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.     

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.     

Postnatal ontogenetic development of neurogenic and myogenic control in the rat portal vein.

The postnatal ontogenetic development of neuro-effector control in vascular smooth muscle of the single-unit type has been studied in the rat portal vein. Contractile activity was recorded isometrically in isolated preparations from rats 2-38 days of age and in adults rats. Spontaneous activity, characteristic of the adult portal vein, appeared abruptly during the third postnatal week. Whereas, induced responses to noradernaline (NA) and acetylcholine (Ach) appeared early during the first week and responses to transmural nerve stimulation occurred at the end of the first week. The appearance of spontaneous activity was accompanied by significant increases in sensitivity to NA (log ED50) and to transmural nerve stimulation (frequency giving half maximum response), but not to Ach. Also, maximum responses for NA and nerve stimulation relative to Ach responses tended to increase during the first three weeks. It is concluded that the development of a mechanism supporting myogenic propagation as revealed by highly synchronized spontaneous contractions is an important factor for promoting the effectiveness of sympathicoadrenergic control in this type of vascular smooth muscle.     

The effect of length on the sensitivity to phenylephrine and calcium in intact and skinned vascular smooth muscle

The length dependency of the sensitivity to activators of the smooth muscle of different blood vessels is not yet fully understood. Muscle preparations of the aorta, the femoral artery and the portal vein of the rabbit were investigated for the length dependency of the sensitivity to phenylephrine and calcium in both intact and triton X- 100 skinned preparations. For intact smooth muscles we found that at increased preparation length, the sensitivity of contraction was increased. The femoral artery showed the largest effect and the portal vein the smallest. In the skinned preparations of the three preparations the calcium sensitivity was not dependent on the preparation length. We conclude that the changes of the sensitivity in intact preparations are not caused by changes of the calcium sensitivity of the contractile proteins.     

Postnatal Ontogenetic Development of Neurogenic and Myogenic Control in the Rat Portal Vein

The postnatal ontogenetic development of neuro-effector control in vascular smooth muscle of the singleunit type has been studied in the rat portal vein. Contractile activity was recorded isometrically in isolated preparations from rats 2–38 days of age and in adults rats. Spontaneous activity, characteristic of the adult portal vein, appeared abruptly during the third postnatal week. Whereas, induced responses to noradrenaline (NA) and acetylcholine (Ach) appeared early during the first week and responses to transmural nerve stimulation occurred at the end of the first week. The appearance of spontaneous activity was accompanied by significant increases in sensitivity to NA (log Ed₅₀) and to transmural nerve stimulation (frequency giving half maximum response), but not to Ach. Also, maximum responses for NA and nerve stimulation relative to Ach responses tended to increase during the first there weeks. It is concluded that the development of a mechanism supporting myogenic propagation as revealed by highly synchronized spontaneous contractions is an important factor for promoting the effectiveness of sympathico-adrenergic control in this type of vascular smooth muscle.     

Role of nitric oxide in the contraction of circular muscle in the rat portal vein

The role of nitric oxide in the electrical and mechanical activities of the rat portal vein was examined in circular muscle preparations with intact endothelium that were isolated from the longitudinal muscle layer. In contrast to the longitudinal muscle preparation, the circular muscle preparation did not show spontaneous phasic contraction. Inhibition of nitric oxide synthesis by Nomega-nitro-L-arginine (L-NNA) induced a tonic contraction. The contraction was inhibited by L-arginine, sodium nitroprusside or nifedipine. L-NNA did not induce contraction in endothelium-damaged preparations. The membrane potential of smooth muscle cells recorded in endothelium-intact preparations showed sporadic action potentials. L-NNA increased the frequency of action potentials without changing the resting membrane potential. The action potentials were inhibited by nifedipine. In the presence of L-NNA, sodium nitroprusside decreased the frequency of the action potentials without changing the resting membrane potential. These results indicated that contraction of rat portal vein circular muscles is inhibited tonically by nitric oxide, at least partly through inhibition of electrical activity.     

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.     

Contractile effects of various vasoactive agents in small rat portal veins and hepatic arteries and the influence of sympathetic denervation on the noradrenaline response

Contractile responses were studied in isolated tubal segments of branches of the rat portal vein (luminal diameter approximately 300 microns) and hepatic artery (luminal diameter approximately 200 microns). Portal veins were approximately three times more sensitive to noradrenaline (NA) than hepatic arteries. 5-hydroxytryptamine contracted hepatic arteries concentration-dependently, whereas it produced only weak and inconsistent contractions in portal veins. Vasopressin effectively contracted hepatic arteries, whereas it had no effect on portal veins. Both vessel types responded to prostaglandin F2 alpha with contractions, although the drug potency was relatively low (EC50 greater than 10(-5) mol l-1). Histamine and carbachol failed to induce (hepatic arteries) or caused only weak (portal veins) contractions. Microsurgical hepatic hilar denervation reduced the catecholamine content of the parenchyma to less than or equal to 25% of controls. In both portal veins and hepatic arteries, the denervation procedure increased the NA sensitivity by factors of 3.1 and 2.0, respectively. In non-denervated livers, cocaine produced a similar increase of the NA sensitivity, whereas the drug had no significant effect in vessels from denervated animals. Thus, there was a marked difference between rat portal veins and hepatic arteries in their responsiveness to several contractile agents. Furthermore, the results of the present study indicate that the adrenergic nerves in both vessel types can be adequately removed by the microsurgical denervation procedure used.     

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)     

Effects of K-free solution on tension development and Na content in vascular smooth muscles isolated from guinea-pig,rat and rabbit

The effects of K-free solution on tension development and cellular Na content were investigated in aortic strips isolated from guinea-pig, rat and rabbit and in rabbit ear artery. Experiments were conducted in the presence of phentolamine (5×10^–7 to 10^–6 M) in order to eliminate the possible influence of endogenous catecholamine. K-free solution produced contractions and increased cellular Na content in these vascular smooth muscle preparations. The rate of increase in K-free contraction of rabbit aorta and ear artery was much slower than that of guinea-pig and rat aortae. The rate of accumulation of Na in rabbit arterial smooth muscle preparations was also much slower than that in guinea-pig and rat aortae. When the relationship between tension development and cellular Na content was compared in these arteries, it was found that an increase in cellular Na produced a greater increase in tension in guinea-pig and rat aortae than in rabbit aorta and ear artery. It is suggested that the slow and small responses of rabbit arterial smooth muscles to K-free solution might be due to a slow accumulation of Na and to a smaller role of transmembrane Na gradient in the regulation of cellular Ca.     

The Role of Intramural Noradrenaline in the Potassium Induced Contracture of Non-Estrogenized Smooth Muscle

The influence of intramural noradrenaline on potassium induced contractures was studied in isolated preparations of the uterus and the portal vein of the rat. Uterine strips of oophorectomized rats responded with contraction followed by transient relaxation when immersed in a medium containing 127 mM potassium chloride. Reserpinization or blockade of β-adrenoceptors with propranolol greatly diminished the transient relaxation. In the isolated portal vein, both noradrenaline depletion with reserpine and α-adrenoceptor blockade (phentolamine) reduced the active tension produced in response to the isotonic potassium solution. These results suggest that intramural noradrenaline plays a significant role in the development of contractures evoked by potassium in the uterus and the portal vein of the rat. In the uterus, the released noradrenaline counteracts the development of active tension, whereas in the portal vein, noradrenaline has a contractile effect which is added to that of potassium. Estrogen treatment reduced (portal vein) or abolished (rat uterus) the contractile effects of intramural noradrenaline. The adrenolytic effect of estrogen is possibly due to reduction in the release of noradrenaline from the sympathetic nerve-endings.     

Species and tissue variation in extraneuronal and neuronal accumulation of noradrenaline

1. The ability of arterial and non-arterial smooth muscle in five tissues (vas deferens, heart, bladder, colon, spleen) in four species (mouse, rabbit, rat, guinea-pig) to accumulate and retain noradrenaline (NA) was measured in thin tissue slices exposed to NA for 30 min, then washed in cold saline solution for 30 min. NA accumulation was assessed histochemically by measuring the fluorescence brightness of the tissue with the Leitz MPV microphotometer. In addition, similar measurements were made on smooth muscle in the cat spleen, on cardiac muscle and on the terminal adrenergic nerves.

2. In general, arterial smooth muscle had a greater capacity to accumulate and retain NA than non-arterial smooth muscle, but there was a great species and organ variability. The ability to accumulate and retain NA was best developed in the mouse, followed by the rabbit, rat and guinea-pig in that order. Among organs the artery to the vas and the coronary arteries showed the greatest retention. Among non-arterial smooth muscle the mouse vas and the rabbit colon were notable.

3. Cardiac muscle accumulates NA during exposure to the amine but, unlike smooth muscle, cannot retain it when washed with NA-free solution.

4. Terminal adrenergic nerves in different tissues show some variability in fluorescence intensity, and this is increased after exposure to NA. This may indicate a variable capacity of these cells to accumulate and retain NA.

     

Mechanisms underlying pre- and postjunctional effects of neuropeptide Y in sympathetic vascular control

The effects of porcine neuropeptide Y (NPY) regarding sympathetic vascular control were studied in vitro on isolated rat blood vessels. The 10(-9)M NPY enhanced (about two-fold) the contractile responses to transmural nerve stimulation (TNS), noradrenaline (NA) and adrenaline (about two-fold) in the femoral artery. Higher concentrations of NPY (greater than 10(-8)M) caused an adrenoceptor-resistant contraction per se. The TNS-evoked [3H]NA efflux was significantly reduced by NPY in a concentration-dependent manner (threshold 10(-9)M). The calcium antagonist, nifedipine, abolished the contractile effects of NPY and the NPY-induced enhancement of NA contractions but did not influence the prejunctional inhibition of [3H]NA release. Receptor-binding studies showed that the ratio of alpha 1-to alpha 2-adrenoceptors in the femoral artery was 30:1. The NPY did not cause any detectable change in the number of alpha 1-or alpha 2-adrenoceptor binding sites or in the affinity of alpha 2-binding sites, as revealed by prazosin- and clonidine-binding, respectively. The NPY also inhibited the TNS-evoked [3H]NA release (by 42-86%) in the superior mesenteric and basilar arteries and in femoral and portal veins. The NPY still depressed TNS-evoked [3H]NA secretion from the portal vein in the presence of phentolamine. The NPY caused a clear-cut contraction in the basilar artery, increased the contractile force of spontaneous contractions in the portal vein, while only weak responses were observed in the superior mesenteric artery and femoral vein. The NA-induced contraction was markedly enhanced by NPY in the superior mesenteric artery, only slightly enhanced in the portal vein and uninfluenced in the femoral vein. In conclusion, in all blood vessels tested, NPY depresses the TNS-evoked [3H]NA secretion via a nifedipine-resistant action. Furthermore, NPY exerts a variable, Ca2+-dependent vasoconstrictor effect and enhancement of NA and TNS contractions.     

Noradrenaline-induced depolarization is smaller in isobaric compared to isometric preparations of rat mesenteric small arteries

The hypothesis was tested that wall tension can influence the membrane potential response to noradrenaline (NA) using isometric and isobaric vessel preparations of rat mesenteric small arteries. The resting membrane potential was significantly less negative in the isobaric (–49.7±0.5 mV, S.E.M., n=12 vessels) compared to the isometric preparation (–56.1±0.7 mV, n=10), although there was no difference in wall tension. The depolarization induced by 10^–5 M NA was 2.6-fold smaller in the isobaric preparation, where wall tension decreased, compared to the isometric preparation, where wall tension increased. Since wall tension decreases under isobaric conditions, but increases under isometric conditions, the latter finding can be explained by assuming that part of the NA-induced membrane potential change is wall tension dependent.     

Increased contractility to noradrenaline and normal endothelial function in mesenteric small arteries from the Goto-Kakizaki rat model of type 2 diabetes

Type 2 diabetes is associated with many circulatory manifestations, including alteration in endothelial function and hypertension. In this study we investigate the morphology and contractile response as well as the endothelial function of resistance arteries from the spontaneously diabetic Goto-Kakizaki (GK) rat, a model of lean type 2 diabetes expressing glucose intolerance. METHODS: Isolated mesenteric small arteries were investigated under isometric conditions in a wire myograph system using noradrenaline (NA) and the endothelium-dependent vasorelaxant acetylcholine (ACh). Media thickness was measured and media lumen ratio calculated. RESULTS: No apparent morphological difference was noted between the arteries from GK rats and control Wistar (CW) rats. When exposed to the maximal NA concentration used (30 microM), arteries from GK rats developed significantly more tension than arteries from CW rats. In the presence of indomethacin (a specific blocker of the COX synthase) and of L-NAME (an inhibitor of eNOS), the response to NA was still significantly greater in GK rat arteries. Under control conditions, arteries from both groups showed intact relaxation to ACh. After incubation with indomethacin and L-NAME, both groups showed a non-NO nonprostaglandin-dependent relaxation to ACh. This relaxation could be blocked by a combination of apamin and charybdotoxin. CONCLUSION: This study shows that mesenteric small arteries from the diabetic GK rat have increased contractile response to NA, along with a normal endothelial function and unaltered morphology.     

A comparative study of neuromuscular transmission in several mammalian muscular arteries

Intracellular recordings were made from the smooth muscle of isolated segments of the rabbit ear artery, rabbit saphenous artery, rat tail artery, and guinea pig mesenteric artery. Resting membrane potentials recorded from cells in all these arteries were the same (–65 to –75 mV) and perivascular nerve stimulation evoked excitatory junction potentials (EJPs). At stimulation frequencies>0.2 Hz facilitation of the EJPs was observed in all but the rat tail artery; in this artery the amplitude of the second EJP in a train was less than the first for stimulation frequencies between 0.3 and 2.0 Hz. Spontaneous excitatory junction potentials (SEJPs) at frequencies up to 20/min were recorded during all impalements in the rat caudal artery. In rabbit ear, saphenous and guinea pig measenteric arteries single, supramaximal intensity stimuli evoked EJPs which were not associated with any detectable contraction. Facilitation of EJPs during trains of stimuli >1 Hz gave rise to graded active responses of up to 50 mV in the saphenous and tail arteries whereas all-or-nothing action potentials of 42–50 mV were recorded in both the rabbit ear and guinea pig mesenteric arteries. Such responses were always associated with contraction. Intraluminal distension of the arteries had no effect on the RMP or response to nerve stimulation.     

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.     

Pre-and postjunctional beta-adrenoceptor mediated effects on transmitter release and effector response in the isolated rat portal vein

Pre-and postjunctional control mechanisms of the portal vein of spontaneously hypertensive rats (SHR) were characterized. Emphasis was placed on the influence of the presynaptic beta-adrenoceptor mediated mechanism for regulation of neuronal noradrenaline (NA) release (studied as tritium overflow) and its consequences for the contractile response under in vitro conditions. It was found that isoprenaline increased, whereas dl/-propranolol decreased the release of neuronal NA during transmural nerve stimulation, while effector responses remained unaltered. d-Propranolol and the beta-1 selective adrenoceptor antagonist, metoprolol, did not affect these two variables. It is concluded that the presynaptic beta-adrenoceptors in the rat portal vein are mainly of the beta-2 type and mediate facilitation of neuronal transmitter release and that concomitant changes of the effector responses of this tissue are below the level of detection under the present experimental conditions.     

Pre- and postjunctional beta-adrenoceptor mediated effects on transmitter release and effector response in the isolated rat portal vein

Pre- and postjunctional control mechanisms of the portal vein of spontaneously hypertensive rats (SHR) were characterized. Emphasis was placed on the influence of the presynaptic beta-adrenoceptor mediated mechanism for regulation of neuronal noradrenaline (NA) release (studied as tritium overflow) and its consequences for the contractile response under in vitro conditions. It was found that isoprenaline increased, whereas dl-propranolol decreased the release of neuronal NA during transmural nerve stimulation, while effector responses remained unaltered. d-Propranolol and the beta-1 selective adrenoceptor antagonist, metoprolol, did not affect these two variables. It is concluded that the presynaptic beta-adrenoceptors in the rat portal vein are mainly of the beta-2 type and mediate facilitation of neuronal transmitter release and that concomitant changes of the effector responses of this tissue are below the level of detection under the present experimental conditions.     

Differentiation of Vasoconstrictor Sensitivity Caused by Prostaglandin E in Peripheral Vascular Beds of the Rabbit

The increase in perfusion pressure in the rabbit ear, hindleg and mesentery caused by close intra-arterial injection of noradrenaline (NA), and the contractile response to NA of the rabbit aortic strip were investigated with respect to their sensitivity to prostaglandin E₁ (PGE₁) and to the prostaglandin synthesis inhibitor indomethacin. PGEj (100–200 ng) potentiated the increases in perfusion pressure caused by NA in the perfused hindleg and mesentery, and the contractile response to NA of the aortic strip by 25–80%, but inhibited the increase in perfusion pressure by NA in the perfused ear by 35–100%. Indomethacin (3–5 10^-5 M) significantly decreased the pressor responses to NA in the hindleg (by 45%) and mesentery (by 55%). This inhibitory effect by indomethacin was completely reversed by PGE₁. The responses to NA in the aortic strip and the perfused ear were unaffected by indomethacin. It is concluded that the process of vasoconstriction in the vascular beds of the rabbit displays qualitative differences concerning its sensitivity to added PGEj. Furthermore, the decreased pressor responses to NA observed in some of the rabbit vascular beds after indomethacin indicate that the sensitivity to NA in these tissues in fact is increased by endogenous prostaglandin-like substances (PLS). The current results thus suggest that endogenous PLS may regulate, at a local level, the vasoconstrictor sensitivity in the rabbit systemic resistance vessels.