A method for studying isolated resistance vessels from rabbit mesentery and brain and their responses to drugs

Development of an ex vivo model for the study of cerebrovascular function utilizing isolated mouse olfactory artery

Objective

Cerebral vessels, such as intracerebral perforating arterioles isolated from rat brain, have been widely used as an ex vivo model to study the cerebrovascular function associated with cerebrovascular disorders and the therapeutic effects of various pharmacological agents. These perforating arterioles, however, have demonstrated differences in the vascular architecture and reactivity compared with a larger leptomeningeal artery which has been commonly implicated in cerebrovascular disease. In this study, therefore, we developed the method for studying cerebrovascular function utilizing the olfactory artery isolated from the mouse brain.

Methods

The olfactory artery (OA) was isolated from the C57/BL6 wild-type mouse brain. After removing connective tissues, one side of the isolated vessel segment (approximately -500 µm in length) was cannulated and the opposite end of the vessel was completely sealed while being viewed with an inverted microscope. After verifying the absence of pressure leakage, we examined the vascular reactivity to various vasoactive agents under the fixed intravascular pressure (60 mm Hg).

Results

We found that the isolated mouse OAs were able to constrict in response to vasoconstrictors, including KCl, phenylephrine, endothelin-1, and prostaglandin PGH₂. Moreover, this isolated vessel demonstrated vasodilation in a dose-dependent manner when vasodilatory agents, acetylcholine and bradykinin, were applied.

Conclusion

Our findings suggest that the isolated olfactory artery would provide as a useful ex vivo model to study the molecular and cellular mechanisms of vascular function underlying cerebrovascular disorders and the direct effects of such disease-modifying pathways on cerebrovascular function utilizing pharmacological agents and genetically modified mouse models.

The ex vivo isolated skeletal microvessel preparation for investigation of vascular reactivity

The isolated microvessel preparation is an ex vivo preparation that allows for examination of the different contributions of factors that control vessel diameter, and thus, perfusion resistance(1-5). This is a classic experimental preparation that was, in large measure, initially described by Uchida et al.(15) several decades ago. This initial description provided the basis for the techniques that was extensively modified and enhanced, primarily in the laboratory of Dr. Brian Duling at the University of Virginia(6-8), and we present a current approach in the following pages. This preparation will specifically refer to the gracilis arteriole in a rat as the microvessel of choice, but the basic preparation can readily be applied to vessels isolated from nearly any other tissue or organ across species(9-13). Mechanical (i.e., dimensional) changes in the isolated microvessels can easily be evaluated in response to a broad array of physiological (e.g., hypoxia, intravascular pressure, or shear) or pharmacological challenges, and can provide insight into mechanistic elements comprising integrated responses in an intact, although ex vivo, tissue. The significance of this method is that it allows for facile manipulation of the influences on the integrated regulation of microvessel diameter, while also allowing for the control of many of the contributions from other sources, including intravascular pressure (myogenic), autonomic innervation, hemodynamic (e.g., shear stress), endothelial dependent or independent stimuli, hormonal, and parenchymal influences, to provide a partial list. Under appropriate experimental conditions and with appropriate goals, this can serve as an advantage over in vivo or in situ tissue/organ preparations, which do not readily allow for the facile control of broader systemic variables. The major limitation of this preparation is essentially the consequence of its strengths. By definition, the behavior of these vessels is being studied under conditions where many of the most significant contributors to the regulation of vascular resistance have been removed, including neural, humoral, metabolic, etc. As such, the investigator is cautioned to avoid over-interpretation and extrapolation of the data that are collected utilizing this preparation. The other significant area of concern with regard to this preparation is that it can be very easy to damage cellular components such as the endothelial lining or the vascular smooth muscle, such that variable source of error can be introduced. It is strongly recommended that the individual investigator utilize appropriate measurements to ensure the quality of the preparation, both at the initiation of the experiment and periodically throughout the course of a protocol.

Role of endothelium and vasoconstrictor prostanoids in norepinephrine-induced vasoconstriction in isolated rat common carotid arteries

We investigated the role of endothelium and vasoconstrictor prostanoids in the norepinephrine (NE)-induced vasoconstriction of isolated rat common carotid arteries (CCAs). Isolated CCAs were cannulated with stainless steel cannulae by the cannula inserting method. NE was administered intra- or extraluminally by a single microinjection. For denudation of endothelium, the intimal surface was gently rubbed with a cotton pellet. NE produced dose-related vasoconstricting responses in isolated arteries with intact endothelium. These responses were attenuated by pretreatment with denudation, OKY046, a thromboxane A2 synthesis inhibitor or indomethacin. The residual responses after denudation were further blocked by OKY046 or indomethacin. All NE-induced responses were blocked by a single injection of prazosin. There were no significant differences in responses between intra- and extraluminal administration of NE. These results show that most vasoconstriction induced by NE via alpha1-adrenoceptors in CCAs is dependent on vasoconstrictor prostanoids distributed both in endothelium and vascular smooth muscle.

Evidence for pH as an important parameter to control when studying the contractility of isolated rabbit cerebral arteries

Isolated rabbit basilar (BA), middle (MCA) and posterior (PCA) cerebral arteries mounted in an isometric Mulvany myograph and stretched to a given level of resting tension, developed a spontaneous slow-rising contraction. This tone presented the main features of a classical myogenic tone since it was not suppressed by repetitive washings, was not dependent on the presence of endothelium and was markedly influenced by the concentration of extracellular calcium. In addition, we observed that the occurrence of myogenic tone was dependent on the pH of the medium buffer. Decreasing pH of the Krebs solution from 7.54 to 6.81 by lowering NaHCO3 concentration from 25 to 5 mM, reduced the amplitude of the myogenic tone developed by the arteries. We investigated the influence of such changes of pH on the contractile response to potassium chloride (KCl) and to 5-hydroxytryptamine (5-HT). We demonstrated that the contractile response to KCl (124 mM) was not affected by the pH of the organ bath whereas the maximum contraction to 5-HT (10 microM) was significantly affected in BA but not in MCA and PCA. Furthermore, we found that three consecutive concentration-response curves to 5-HT were reproducible when obtained at pH 7.15 and 7.30 for BA and MCA. In PCA, 5-HT-induced responses were reproducible at pH 7.30 whereas the sensitivity of the repeated response curves to 5-HT was reduced at pH 7.15. We also noted a larger variability of the response to 5-HT for the three arteries at this pH.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of vasopressin and oxytocin on canine cerebral circulation in vivo

In vivo experiments on the vasoactive effects of vasopressin and oxytocin on cerebral circulation were carried out in anesthetized dogs, using an electromagnetic flowmeter to measure vertebral blood flow and angiography to measure the internal diameter of the basilar artery. Direct bolus infusion of 1 pmol to 1 nmol of vasopressin or 10 pmol to 10 nmol of oxytocin into a femoral-vertebral artery shunt produced a dose-dependent decrease in vertebral artery blood flow without significantly affecting mean arterial blood pressure. Vasopressin was more potent than endothelin and neuropeptide Y, which have also been demonstrated to induce long-lasting decreases in vertebral artery blood flow. However, direct bolus infusion of vasopressin (100 pmol and 1 nmol) or oxytocin (1 nmol and 10 nmol) into the vertebral artery dilated major vessels including the vertebral, anterior spinal, and basilar arteries, as well as the circle of Willis and its main branches, while endothelin (1 nmol) and neuropeptide Y (5 nmol) caused no change in the diameters of major cerebral arteries. The V1 antagonist d(CH2)5tyrosine(methyl) arginine vasopressin suppressed the effects of both vasopressin and oxytocin. Vasopressin was over 10 times as potent as oxytocin in both assays. The vasodilatory effect of vasopressin, which may be mediated by an endothelium-dependent mechanism, was functionally damaged in dogs after experimental subarachnoid hemorrhage. These data suggest regional differences in the sensitivity and responsiveness of vasculature to vasopressin and oxytocin, and specifically that both peptides act through V1 receptors to decrease the resistance of large vessels and increase the resistance of small vessels.

Responses of isolated rabbit thoracic aortae to endothelin-1 and several vasoactive substances. A new blood perfusion circuit under the controlled conditions of resistance and flow

Analysis of the reactivity of isolated rabbit thoracic aorta to isoproterenol and norepinephrine injected into a blood perfusion circuit intraarterially (i.a.) under controlled conditions of resistance and flow using conscious support rabbits revealed that with pressure loads of 10-20 mmHg for a vessel-holding chamber and 60 mmHg for a Starling pneumatic resistance set, a suitable vasodilation or vasoconstriction can be obtained. This leads us to assume that the system could be used to assess the direct influence of vasoactive substances on vasomotor tone and, furthermore, the support rabbit could be used for systemic hemodynamic variables and behavior observation. The i.a. injection of increasing doses of endothelin-1 (ET) (0.01-0.3 microgram) caused a weak but long-lasting and dose-dependent vasoconstriction of the thoracic aorta; its effect was less potent than that of angiotensin II or norepinephrine when their peak responses were compared. The duration of vasoconstriction caused by 0.3 microgram i.a. of ET was approximately 30 min and much longer than that of angiotensin II and norepinephrine. Histamine, serotonin, and prostaglandin E2 produced noticeable dose-dependent vasoconstriction. When ET (0.1-3 micrograms) was injected i.v. to conscious rabbits, the systemic blood pressure, which first dropped and then rose, was accompanied by significant changes in the heart rate in a reciprocal way. The observed rise in the blood pressure with the accompanying decrease in the heart rate lasted for at least 30 min following 1 microgram i.v. of ET.(ABSTRACT TRUNCATED AT 250 WORDS)

In vivo microscopy of the cerebral microcirculation using neonatal allografts in hamsters

Studies were performed to characterize the morphology and vascular reactivity of the allografted cerebral microcirculation. Cerebral cortical tissue was allografted into the cheek pouch of the hamster so that cerebral parenchymal vessels could be studied. The vascular morphology was characterized by a large number of looping vessels. The ultrastructural examination indicated viable cerebral tissue containing typical vessels, that is, "tight" junctions, not like those of the cheek pouch. Also, the microvasculature was impermeable to 150, 70, and 20 kDa fluorescein isothiocyanate dextrans. Angiotensin II and norepinephrine caused constriction of the cerebral vessels whereas adenosine caused dilation. Isoproterenol did not affect cerebral arterioles; however, it dilated cheek pouch arterioles. Thus, this preparation provides a satisfactory model for studying the living cerebral microcirculation.

Humoral regulation of blood flow to choroid plexus: role of arginine vasopressin

The goal of this study was to examine humoral mechanisms that regulate blood flow to the choroid plexus. We determined the effects of arginine vasopressin on blood flow (microspheres) to the choroid plexus in anesthetized and awake rabbits. In anesthetized rabbits, blood flow to the choroid plexus was 342 +/- 31 (mean +/- SEM) ml/min/100 g under control conditions. Intravenous infusion of vasopressin at 4 and 40 mU/kg increased plasma vasopressin levels from 11 +/- 1 to 55 +/- 15 and 441 +/- 120 pg/ml, respectively, and blood flow to the choroid plexus decreased by 48 +/- 6% and 70 +/- 4%. Cerebral blood flow was not affected by infusion of vasopressin. Similar responses to infusion of vasopressin were observed in awake rabbits. The V1 antagonist [d(CH2)5Tyr(Me)AVP] (10 micrograms/kg i.v.) had no effect on resting blood flow, but abolished the effect of vasopressin on blood flow to the choroid plexus. Vasoconstrictor responses of the choroid plexus to intravenous infusion of phenylephrine were not attenuated by the V1 antagonist. Thus, circulating vasopressin, at plasma levels that are observed under physiological and pathophysiological conditions, has marked effects on blood flow to the choroid plexus. These effects appear to be mediated through a V1 receptor. We speculate that vasopressin may play an important role in regulation of blood flow to the choroid plexus and perhaps in the regulation of cerebrospinal fluid production.

Responses of isolated canine and simian basilar arteries to thiopentone by a newly designed pharmacological method for measuring vascular responsiveness

The stainless steel cannula inserting method was modified for application to observe vascular responses to thiopentone in the isolated basilar arteries of the dog and monkey. In the dog, thiopentone (0.01-3 mg) induced a monophasic vasoconstriction in a dose-dependent manner. On the other hand, in the monkey, thiopentone (0.01-3 mg) showed a biphasic vascular response, i.e., an initial vasoconstriction followed by a vasodilatation in a dose-dependent manner. Thiopentone usually produced much more potent vasoconstriction in the dog than that in the monkey, while potassium chloride made little difference of vasoconstriction between the dog and monkey basilar arteries. These findings suggest that thiopentone exerts a direct constrictive effect on cerebral vessels, the actions of which decrease the total cerebral blood volume and the brain bulk, allowing a reduction in intracranial pressure.

Effect of chronic denervation on pharmacological responsiveness of coronary vessels

We have hypothesized that the well-established regional heterogeneity of the properties of the vascular smooth muscle results from peculiarities of the microenvironment in every region. In particular, the coronary vasculature has well-established differences between large and small coronary arteries in their responsiveness to alpha and beta agonists, nitroglycerin and adenosine. To test our hypothesis, we altered the micro-environment of coronary vessels by chronic surgical sympathectomy in dogs. Our in vitro studies on vessels from normally innervated hearts confirmed previous studies and showed that in large vessels, alpha and beta epinephrine or norepinephrine responses can be demonstrated; the alpha effect is dominant and epinephrine is a more potent alpha agonist than is norepinephrine. In small vessels only a beta catecholamine effect can be demonstrated. Chronically sympathectomized blood vessels show an alpha norepinephrine effect in small vessels. Denervation caused a reduction in the sensitivity to K+-induced contraction in both large and small vessels. In large vessels the responses to nitroglycerin were not affected by denervation, whereas in small vessels the sensitivity to adenosine was reduced. These results indicate that chronic denervation alters the pharmacological responsiveness of the vasculature to various agonists indicating that vascular smooth muscle possesses the ability to respond to long-term modulatory influences arising in the immediate environment.

Vasopressin causes endothelium-dependent relaxations of the canine basilar artery

The effect of synthetic 8-arginine vasopressin (vasopressin) was studied in isolated canine basilar, left circumflex coronary, and femoral arteries of the dog. Vascular rings with and without endothelium were suspended for isometric tension recording in physiological salt solution. The removal of the endothelium was confirmed by the absence of relaxations induced by either thrombin (basilar arteries) or acetylcholine (coronary and femoral arteries). In the basilar artery, vasopressin induced concentration-dependent inhibition of myogenic tone. In basilar and coronary arteries, the hormone caused concentration-dependent relaxations during contractions evoked by prostaglandin F2 alpha. In femoral arteries, vasopressin caused contraction. After removal of the endothelium, the inhibitory responses to vasopressin were abolished in basilar arteries and significantly reduced in left circumflex coronary arteries. The contractions of femoral arteries were not affected by endothelium removal. The V1-vasopressinergic antagonist d(CH2)5Tyr(Me)AVP prevented the inhibitory response to vasopressin, but did not alter endothelium-dependent relaxations of basilar arteries caused by adenosine diphosphate. These results demonstrate that the endothelial cells mediate relaxation induced by vasopressin via specific V1-vasopressinergic receptors.

A new method for measuring vascular responsiveness of relatively larger arteries of dogs

A new model of isolated vascular preparation was developed in the dog. Several canine arteries were isolated and suspended in a bath and perfused under a constant flow rate with Ringer-Locke solution. In each experiment, resting perfusion pressure was set at a constant level at 40-60 mm Hg. Drugs were administered in the endothelial side of the artery through a cannulated tubing, and the response was obtained as changes in the perfusion pressure. By inserting the polyethylene cannula into the artery, the space between the luminar wall of the artery and the cannula was decreased enough to obtain a suitable perfusion pressure, while the cannula was filled with the fluid by pumping. Although some other methods have been used to observe contractile responses of the artery, this method might be superior to them because: 1) drugs act only on the luminar wall; 2) vascular responses to drugs, not only constriction, but also dilatation, in nontreated preparations, are readily observed in a relatively larger artery (2-3 mm I.D.); 3) the vascular reactivity is kept in a stable condition over 7-8 hours at 37 degrees C. We used this preparation to examine the contractile responses of the common, internal, external carotid, and femoral arteries to 5-hydroxytryptamine.

Mechanical properties of rat cerebral arteries as studied by a sensitive device for recording of mechanical activity in isolated small blood vessels

A sensitive device for recording of mechanical activity in isolated small blood vessels with calibres down to 100 microns is described. This equipment was used to examine the mechanical properties of rat cerebral arteries. The ultrastructure of the preparations was investigated by light-, transmission, and scanning electron-microscopy. In general the walls of the middle cerebral and basilar arteries consisted of 3 layers of smooth muscle cells, which occupied approximately 80% of the total wall thickness. The present technique preserved the integrity of the vessel wall and caused no observable damage to the smooth muscle or endothelial cells. Neither the basilar nor the middle cerebral arteries developed spontaneous phasic contractions under standard conditions. Potassium excess (124 mM) induced a biphasic contractile response characterized by a fast and partly transient increase in tension (phase A), followed by a slowly developing sustained contraction (phase B). The responses to K+ were strong, highly reproducible and not influenced by pH changes in the range 6.9-7.8, making K+-stimulation suitable for testing of vascular contractility. Length-tension measurements were performed on relaxed and K+-activated basilar arteries. The mechanical behaviour of the vessels conformed to a sliding-filament model of muscular contraction. Using the "Maxwell model" of a muscle, the length at which the contractile element produced maximum active tension was established. The passive wall tension at this length (approximately 1 mN/mm) averaged only about 20% of the total wall tension the arteries were capable of producing when activated by K+. Under isometric conditions the K+-contracted basilar artery developed a maximum active wall stress of approximately 240 mN/mm2. In the light of the mechanical data obtained from the length-tension measurements, the optimum resting wall tension for registration of vascular responses is discussed. It appears that the present in vitro system can be of great value in investigations of the smooth muscle function in small blood vessels.

Membrane properties and excitatory neuromuscular transmission in the smooth muscle of dog cerebral arteries

1 Drug actions on electrical and mechanical properties of smooth muscle cells and neuromuscular transmission in the canine cerebral arteries were investigated by use of microelectrode and isometric tension recording methods. 2 In the basilar and middle cerebral arteries, the resting membrane potentials were--49.4 mV and -51.7 mV, respectively, the length constants 0.57 mm and 0.45 mm, respectively and the time constants 142 ms and 118 ms, respectively. 3 Outward current pulses did not evoke the spike in either artery but did evoke the spike under conditions of pretreatment with 10 mM tetraethylammonium (TEA). 4 The maximum slope of depolarization produced by a ten fold increase in [K]o plotted on a log scale was 40.1 mV in the basilar artery and 42.2 mV in the middle cerebral artery. 2-Nicotinamidoethyl nitrate, the K-permeability accelerator, had no effect on the membrane potential. 5 K-free or ouabain [10(-5)M] treatment slightly depolarized the membrane. Re-addition of K [5.9 mM] hyperpolarized the membrane by several mV. Thus, the contribution of an active Na-K pump in the membrane potential seems to be small. 6 In both arteries, acetylcholine, adenosine, noradrenaline and isoprenaline in concentrations up to 10(-5)M did not modify the membrane potential and resistance, while 5-hydroxytryptamine (over 10(-8)M) and ATP (over 10(-5)M) depolarized the membrane, decreased the membrane resistance and produced a dose-dependent contraction. Adenosine suppressed the contraction evoked by excess [K]o (39.8 mM). 7 Perivascular nerve stimulation produced excitatory junction potentials (e.j.ps). Often e.j.ps were followed by a hyperpolarization. Repetitive stimulation produced facilitation after several stimuli and depression followed. In some cells, this depression appeared without facilitation. 8 The e.j.ps ceased with pretreatment with guanethidine (10(-6)M) or tetrodotoxin (3 X 10(-7)M), while phentolamine (10(-7)M) and yohimbine (10(-7)M) enhanced the amplitude of e.j.ps. ATP (10(-5)M) and noradrenaline (10(-6)M) suppressed and prazosin had little effect on the e.j.ps. Atropine (10(-6)M) also had no effect on the e.j.ps. 9 Specific features of the cerebral artery and systemic vascular beds were compared, and the features of adrenoceptors on the smooth muscle membrane were compared with findings in other vascular beds.

Correlated electrical and mechanical responses of isolated rabbit pial arteries to some vasoactive drugs

Simultaneous measurements were made of spike activity and perfusion pressure (PA) in intact segments of rabbit middle cerebral artery in vitro. The segments were mounted on a Teflon tube designed so that the perfusing solution flowed in the annular space between the tube and the artery wall, thus magnifying the PA changes occurring when the artery constricted or dilated. A widened portion of the Teflon tube immobilized 1--2 mm of the artery segment for electrical recording with fine glass microelectrodes. Spontaneous spike activity (extra- and intracellular) was regularly observed. When a steady PA and spike discharge was obtained, tests were performed by substituting for the normal perfusion liquid, solutions containing 5 microgram/ml norepinephrine, 5 microgram/ml angiotensin II or 7.5 microgram/ml isoproterenol. Norepinephrine and angiotensin each increased spike frequency (+ 293 and + 126%) and PA (+ 6.6 and + 7.9 mm Hg) whereas isoproterenol decreased spike frequency (-89%) and PA (-22.9 mm Hg). These results a) confirm the presence of receptors to these agents in pial arteries, and b) demonstrate a high degree of correlation between membrane electrical events and mechanical activity of these spontaneously-active myovascular cells.

Selective inhibitory actions of sodium-p-benzyl-4-[1-oxo-2-(4-chlorobenzyl)-3-phenyl propyl] phenyl phosphonate (N-0161) and indomethacin on the biosynthesis of prostaglandins and thromboxanes from arachidonic acid

1 Sodium p-benzyl-4-[1-oxo-2-(4-chlorobenzyl)-3-phenyl propyl]phenyl phosphonate (N-0164) selectively inhibited the formation of thromboxane-A(2) from prostaglandin endoperoxides by human platelet microsomes in a dose-dependent manner (IC(50) 2.2 x 10(-5) M or 11.6 mug/ml).2 N-0164 was approximately 15 to 20 times as potent as indomethacin as an inhibitor of thromboxane-A(2) formation. In contrast, indomethacin was 20 times as potent as N-0164 as an inhibitor of prostaglandin endoperoxide formation from arachidonic acid (IC(50) 2.6 x 10(-5) M or 9.4 mug/ml).3 Spiral strips of dog coronary arteries relaxed in the presence of prostaglandin endoperoxides and were contracted by prostaglandin E(2) and thromboxane-A(2) and were therefore used to distinguish between prostaglandins and their intermediate precursors, the endoperoxides.4 Neither indomethacin nor N-0164 (both 50 mug/ml) significantly inhibited the formation of prostaglandin-like activity from the endoperoxides following incubation with indomethacin-pretreated rabbit kidney medulla microsomes.5 It is not known whether this action of N-0164 is related to its ability to antagonize certain actions of prostaglandins (and related compounds) or whether N-0164 can penetrate the cell membrane to inhibit thromboxane formation in the intact cell.6 Selective inhibition of thromboxane formation by drugs such as N-0164 may be useful both clinically and as a pharmacological tool to elucidate the patho-physiological roles of the thromboxanes.

Inhibition by Cd2+ verapamil and papaverine of Ca2+-induced contractions in isolated cerebral and peripheral arteries of the dog

1 In helically cut strips of canine cerebral arteries exposed to Ca2+-free media and depolarized by K+, the addition of Ca2+ caused biphasic (transient and sustained) contractions, while in coronary and mesenteric arteries, the additon of Ca2+ produced a sustained contraction sometimes preceded by a slight transient contraction. 2. These Ca2+-induced contractions were attenuated by Cd2+ (50 to 100 micron) in a dose-dependent manner, the attenuation being greater in cerebral than in coronary and mesenteric arteries. The inhibitory effect of Cd2+ was prevented and partially reversed by 1 mM cysteine. 3. Verapamil and papaverine were also effective in attenuating the Ca2+-induced contrations in cerebral and peripheral arteries: susceptibility to verapamil was in the order, cerebral greater than coronary greater than mesenteric, while that to papaverine was in the order, cerebral=coronary greater than mesenteric. 4. It may be concluded that the agents that interfere with trans-membrane influxes of Ca2+ cause a greater relaxation in cerebral than in peripheral arteries, as is seen with papaverine, a non-specific vasodilator.

The reactivity of canine cerebral arteries to O2 and CO2 in vitro

The responses of canine middle cerebral arteries to changes in pCO2 and pO2 were tested in vitro. It was found that there was no response to changes in pCO2 from 38.1 mm. Hg to 26.6 mm. Hg, but there was some constriction of the vessels with lowering of the pCO2 below 26.6 mm. Hg and there was minimal dilatation of the vessels when the pCO2 was increased from 38.1 mm. Hg to 87.2 mm. Hg. There was no response to changes in pO2 from more than 55 mm. Hg to 59.6 mm. Hg, but when pO2 was lowered below 50 mm. Hg there was a sudden, massive constriction of the arteries tested. It is postulated that this constriction is due to build-up of a substance (substances) during a period of hypoxia (pO2 less than 50 mm. Hg). The significance of the results obtained are discussed.

Determinants of response of pial arteries to norepinephrine and sympathetic nerve stimulation

Feline pial arteries larger than 100 mu in diameter constricted in response to cervical sympathetic nerve stimulation suggests or in response to topical application of norepinephrine. Smaller pial arteries were unresponsive to norepinephrine. This unresponsiveness persisted when norepinephrine was dissolved in CSF with high calcium ion concentration, or in CSF with both high calcium ion and zero magnesium ion concentration, or when it was dissolved in the acid fluid used by Wahl et al. and applied by constant infusion or by intermittent application. Comparison of the responses of the larger pial vessels to norepinephrine and to sympathetic nerve stimulation that maximal activation of sympathetic nerves achieves a concentration of released norepinephrine equal to 5.9 x 10(-6) M. The constriction of the larger pial vessels in response to sympathetic nerve stimulation could account for modest reductions in cerebral blood flow.

Chronic cerebral arterial spasm. The role of intracranial pressure

The author used isolated rabbit common carotid and femoral arteries perfused at a constant pressure of 90 mm Hg to examine the variation of flow (F) with transmural pressure (TMP). When the TMP was reduced below 50 to 60 mm Hg in arteries with normal smooth muscle tone, arterial resistance increased significantly causing a reduction in flow. It is suggested that the diffuse arterial narrowing that occurs in patients with severe intracranial hypertension may be the result of a similar reduction in TMP. In the presence of active vasoconstriction, any increase in extraluminal (intracranial) pressure (ICP) resulted in a substantial increase in arterial resistance and subsequent reduction of flow. This F-TMP relationship depended only on the initial degree of constriction and was independent of the vasoconstrictor used to achieve this constriction and of the artery in which this constriction was produced. A review of the literature suggests that human cerebral arteries normally exhibit only mild constrictions in response to subarachnoid blood during the chronic phase of spasm. In the present study, a mild constriction in the absence of increased ICP or a moderate increase in ICP (45 mm Hg) in the absence of constriction produced minor reductions in arterial diameter and an average flow reduction of only 5% to 10%. However, when ICP was increased to 45 mm Hg in the presence of a mild constriction, severe arterial narrowing resulted and flow was reduced by 50%. Therefore, it is suggested that chronic arterial spasm is the result of a mild constriction which is amplified by the simultaneous occurrence of increased ICP. Phenoxybenzamine was found to be effective in reversing and preventing these contractions. The improvement in flow produced by phenoxybenzamine decreased as the TMP was reduced below 60 mm Hg. The effects of both diffuse and local spasm on cerebral blood flow are discussed.

Simple isolated perfused artery preparation: vasoconstrictor evaluation

Small mesenteric arteries free of all extraarterial tissues were obtained from anesthetized dogs and perfused in vitro with Krebs solution. Vasoconstrictor responses of these arteries to intraarterial levarterenol and epinephrine were dose related and equivalent to those of arteries surrounded by fat and other tissues. Responsiveness was stable for at least 60 min. This simple preparation is useful for the study of vasoconstrictor phenomena uncomplicated arterial tissue.

Comparative effectiveness of alpha-blockade and beta-stimulation in modifying experimental basilar arterial spasm

✓The comparative effectiveness of isoproterenol (Isuprel), phentolamine (Regitine), and phenoxybenzamine (Dibenzyline) in modifying basilar arterial spasm induced by the topical application of arterial blood was studied in cats. None of these agents was effective in inhibiting initial peak constriction. Each reduced the duration of the vasospasm. The results indicate that both alpha-blockade and beta-stimulation can alter the course of blood-induced cerebral vasospasm.

Localized neurogenic vasoconstriction of the basilar artery

Ring segments from the caudal end of the basilar artery of the rabbit contract to electrical stimulation of their intramural nerve supply more than those from adjacent parts of the vertebral and the rostral three-fourths of the basilar arteries. The magnitude of developed neurogenic tension is approximately one-fourth that of the ear artery, a highly reactive muscular artery. The position of this reactive arterial segment suggests its role in the regulation of cerebral blood flow.

The effects of arterial blood pressure on the regional cerebral blood volume by X-ray fluorescence

Cerebral blood flow (CBF) remains constant over a wide range of arterial blood pressure. This is thought to be accomplished by changes in the diameter, and therefore the volume, of the cerebral resistance vessels. To test this hypothesis, regional cerebral blood volume (rCBV) was measured in vivo in Rhesus monkeys over a range of mean arterial blood pressures (MABP) of 35 to 200 torr. Multiple measurements were made in each animal by the method of stimulated x-ray fluorescence. A significant linear relationship of rCBV = 6.26 (±0.47 SD) -0.015 (±0.004 SD) MABP was found. For each one torr change in the MABP, there is a change in rCBV of 0.015 cc/100 gm of brain tissue over a range of MABP of 35 to 200 torr. An additional observation of this investigation was that autoregulation of the cerebral blood flow (CBF) is perturbed for a period lasting up to 15 minutes after the intravenous injection of Renografin 76 R .

Humoral responses of smooth muscle from rabbit subarachnoid artery compared to kidney, mesentery, lung, heart, and skin vascular smooth muscle

Smooth muscle strips from rabbit subarachnoid arteries did not respond to catecholamines in concentrations which caused strips from skin, mesentery, and kidney vessels to contract. The strips from subarachnoid arteries responded to serotonin, histamine, and angiotensin II but not to bradykinin, acetylcholine, methacholine, and adenosine phosphate compounds. Heart and lung vascular strips did not respond to catecholamines but responded to acetylcholine; cardiac strips responded to dilute methacholine. Lung strips often failed to respond to histamine in usual concentrations and did respond to bradykinin. The subarachnoid strips had a distinactive pattern of respone as compared to vascular tissue from other organs. Results of other isolated vessel studies are reviewed along with pertinent investigations of cerebral autoregulation, vasospasm, and autonomic nerves. The possible significance of these findings is discussed.

Effect of 1-norepinephrine on the diameter of pial arterioles and arteries in the cat

The influence of norepinephrine on the diameter of single pial arteries and arterioles was investigated by adding the drug to the perivascular space with micropipettes. The mock spinal fluid solution in which the norepinephrine was dissolved contained 0, 11, or 22 mEq/liter of bicarbonate. These concentrations of bicarbonate were by themselves found to dilate, cause no change, and constrict the pial vessels, respectively. Concentration-response curves with 11 mEq/liter of bicarbonate over the concentration range of 5 x 10 -4 to 5 mmoles/liter of norepinephrine showed significant constriction at 5 x 10 -2 mmoles/liter and maximal constriction (40% of diameter) at 2.5 mmoles/liter. In bicarbonate-free solution, the slope of the concentration-response curves was less, and at 22 mEq/liter of bicarbonate norepinephrine had no effect. The pial arteries seemed to be less sensitive than the mesenteric and the cremasteric arteries to norepinephrine. The present data demonstrate the existence of norepinephrine receptors on the pial arterial smooth muscle cells, which satisfies the major requirement for the possible existence of a sympathetic control of cerebral blood flow.

Unresponsiveness of pial precapillary vessels to catecholamines and sympathetic nerve stimulation

A systematic analysis of the possible existence of neurogenic control of precapillary pial vessels was made in three species (cat, dog, and rabbit). In all of these animals, pial vessels failed to respond to externally applied isoproterenol or norepinephrine in high concentrations (up to 100 µg/ml), although the vessels did dilate in response to externally applied histamine. Adrenergic nerve endings on the pial vessels were demonstrated by fluorescent histochemical techniques specific for catecholamines. However, in the absence of changes in arterial blood pressure and arterial blood carbon dioxide tension, pial precapillary vessels showed no change in caliber in response to stimulation of the ipsilateral superior cervical ganglion. These results show that pial precapillary vessels are not subject to vasoconstriction probably because they lack sufficient receptors for the catecholamine neurotransmitter.

thermoregulatory effects of intraventricular injection of noradrenaline in the mouse and the influence of ambient temperature

1. At an ambient temperature of 20 degrees C, intraventricular injection of noradrenaline in the mouse resulted in hypothermia accompanied by a fall in metabolic rate and by cutaneous vasodilatation. Subcutaneous injection of noradrenaline resulted in hyperthermia with raised metabolic rate and cutaneous vasodilatation.2. The hypothermia and fall in oxygen consumption rate following intraventricular noradrenaline were prevented by pre-treatment with subcutaneous propranolol, while the cutaneous vasodilatation was un-affected. However, the effects of subcutaneously injected noradrenaline were completely abolished by subcutaneous propranolol. Intraventricular propranolol did not modify the hypothermic effect of intraventricular noradrenaline.3. The direction of the effect on body temperature of intraventricular noradrenaline was dependent upon ambient temperature; hypothermia occurring at low (15 degrees C) and hyperthermia at high (36 degrees C) ambient temperatures. However, when the possibility of any peripheral action of noradrenaline escaping into the systemic circulation was prevented by prior subcutaneous injection of propranolol, significant hypothermia could be detected at temperatures as high as 32 degrees C.4. The possibility that the effects of intraventricular noradrenaline could be due to complete abolition of central temperature regulation was further excluded by the occurrence of thermal salivation in all animals during experiments performed at 36 degrees C.5. It is suggested that, in the mouse, the hypothermic actions of intraventricular noradrenaline are due to a central effect, while its hyperthermic effects at high ambient temperature are due to escape of noradrenaline into the peripheral circulation. The hypothermia could be the result of selective activation of central heat loss mechanisms.6. Intraventricular noradrenaline was without effect on brain plasma-space although exposure to 100% oxygen caused a detectable fall.

Vascular resistance in the perfused isolated rat tail

1. A constant flow perfusion system using the isolated rat tail has been developed to facilitate the study of resistance vessel behaviour and the action of vasoactive drugs.2. Baseline resistance remains stable for several hours and dose response curves to bolus injections of pressor agents are reproducible when dialysed bovine serum albumen is used in the perfusion medium to maintain osmotic pressure.3. Noradrenaline, adrenaline, serotonin, vasopressin, angiotensin II, high potassium concentrations and sympathetic nerve stimulation constricted the vascular bed.4. Angiotensin I, bradykinin, histamine, acetylcholine and isoprenaline did not alter vascular resistance under baseline conditions.5. Maximal sensitivity to noradrenaline occurred at 32 degrees to 34 degrees C. Below 30 degrees C, resting tone increased and the pressor effect of noradrenaline was prolonged.6. Low concentrations of (+/-)-propranolol in the perfusate enhanced adrenaline and noradrenaline vasoconstriction, high concentration of (+/-)-propranolol had a direct pressor effect and did not affect catecholamine responses.7. The preparation is a simple and relatively inexpensive adjunct to established methods of studying resistance vessel behaviour under varying experimental conditions.

Myogenic tone in isolated perfused vessels. Occurrence among vascular beds and along vascular trees

In situ studies yield only indirect evidence as to whether non-neurogenic vascular tone results from an intrinsic myogenic tendency of the smooth muscle cell to contract or is due to stimulation of the muscle by some vasoactive humoral influence of the environment. In the current study, small resistance vessels from different vascular beds, from various levels of the arterial tree, and from several species were isolated and perfused with physiological salt solution. Under these conditions the environment is devoid of any vasoactive, in-situ, humoral influence, and tonic contraction would have to be of myogenic origin. Myogenic tone was consistently present in smooth muscle from some vascular beds, whereas it was absent from other beds. Myogenic tone was more prevalent in smaller vessels (50 to 100µ o.d.) than in larger ones (100 to 400µ o.d.). Significant differences in this property were observed among the species studied. We conclude that, contrary to the properties classically assigned it as a multiunit smooth muscle, most vascular smooth muscle from resistance vessels is intrinsically and spontaneously active.

The effects of chloralose-urethane and sodium pentobarbitone anaesthesia on the local and autonomic components of the circulatory response to arterial hypoxia

1. The circulatory and respiratory responses to severe arterial hypoxia were studied in normal rabbits, ;de-efferented' rabbits without functioning autonomic effectors, and atropinized animals before anaesthesia and during chloralose-urethane and sodium pentobarbitone anaesthesia. Net systemic autonomic activity and autonomic activity to the heart was assessed from a comparison of the responses of the various preparations.2. In the normal spontaneously breathing animal each anaesthetic had a similar mode of action, and modified qualitatively the circulatory response present before anaesthesia. In the ;de-efferented' animal the circulatory response was determined by the local effects of hypoxia, and was altered only quantitatively during anaesthesia.3. In the normal unanaesthetized animal the reflex changes in autonomic activity during hypoxia consisted of a large increase in vagal efferent activity, a decrease in cardiac sympatho-adrenal activity, and an increase in total sympatho-adrenal constrictor activity.4 In hypoxia during anaesthesia the vagal efferent activity no longer increased, but the change in sympatho-adrenal activity to heart and systemic circulation was the same as before anaesthesia in the spontaneously breathing animal. During anaesthesia with controlled ventilation systemic sympatho-adrenal activity increased further, and bradycardia again developed. The bradycardia was now due exclusively to reduction in cardiac sympathetic activity and not to an increase in vagal efferent activity.