of resting heart rate and peripheral circulation exists in early fetal life and increases as the fetus reaches maturity, and both adrenergic receptors respond strongly to stimuli in immature, premature, and mature fetuses; d) in immature fetuses, veratridine does not elicit a vagally mediated reflex; instead, it produces a centrally mediated alpha- and beta-adrenergic stimulation; e) the fetal cardiovascular response to any given test is dampened by the existence of the various vascular shunts, the umbilicoplacental circulation and, possibly, by incomplete maturation of vasomotor tone.

Systemic and pulmonary hemodynamic responses to adrenergic and cholinergic agonists during fetal development

Systemic and pulmonary hemodynamic responses to adrenergic and cholinergic agonists were investigated in fetal lambs between 60 days and term gestation. The cardiovascular response to these agents increases with fetal age, and the increase is related to maturation of the effector system rather than the vascular receptors. The fetal pulmonary vascular bed and the ductus arteriosus are the primary components responding to acetylcholine; the systemic response is secondary to the occuring in the lung. Both fetal systemic and pulmonary vascular beds are under alpha-adrenergic control whereas the fetal heart is under beta-adrenergic control.

Effects of phenoxybenzamine and norepinephrine on transmitter release in the pulmonary artery of the rabbit

The effects of phenoxybenzamine and norepinephrine (NE) on the release of tritiated NE were studied in spiral strips of the rabbit pulmonary artery. Phenoxybenzamine increased the tritiated NE efflux induced by transmural stimulation at 4 Hz by a factor of three. The magnitude of this effect was inversely related to the frequency of stimulation. NE (2.5 times 10- minus 6 M) decreased the enhancing effect of short exposure but not long exposure to phenoxybenzamine, probably by surmounting the phenoxybenzamine blockade of the alpha-adrenergic receptors on the adrenergic nerve terminal. The transmitter output per pulse increased with the frequency from 4 to 16 HZ and decreased at 32 HZ. This evidence suggests that a negative feedback mechanism does operate in this vascular preparation but, probably because of the wide neuromuscular cleft, is less effective than in tissues with a narrower synaptic interval.

Norepinephrine uptake, smooth muscle sensitivity, and metabolizing enzyme activity in rabbit veins

Some parameters of the adrenergic neuroeffector mechanism were measured in vitro in the central ear (near the base of the ear), common jugular, pulmonary, brachial, femoral, renal, superficial cervical, cephalic, and small saphenous veins, a branch of the deep circumflex iliac vein, the parietal branch of the internal iliac vein, branches of the anterior mesenteric vein, the inferior vena cava (immediately distal to the left renal vein), and a subcutaneous vein of the back. Although extraneuronal uptake of 3 H-norepinephrine was the same in all of the veins except the mesenteric, the neuronal uptake of norepinephrine varied widely. A number of veins, including the femoral and superficial cervical veins, showed no neuronal uptake, and the uptake of others, including the cephalic and mesenteric veins, was greater than that measured in any previously studied vascular tissue. The median effective dose for the contractile effect of norepinephrine on the veins was on the same order of magnitude as that for the aorta and the ear artery. Catechol-O-methyl transferase, but not monoamine oxidase, activity appeared to be related to innervation density. It is concluded that veins show a remarkable variation in the dimensions of their adrenergic parameters, particularly those related to innervation density.

Innervation pattern and neurogenic response of rabbit veins

The adrenergic neuroeffector mechanism has been assessed in 14 different veins of the rabbit chosen to represent vessels of different, function and regional location. The pattern, distribution and density of the adrenergic innervation were assessed by fluorescence histochemistry. The magnitude of the neurogenic contractile response of isolated segments was studied <i>in vitro</i> in relation to the maximum response to <i>1</i>-norepinephrine. The diameter and wall thickness of the veins was determined. The amount of adrenergic innervation determined histochemically was related to the size of the contractile response. Both fluorescence and response were absent in the superficial cervical, femoral, subcutaneous from back, brachial and deep circumflex iliac branch veins. The amount of adrenergic innervation increased in order in the renal, inferior vena cava, pulmonary, jugular, parietal branch of internal iliac, mesenteric, central ear, cephalic and small saphenous veins. In the latter two veins, innervation was not restricted to the adventitio-medial junction but entered the media. The pattern of innervation and contractile response may be related to the functional need for active venoconstriction in the various parts of the capacitance section of the vascular system.

Evidence for uneven alpha-receptor distribution in the rat portal vein

The smooth muscle of the rat portal vein which supports propagation becomes supersensitive to exogenous l -norepinephrine after elimination of the vessel's intramural neuronal uptake mechanism. Such supersensitivity is generally associated with intimate neuromuscular connections. Since the adrenergic vasomotor fibers in the portal vein do not enter the muscle tissue, it has been proposed that muscle cells close to the nerve terminals must determine the response to norepinephrine. With the aim of elucidating the mechanism behind the prejunctional supersensitivity, the total uptake of labeled norepinephrine in the portal vein was analyzed and its distribution within the vessel wall was evaluated directly using isotopically labeled frozen sections. The total uptake of the portal vein, related to its endogenous norepinephrine content, was comparable with that in other vascular and nonvascular tissues. The 3 H-uptake profile of the vessel wall showed that the neurogenic uptake was confined to the narrow adrenergic plexus between the longitudinal and the circular muscle layers. The neuronal uptake mechanism therefore could only influence the concentration of exogenous norepinephrine at muscle cells close to the nerve terminals. These results support the hypothesis that the prejunctional supersenstivity in the rat portal vein indicates that the alpha receptors are located on muscle cells in the vicinity of the adrenergic nerve terminals.

Comparison of adrenergic mechanisms in an elastic and a muscular artery of the rabbit

Elastic and muscular arteries are known to differ in function and in the magnitude of their response to vasoconstrictor influences. With the isolated thoracic aorta and ear artery of the rabbit as representative arteries, the morphological, physiological, and pharmacological correlates of these differences have been sought among their presynaptic adrenergic mechanisms. The adrenergic nerve plexus in the ear artery is wider and the nodes are denser than they are in the aorta. There is some evidence consistent with the hypothesis that the thicker the plexus, the greater the reuptake of released transmitter, the smaller the transmitter overflow, and the lower the efficiency of uptake of exogenous norepinephrine. Measurements of fractional release of tritiated norepinephrine suggest that qualitative differences in the adrenergic transmitter storage and release mechanisms may exist between the two vessels. Thus the considerable functional difference between the two vessels is in part, at any rate, a consequence of adrenergic mechanisms which differ both quantitatively and qualitatively from each other.

[3H]adenosine triphosphate: release during stimulation of enteric nerves

The isolated taenia coli of the guinea pig takes up tritiated adenosine, adenosine monophosphate, adenosine diphosphate, and adenosine triphosphate, in preference to tritiated inosine and adenine. After uptake, [(3)H]adenosine is converted and retained primarily as [(3)H]adenosine triphosphate. Tritium is released from taenia coli treated with [(3)H]adenosine upon activation of the nonadrenergic inhibitory nerves. These results are consistent with the previous evidence that adenosine triphosphate may be the transmitter from the nerves.

Distribution theory of resistance of neurogenic vasoconstriction to alpha-receptor blockade in the rabbit

The effects of α-receptor-blocking agents on the contractile responses of isolated rabbit arteries to sympathetic nerve stimulation and exogenous l -norepinephrine ( l -NE) were compared. In the pulmonary artery and aorta, yohimbine, phentolamine, and phenoxybenzamine blocked the response to nerve stimulation less than that to an equipotent dose of l -NE. This resistance of neurogenic response was independent of the frequency and number of stimuli and persisted after inhibition of the nerve l -NE uptake by cocaine. The neurogenically released transmitter l -NE probably forms a high concentration near the adventitia-media junction, whereas the exogenous NE is distributed evenly throughout the thickness of media. Thus higher concentrations of α-receptor-blocking agents would be needed to block the effect of neurogenic l -NE than to block that of exogenous l -NE. This explanation of the resistance was thought to be more appropriate to the large vessels tested than that based on neuroeffector proximity.