Cerebrovascular responses to haemorrhagic hypotension in anaesthetized cats. Effects of alpha-adrenoceptor antagonists

Effect of pregnancy and nitric oxide on the myogenic vasodilation of posterior cerebral arteries and the lower limit of cerebral blood flow autoregulation

Hemorrhage during parturition can lower blood pressure beyond the lower limit of cerebral blood flow (CBF) autoregulation that can cause ischemic brain injury. However, the impact of pregnancy on the lower limit of CBF autoregulation is unknown. We measured myogenic vasodilation, a major contributor of CBF autoregulation, in isolated posterior cerebral arteries (PCAs) from nonpregnant and late-pregnant rats (n = 10/group) while the effect of pregnancy on the lower limit of CBF autoregulation was studied in the posterior cerebral cortex during controlled hemorrhage (n = 8). Pregnancy enhanced myogenic vasodilation in PCA and shifted the lower limit of CBF autoregulation to lower pressures. Inhibition of nitric oxide synthase (NOS) prevented the enhanced myogenic vasodilation during pregnancy but did not affect the lower limit of CBF autoregulation. The shift in the autoregulatory curve to lower pressures during pregnancy is likely protective of ischemic injury during hemorrhage and appears to be independent of NOS.

Cerebral blood flow autoregulation in experimental liver failure

Patients with acute liver failure (ALF) display impairment of cerebral blood flow (CBF) autoregulation, which may contribute to the development of fatal intracranial hypertension, but the pathophysiological mechanism remains unclear. In this study, we examined whether loss of liver mass causes impairment of CBF autoregulation. Four rat models were chosen, each representing different aspects of ALF: galactosamine (GlN) intoxication represented liver necrosis, 90% hepatectomy (PHx90) represented reduction in liver mass, portacaval anastomosis (PCA) represented shunting of blood/toxins into the systemic circulation thus mimicking intrahepatic shunting in ALF, PCA+NH(3) provided information about the additional effects of hyperammonemia Rats were intubated and sedated with pentobarbital. We measured CBF with laser Doppler, intracranial pressure (ICP) was measured in the fossa posterior and registered with a pressure transducer, brain water was measured using the wet-to-dry method, and cerebral glutamine/glutamate was measured enzymatically. The CBF autoregulatory index in both the GlN and PHx90 groups differed significantly from the control group. Conversely, CBF autoregulation was intact in the PCA and PCA+NH(3) groups despite high arterial ammonia, high cerebral glutamine concentration, and increased CBF and ICP. Increased water content of the brainstem or cerebellum was not associated with defective CBF autoregulation. In conclusion, impairment of CBF autoregulation is not caused by brain edema/high ICP. Nor does portacaval shunting or hyperammonemia impair autoregulation. Rather, massive liver necrosis and reduced liver mass are associated with loss of CBF autoregulation.

Cerebrovascular responsiveness to steady-state changes in end-tidal CO2 during passive heat stress

This study tested the hypothesis that passive heat stress alters cerebrovascular responsiveness to steady-state changes in end-tidal CO(2) (Pet(CO(2))). Nine healthy subjects (4 men and 5 women), each dressed in a water-perfused suit, underwent normoxic hypocapnic hyperventilation (decrease Pet(CO(2)) approximately 20 Torr) and normoxic hypercapnic (increase in Pet(CO(2)) approximately 9 Torr) challenges under normothermic and passive heat stress conditions. The slope of the relationship between calculated cerebrovascular conductance (CBVC; middle cerebral artery blood velocity/mean arterial blood pressure) and Pet(CO(2)) was used to evaluate cerebrovascular CO(2) responsiveness. Passive heat stress increased core temperature (1.1 +/- 0.2 degrees C, P < 0.001) and reduced middle cerebral artery blood velocity by 8 +/- 8 cm/s (P = 0.01), reduced CBVC by 0.09 +/- 0.09 CBVC units (P = 0.02), and decreased Pet(CO(2)) by 3 +/- 4 Torr (P = 0.07), while mean arterial blood pressure was well maintained (P = 0.36). The slope of the CBVC-Pet(CO(2)) relationship to the hypocapnic challenge was not different between normothermia and heat stress conditions (0.009 +/- 0.006 vs. 0.009 +/- 0.004 CBVC units/Torr, P = 0.63). Similarly, in response to the hypercapnic challenge, the slope of the CBVC-Pet(CO(2)) relationship was not different between normothermia and heat stress conditions (0.028 +/- 0.020 vs. 0.023 +/- 0.008 CBVC units/Torr, P = 0.31). These results indicate that cerebrovascular CO(2) responsiveness, to the prescribed steady-state changes in Pet(CO(2)), is unchanged during passive heat stress.

The effect of S. pneumoniae bacteremia on cerebral blood flow autoregulation in rats

In the present study, we studied the effect of bacteremia on cerebral blood flow (CBF) autoregulation in a rat model of pneumococcal bacteremia and meningitis. Anesthetized rats were divided into five groups (A to E) and inoculated with pneumococci intravenously and normal saline intracisternally (group A, N=10); saline intravenously and pneumococci intracisternally (group B, N=10); pneumococci intravenously and pneumococci intracisternally (group C, N=5); saline intravenously, antipneumococcal antibody intravenously (to prevent bacteremia), and pneumococci intracisternally (group D, N=10); or saline intravenously and saline intracisternally (group E, N=10), respectively. Positive cultures occurred in the blood for all rats in groups A, B, and C, and in the cerebrospinal fluid for all rats in groups D and E. Twenty-four hours after inoculation, CBF was measured with laser-Doppler ultrasound during incremental reductions in cerebral perfusion pressure (CPP) by controlled hemorrhage. Autoregulation was preserved in all rats without meningitis (groups A and E) and was lost in 24 of 25 meningitis rats (groups B, C, and D) (P<0.01). In group A, the lower limit was higher than that of group E (P<0.05). The slope of the CBF/CPP regression line differed between the meningitis groups (P<0.001), being steeper for group B than groups C and D, with no difference between these two groups. The results suggest that pneumococcal bacteremia in rats triggers cerebral vasodilation, which right shifts the lower limit of, but does not entirely abolish, CBF autoregulation in the absence of meningitis, and which may further aggravate the vasoparalysis induced by concomitant pneumococcal meningitis.

Reflex-mediated reduction in human cerebral blood volume

Adrenergic nerves innervate the human cerebrovasculature, yet the functional role of neurogenic influences on cerebral hemodynamics remains speculative. In the current study, regional cerebrovascular responses to sympathoexcitatory reflexes were evaluated. In eight volunteers, contrast-enhanced computed tomography was performed at baseline, -40 mmHg lower body negative pressure (LBNP), and a cold pressor test (CPT). Cerebral blood volume (CBV), mean transit time (MTT), and cerebral blood flow (CBF) were evaluated in cortical gray matter (GM), white matter (WM), and basal ganglia/thalamus (BGT) regions. Lower body negative pressure resulted in tachycardia and decreased central venous pressure while mean arterial pressure was maintained. Cold pressor test resulted in increased mean arterial pressure concomitant with tachycardia but no change in central venous pressure. Neither reflex altered end-tidal carbon dioxide. Cerebral blood volume was reduced in GM during both LBNP and CPT (P<0.05) but was unchanged in WM and BGT. Mean transit time was reduced in WM and GM during CPT (P<0.05). Cerebral blood flow was only modestly affected with either reflex (P<0.07). The combined reductions in GM CBV (approximately -25%) and MTT, both with and without any change in central venous pressure, with small CBF changes (approximately -11%), suggest that active venoconstriction contributed to the volume changes. These data demonstrate that CBV is reduced during engagement of sympathoexcitatory reflexes and that these cerebrovascular changes are heterogeneously distributed.

Pre- and postjunctional alpha(2)-adrenergic receptors in fetal and adult ovine cerebral arteries

In ovine cerebral arteries, adrenergic-mediated vasoconstrictor responses differ significantly with developmental age. We tested the hypothesis that, in part, these differences are a consequence of altered alpha(2)-adrenergic receptor (alpha(2)-AR) density and/or affinity. In fetal (approximately 140 days) and adult sheep, we measured alpha(2)-AR density and affinity with the antagonist [(3)H]idazoxan in main branch cerebral arteries and other vessels. We also quantified contractile responses in middle cerebral artery (MCA) to norepinephrine (NE) or phenylephrine in the presence of the alpha(2)-AR antagonists yohimbine and idazoxan and contractile responses to the alpha(2)-AR agonists clonidine and UK-14304. In fetal and adult cerebral artery homogenates, alpha(2)-AR density was 201 +/- 18 and 52 +/- 6 fmol/mg protein, respectively (P < 0.01); however, antagonist affinity values did not differ. In fetal, but not adult, MCA, 10(-7) M yohimbine significantly decreased the pD(2) for NE-induced tension in the presence of 3 x 10(-5) M cocaine, 10(-5) M deoxycorticosterone, and 10(-6) M tetrodotoxin. In fetal, but not adult, MCA, UK-14304 induced a significant decrease in pD(2) for the phenylephrine dose-response relation. In addition, stimulation-evoked fractional NE release was significantly greater in fetal than in adult cerebral arteries. In the presence of 10(-6) M idazoxan to block alpha(2)-AR-mediated inhibition of prejunctional NE release, the fractional NE release was significantly increased in both age groups. We conclude that in fetal and adult ovine cerebral arteries, alpha(2)-AR appear to be chiefly prejunctional. Nonetheless, the fetal cerebral arteries appear to have a significant component of postjunctional alpha(2)-AR.

Effect of alpha-adrenergic blockade on the cerebrovascular response to increased intracranial pressure after hemorrhage

OBJECT

In this study the authors tested the hypothesis that hemorrhagic hypotension and high intracranial pressure induce an increase in cerebrovascular resistance that is caused by sympathetic compensatory mechanisms and can be modified by alpha-adrenergic blockade.

METHODS

Continuous measurements of cerebral blood flow were obtained using laser Doppler microprobes placed in the cerebral cortex in anesthetized pigs during induced hemorrhagic hypotension and high cerebrospinal fluid pressure. Eight pigs received 2 mg/kg phentolamine in 10 ml saline, and 13 pigs served as control animals. During high intracranial pressure occurring after blood loss, cerebral perfusion pressure (CPP) (p < 0.01) and cerebral blood flow (p < 0.01) decreased in both groups. Cerebrovascular resistance increased (p < 0.05) in the control group and decreased (p < 0.005) in the phentolamine-treated group. The cerebrovascular resistance was significantly lower in the phentolamine-treated group (p < 0.05) than in the control group. Cerebrovascular resistance increased at lower CPPs in the control group (linear correlation, r = 0.39, p < 0.01) and decreased with decreasing CPP in the phentolamine-treated group (linear correlation, r = 0.76, p < 0.001).

CONCLUSIONS

This study shows that the deleterious effects on cerebral hemodynamics induced by blood loss in combination with high intracranial pressure are inhibited by alpha-adrenergic blockade. This suggests that these responses are caused by alpha-adrenergically mediated cerebral vasoconstriction.

The optimal test dose of epinephrine for epidural injection with lidocaine solution in awake patients premedicated with oral clonidine

We attempted to determine the optimal test dose of epinephrine for use with epidural anesthesia in awake patients premedicated with clonidine. Eighty-eight adult patients were randomized into two groups [oral premedication with clonidine 5 microg/kg (CLON) or no premedication (CONT)]. Before induction of general anesthesia, heart rate (HR) and blood pressure (BP) were measured for 3 min after the i.v. injection of 3 mL of 1.5% lidocaine containing epinephrine (0, 1.25, 2.5, 5, 7.5, or 15 microg) in a randomized, double-blind manner. We calculated 95% confidence intervals for the peak HR and BP increases induced by each dose of epinephrine. At 7.5 microg, epinephrine induced a significantly greater increase in HR and BP in CLON than in CONT. The 95% confidence interval for the HR change induced by 7.5 microg of epinephrine in CLON was nearly the same as the accepted standard dose of epinephrine (15 microg) in CONT. We conclude that premedication with clonidine enhances HR and BP responses to the i.v. administration of epinephrine-containing epidural test solutions. Consequently, 7.5 microg of epinephrine may be sufficient to enable detection of accidental injection into a blood vessel in awake patients premedicated with clonidine 5 microg/kg.

IMPLICATIONS

Clonidine, a commonly used preanesthetic medication, alters patients' cardiovascular responses to drugs such as epinephrine. Our randomized, double-blind study suggests that, in awake patients receiving oral clonidine premedication, 7.5 microg of epinephrine (half the usual dose) is adequate as an indicator of accidental injection into the epidural vessels during epidural anesthesia.

Effect of hemorrhagic hypotension on cortical oxygen pressure and striatal extracellular dopamine in cat brain

This study investigated the relationships between blood pressure, cortical oxygen pressure, and extracellular striatal dopamine in the brain of adult cats during hemorrhagic hypotension and retransfusion. Oxygen pressure in the blood of the cortex was measured by the oxygen dependent quenching of phosphorescence and extracellular dopamine, dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) by in vivo microdialysis. Following a 2 h stabilization period after implantation of the microdialysis probe in the striatum, the mean arterial blood pressure (MAP) was decreased in a stepwise manner from 132 +/- 2 Torr (control) to 90 Torr, 70 Torr and 50 Torr, holding the pressure at each level for 15 min. The whole blood was then retransfused and measurements were continued for 90 min. As the MAP was lowered there was a decrease in arterial pH, from a control value of 7.37 +/- 0.05 to 7.26 +/- 0.06. The PaCO2 decreased during bleeding from 32.3 +/- 4.8 Torr to 19.6 +/- 3.6 Torr and returned to 30.9 +/- 3.9 Torr after retransfusion. The PaO2 was 125.9 +/- 15 Torr during control conditions and did not significantly change during bleeding. Cortical oxygen pressure decreased with decrease in MAP, from 50 +/- 2 Torr (control) to 42 +/- 1 Torr, 31 +/- 2 Torr and 22 +/- 2 Torr, respectively. A statistically significant increase in striatal extracellular dopamine, to 2,580 +/- 714% of control was observed when MAP decreased to below 70 Torr and cortical oxygen pressure decreased to below 31 Torr. When the MAP reached 50 Torr, the concentration of extracellular dopamine increased to 18,359 +/- 2,764% of the control value. A statistically significant decrease in DOPAC and HVA were observed during the last step of bleeding. The data show that decreases in systemic blood pressure result in decrease in oxygen pressure in the microvasculature of the cortex, suggesting vascular dilation is not sufficient to result in a full compensation for the decreased MAP. The decrease in cortical oxygen pressure to below 32 Torr is accompanied by a marked increase in extracellular dopamine in the striatum, indicating that even such mild hypoxia can induce significant disturbance in brain metabolism.

Differential effects of increasing doses of alpha-trinositol on cerebral blood flow autoregulation

The effect of neuropeptide Y inhibition with alpha-trinositol on the cerebral blood flow autoregulation was studied in Wistar Kyoto rats. alpha-Trinositol was tested in two doses: one dose (5 mg kg-1 hr-1) selectively affecting neuropeptide Y and one higher dose (50 mg kg-1 hr-1) affecting both neuropeptide Y and the adrenergic response. The cerebral blood flow was measured with the intracarotid 133xenon injection method in halothane nitrous oxide-anaesthetized animals. Blood pressure was raised by norepinephrine infusion and lowered by controlled haemorrhage in separate groups of rats. In addition we examined the effect of alpha-trinositol on neuropeptide Y-induced contraction of cerebral vessels in vitro. The in vitro study demonstrated inhibition of neuropeptide Y-induced contraction with a alpha-trinositol dose selective of neuropeptide Y. The in vivo study demonstrated that cerebral blood flow autoregulation was preserved after both doses of alpha-trinositol. alpha-Trinositol in the low neuropeptide Y-selective dose (5 mg kg-1 hr-1) did not affect the blood pressure limits of cerebral blood flow autoregulation, but the higher dose (50 mg kg-1 hr-1) of alpha-trinositol shifted the upper blood pressure limit of cerebral blood flow autoregulation towards lower blood pressures, an effect probably due to inhibition of both the adrenergic and neuropeptide Y systems.

Dexmedetomidine-induced decrease in cerebral blood flow is attenuated by verapamil in rats: a laser Doppler study

This study was performed to examine the changes in local cortical blood flow (CoBF) after simultaneous administration of an alpha 2 adrenergic agonist (dexmedetomidine) and a calcium channel antagonist (verapamil) to urethane-anaesthetized rats. Dexmedetomidine (100 micrograms.kg-1) given intraperitoneally alone resulted in decreases in mean arterial blood pressure (MABP) (F[27,140] = 3.43; P < 0.01) and CoBF (F[27,140] = 4.22; P < 0.01), whereas the heart rate (HR) was increased (F[27,140] = 2.33; P < 0.01). Verapamil (2.5 mg.kg-1) given subcutaneously reduced the MABP (F[27,140] = 3.41; P < 0.01), but the HR and CoBF were not changed. Combined administration of the drugs decreased MAPB (F[27,140] = 5.37; P < 0.01), with no changes in CoBF and HR. The present data indicate that the calcium channel antagonist verapamil did not potentiate the haemodynamic effects of dexmedetomidine in rats, but rather attenuated the effect of dexmedetomidine on CoBF. This favourable interaction suggests a potential therapeutic role of these agents in maintaining cardiovascular stability during surgical interventions.

Examination of the involvement of neuropeptide Y (NPY) in cerebral autoregulation using the novel NPY antagonist PP56

The contribution of neurotransmitters known to be present in the cervical sympathetic nervous system to cerebral autoregulation was evaluated in the anaesthetised cat using a continuous measurement of cerebral cortical perfusion with laser Doppler flowmetry and an in vitro pial vessel preparation. Autoregulation was tested by venesection and fluid administration to achieve changes in blood pressure from -40% of resting control levels to +80% and flow was monitored. Between -20% and +50% there was no significant alteration in cortical blood flow with perfusion following blood pressure passively outside these ranges. The non-competitive neuropeptide Y antagonist PP56 shifted the level at which the change in flow was passively dependent on blood pressure from +60% to +38%. The pial vessel study demonstrated that PP56 shifted the dose-response curve for the vasoconstrictor effect of NPY with a maximal reduction of 22 +/- 6%. These data suggest that the cervical sympathetic nerves with NPY play an active role in cerebral autoregulation. Furthermore in view of the longer time course of action of neuropeptide Y, it is an ideal transmitter candidate to be involved in cerebral autoregulation and any compound that blocks its action must be considered to potentially alter the normal cerebrovascular physiology.

Adrenergic and non-adrenergic cardiovascular effects of thyrotropin-releasing hormone (TRH) in the anaesthetized rabbit

The effects of thyrotropin-releasing hormone (TRH) on regional blood flows were studied in urethane-anaesthetized rabbits. Experiments were performed both with and without adrenergic antagonist pretreatment. The tracer microsphere method was used to measure blood flow. TRH (0.1 mg kg-1) caused an increase in mean arterial blood pressure (MAP) from 9.8 +/- 1 to 11.8 +/- 0.8; a higher dose (1 mg kg-1) increased the blood pressure to 15.2 +/- 1 kPa (P less than 0.001). Total cerebral blood flow (CBFtot) increased to 137 +/- 10% (P less than 0.05) of control at the lower dose and to 214 +/- 16% (P less than 0.001), at the higher dose. A reduction in blood flow at both doses of TRH in several peripheral organs indicates that the pressor effect was mainly due to an effect on the peripheral vascular resistance. In prazosin-pretreated animals in which the MAP was normalized by ligation of the thoracic aorta, TRH elicited an increase in the CBFtot to 131 +/- 12% (P less than 0.05) of control. In the iris, TRH caused vasodilation in prazosin-pretreated animals. In experiments with combined alpha- and beta-adrenergic blockade, a non-adrenergic vasoconstricting effect of TRH was seen in some peripheral organs. The results indicate that TRH activates the sympathetic nervous system thus causing an increased vascular resistance and MAP; these effects are mediated mainly by an alpha 1-adrenergic mechanism. In the spleen, the gastric mucosa and the adrenal glands, the vasoconstriction caused by TRH was partly non-adrenergic. The vasodilation seen in the small intestine and the anterior uvea after TRH treatment and adrenoceptor blockade may be explained by effects on the parasympathetic nervous system. The vasodilating effect of TRH in the brain does not seem to involve alpha 1- or beta-adrenergic mechanisms.

Spontaneous flow oscillations in the cerebral cortex during acute changes in mean arterial pressure

The purpose of this study was to characterize spontaneous oscillations of blood flow in the cerebral cortex of anesthetized rats under control conditions and after mean arterial pressure was altered by various means. Blood flow was monitored using a laser-Doppler flowmeter through the closed cranium. Spontaneous flow oscillations with amplitudes of 14-30% of the mean flow and frequencies of 4-11 cycles/min were recorded when arterial pressures were less than 90 mm Hg. Stepwise hemorrhagic hypotension and unilateral carotid occlusion increased the amplitude of oscillations. The amplitude of oscillations was negatively correlated with the level of mean arterial pressure after manipulation with norepinephrine or sodium nitroprusside. The oscillations were reversibly abolished during dilation of the cerebral circulation by elevating the inspired carbon dioxide content to 5%. The frequency of flow oscillations was very stable during all of the above maneuvers except during the infusion of norepinephrine, which increased the oscillation frequency slightly. The results suggest that flow oscillations are determined primarily by cerebral arterial pressure.

The role of neuroeffector mechanisms in cerebral hyperperfusion syndromes

Cerebral hyperperfusion, a state in which blood flow exceeds the metabolic needs of brain, may complicate a number of neurological and neurosurgical conditions. It may account for the propensity with which hemorrhage, cerebral edema, or seizures follow embolic stroke, carotid endarterectomy, or the excision of large arteriovenous malformations, and for some of the morbidity that accompanies acute severe head injury, prolonged seizures, and acute severe hypertension. Hyperperfusion syndromes have in common acute increases in blood pressure, vasodilatation, breakdown of the blood-brain barrier, and the development of cerebral edema. These common features suggest the possibility that they share the same pathogenic mechanisms. It was believed until recently that reactive hyperemia was caused primarily by the generation of vasoactive metabolites, which induced vasodilatation through relaxation of vascular smooth muscle. However, the authors have recently established that the release of vasoactive neuropeptides from perivascular sensory nerves via axon reflex-like mechanisms has a significant bearing upon a number of hyperperfusion syndromes. In this article, the authors summarize their data and discuss possible therapeutic implications for blockade of these nerves or their constituent neuropeptides.

Influence of Bay K 8644 on vascular responses mediated by alpha 1- and alpha 2-adrenoceptors

1. The calcium channel activator Bay K 8644 increased the potency of noradrenaline in cat middle cerebral (alpha 2-adrenoceptors) and mesenteric (atypical or mixed alpha 1- and alpha 2-adrenoceptor population) arteries, but not in rat middle cerebral and mesenteric arteries (alpha 1-adrenoceptors). 2. In cat arteries, exposure to 15 mM K+ solution shifted the noradrenaline concentration-response curve to the left in an almost identical manner as did Bay K 8644. 3. Bay K 8644 completely reversed the relaxation produced by nifedipine in K+-contracted cat middle cerebral arteries, whereas the relaxation induced by verapamil, diltiazem or flunarizine was only partially reversed. This suggests a specific interaction between Bay K 8644 and the dihydropyridine receptors on the calcium channels. 4. It is concluded that the degree to which noradrenaline promotes calcium influx through membrane channels is at least partly related to the alpha-adrenoceptor subtype mediating the response.

Theoretical and functional studies on alpha 1-and alpha 2-adrenoreceptors: an examination using the Schild plot

1 The influence on the shape of the Schild plot by a two-receptor system was studied in both functional and theoretical studies. In the functional studies, the alpha-adrenoreceptors in cat lingual arteries were studied since both alpha 1- and alpha 2-adrenoreceptors have been suggested to contribute to the noradrenaline-induced contractile response in this tissue. 2 The Schild plots constructed using noradrenaline (NA) as agonist and prazosin as antagonist gave a straight line with a slope close to unity. In contrast, the corresponding Schild plot constructed for rauwolscine appeared to be biphasic. 3 The results obtained in the functional study were discussed in view of Schild plots obtained from a theoretical model which was designed to take into consideration the presence of two distinct types of receptor, the percentile proportions of which could be altered. 4 The theoretical model indicates that the shape of the Schild plot can vary considerably depending on the relative contribution of each receptor subtype to the contractile response, the selectivity of the antagonist, and the range and number of antagonist concentrations used. 5 It is suggested that a response is predominantly mediated by alpha 1-adrenoreceptors and less by alpha 2-adrenoreceptors when the Schild plot for prazosin gives a slope close to unity and has a pA2-value representative for alpha 1-adrenoreceptors, and rauwolscine gives a slope less than unity and a significant shift of the NA concentration-response curve in concentrations around 10(-8) M. When the reverse is true (but with a significant shift caused by prazosin already at a concentration around 10(-9) M) the contraction elicited by NA is proposed to be predominantly mediated by alpha 2-adrenoreceptors and less by alpha 1-adrenoreceptors.

Exogenous norepinephrine constricts cerebral arterioles via alpha 2-adrenoceptors in newborn pigs

The purpose of this study was to determine whether exogenous norepinephrine mediates cerebrovascular constriction via alpha 1- or alpha 2-adrenoceptors in anesthetized neonatal pigs. Diameters of pial arterioles in anesthetized piglets, 1--6 days old, were investigated using a "closed" cranial window. We examined constrictor effects of norepinephrine on pial arterioles in the absence and presence of relatively selective alpha 1-(prazosin) and alpha 2-(yohimbine) adrenoceptor antagonists (1 mg/kg i.v.). Yohimbine and prazosin inhibited pial arteriolar constriction induced by topical application of clonidine and phenylephrine (10(-6) and 10(-4) M, respectively), and yohimbine did not affect the response to topical phenylephrine. In one group diameter was 188 +/- 13 (mean +/- SEM) micron during control and 146 +/- 12 micron during 10(-5) M norepinephrine (22 +/- 5% constriction). Following yohimbine the same vessels did not constrict significantly. In another group 10(-5) M norepinephrine constricted arterioles by 22 +/- 5%, and this response was unaffected by prazosin (24 +/- 5% constriction). We conclude that pial arterioles are responsive to both alpha 1- and alpha 2-adrenoceptor agonists, that intravenous administration of prazosin and yohimbine results in these drugs crossing the blood-brain barrier and inhibiting constrictor effects of agonists, and that norepinephrine constricts pial arterioles predominantly via alpha 2-adrenoceptors.