Unimportance of perivascular H+ AND K+ activities for the adjustment of pial arterial diameter during changes of arterial blood pressure in cats

Does NO regulate the cerebral blood flow response in hypoxia?

OBJECTIVES

To study whether nitric oxide (NO) affects the CBF response to hypoxic and carbon monoxide (CO) hypoxia.

MATERIAL AND METHODS

We incrementally reduced arterial oxygen content in rats, by decreasing the concentration of inspired oxygen (20 rats) or by repeated CO inhalation (20 rats), and measured local CBF using the hydrogen clearance method. Ten animals of each group received 80 mg/kg NO synthase (NOS) inhibitor N-monomethyl-L-arginine intravenously prior to hypoxia, while 10 rats served as controls.

RESULTS

Inhibition of NOS decreased mean CBF by 30% and increased cerebrovascular resistance by 70%. Under hypoxic hypoxia, mean oxygen reactivity of CBF (relative change of CBF to a change of arterial oxygen content) was 7.8%/vol% in control animals and 3.3%/vol% after NOS inhibition (P < 0.02). Under CO hypoxia, mean oxygen reactivity was 7.3%/vol% in control animals and 5.1%/vol% after NOS inhibition (P < 0.05). Inhibition of NOS diminished significantly the cerebral vasodilatory response during hypoxic hypoxia (P < 0.05) but only to a lesser extent during CO hypoxia.

CONCLUSION

These observations suggest that NO is involved in cerebral oxygen vasoreactivity, particularly in severe hypoxia.

Rat brain glucose and energy metabolites: effect of +Gz (head-to-foot inertial load) exposure in a small animal centrifuge

A unique small animal centrifuge with on-line physiological monitoring and brain tissue collection (in < 1 s) capability was used to investigate the effect of increasing +Gz levels, exposure duration, number of exposures, and time course of metabolic changes in the rat brain. To determine the +Gz tolerance, rats were exposed to +7.5 to 25 Gz (30 s each) and EEG was monitored. G-induced loss of consciousness (G-LOC) defined as isoelectric EEG (I-EEG) occurred only at +22.5 and 25 Gz within 14.5 +/- 3 s. To study the effect of increasing +Gz levels on metabolism, rats were exposed to either 0.5 (control) or +7.5 to 25 Gz (30 s each), and brains were collected 1 min postcentrifugation by freeze fixation. A significant increase in lactate (> or = +7.5 Gz) and a decrease in glucose, creatine phosphate (Cr-P), and ATP levels were observed at +15 Gz and higher. The effect of exposure duration was investigated by exposing the rats to +22.5 Gz for 15-60 s. Brain lactate levels increased six-fold while glucose decreased (75%) following the 60-s exposure. The level of Cr-P and ATP decreased significantly after the 15- and 30-s exposures with no further changes at longer +Gz exposures. For time course studies, brains were collected both during (5-35 s) and after (1-15 min) a +25 Gz exposure. A significant decrease in Cr-P occurred within 5 s, but changes in glucose, ATP, and lactate required 15 s. All metabolites returned to control levels within 3 min, except lactate and adenosine, which required 15 min.(ABSTRACT TRUNCATED AT 250 WORDS)

Persistence of impaired autoregulation of cerebral blood flow in the postictal period in piglets

Cerebral blood flow (CBF) autoregulation was evaluated in the postictal period in unanesthetized term newborn piglets with a mean age of 5.7 +/- 3 days. Seven animals (group 1) received 1 mg/kg bicuculline to induce brief generalized seizures, and six control animals (group II) received saline. Twenty to 90 min after the end of seizure activity in group I or saline injection in group II, CBF was measured by the radioactive labeled microspheres method at three levels of mean arterial blood pressure (MABP) obtained by controlled blood withdrawal within the normal range for autoregulation. In the postictal period, baseline CBF was higher in group I than in group II (85 +/- 21 vs. 48 +/- 7 ml/min/100 g, p less than 0.001). During hypotension, total CBF was positively correlated with variations of MABP in group I (r = 0.739, p less than 0.01) but not in group II. Regional CBF showed the same correlation with MABP. These data show that after seizures in piglets, the rise in CBF is associated with a persistent impairment of CBF autoregulation. These hemodynamic alterations may be relevant in the pathogenesis of hemorrhagic or ischemic brain lesions.

The role of the endothelial dependent relaxing factor in the regulation of cerebral circulation

It has recently been demonstrated that vessel dilation induced by several physiological agents is dependent on an intact vascular endothelium. In order to explain this endothelium dependence, it has been hypothesized that a still unknown chemical substance, generically named Endothelium Dependent Relaxing Factor (EDRF) is necessary for physiological vasodilation. The role of this EDRF in the cerebrovascular physiology is not yet well understood. In this article the cerebrovascular physiological control is reviewed in relationship with possible EDRF actions. The importance of endothelial lesions in the cerebrovascular responses is discussed.

Cerebrovascular reactivity to angiotensin and angiotensin-converting enzyme activity in cerebrospinal fluid

The purpose of the present study was to test the vasomotor effect of angiotensin I (A I) and angiotensin II (A II) in feline cerebral arteries and to examine the presence of angiotensin converting enzyme (ACE) activity in the vessel wall and cerebrospinal fluid (CSF). A II (10(-8) -10(-5) M) induced concentration-dependent contractions of feline pial arteries (resting diameter, 98-286 microns) in situ with a maximum of 34% at 10(-4) M A II. A I produced dose-related contractions being approximately 20 times less potent than A II. The action of A I was significantly attenuated by the ACE inhibitor captopril (10(-5) M). These findings demonstrate the presence of ACE activity in the vessel wall and/or its surroundings. ACE activity was also found in feline CSF sampled from the cisterna cerebellomedullaris. Bradykinin (BK) was broken down and A I converted to A II by CSF, both effects being inhibited by captopril. This was demonstrated using bioassay and high-performance liquid chromatography. Considering the present and previous studies we conclude that the presence of ACE in the vessel wall and CSF is necessary for the conversion of A I to A II. Although ACE in CSF is able to degrade BK it appears not to be important for the metabolism of BK acting from the perivascular side of pial arteries in situ.

Change of cerebrovascular reactivity after cortical spreading depression in cats and rats

The purpose of the present study was to examine the pial arteriolar diameter and evoked vascular responses after single episodes of cortical spreading depression (CSD) in rats and cats in order to elucidate the mechanisms of the persistent change of cortical perfusion which succeeds CSD. This problem is of potential clinical interest also since CSD may be involved in migraine pathophysiology. Using an open cranial window technique, pial arteriolar diameters were measured with an image splitting method. Vascular reactivity was tested by local perivascular microapplication of mock cerebrospinal fluid (CSF) containing high and low levels of K+, high and low pH, adenosine and bradykinin before and after CSD which was triggered by intracortical injection of KCl. During CSD a monophasic vasodilatation of 26.0 +/- 3.7% (mean +/- S.E.M.; cat) or 64.6 +/- 3.9% (rat) was observed. Following CSD, the cat developed persistent vasodilatation (16.7 +/- 1.9%) while the rat exhibited vasoconstriction (12.1 +/- 1.8%). Both species displayed a severely impaired responsiveness to constrictor and dilating stimuli as compared to pre-CSD values. The responses were reduced by 28-84%, dependent on the substance tested. It is concluded that vascular reactivity is severely impaired after CSD (15-75 min) and that this might explain the impaired coupling between flow and metabolism after CSD.

Effects of enkephalins, morphine, and naloxone on pial arteries during perivascular microapplication

The effects of the opiate receptor agonists, enkephalins and morphine, and the antagonist, naloxone, on cerebrovascular resistance vessels was investigated in situ by employing perivascular microapplication. Feline pial arteries with a resting diameter of 66-294 micron were tested. Vascular diameter was measured using television image splitting. Concentration-response curves revealed no change of diameter when Leu-enkephalin, D-Ala2-Leu-enkephalinamide, D-Ala2-Met-enkephalinamide, and morphine were applied in concentrations of 10(-11)-10(-5) M. Considering the concentrations of enkephalins that have been found in natural cerebrospinal fluid or that can be expected in the vicinity of enkephalinergic synapses, the data obtained with the lower concentrations indicate that enkephalins are probably not important for the regulation of pial arterial resistance. At 10(-4) M only the dilation (4.3%) elicited by D-Ala2-Leu-enkephalinamide was statistically significant (p less than 0.01). All four agonists at 10(-3) M induced significant dilatations varying between 5.4 and 13.6%. Naloxone exerted no vascular effect per se at 10(-5) and 10(-4) M but a dilatation of 15.3% at 10(-3) M. The latter can be explained by a partial agonist action. During simultaneous administration, naloxone (10(-4) M) reduced the dilatations induced by 10(-4) and 10(-3) M D-Ala2-Leu-enkephalinamide dose dependently. This indicates that mu- and delta-opioid receptors, probably located at the vascular smooth muscle cell, were involved in the mediation of the dilatation induced by the highest concentrations of the compounds.

Effect of adenosine and its stabile analogue 2-chloroadenosine on cerebrocortical microcirculation and NAD/NADH redox state

In the present study, the effects of topically applied adenosine (ADO) and its stabile analogue 2-chloroadenosine (CADO) on cerebrocortical microcirculation and NAD/NADH redox state (oxidized/reduced nicotinamide adenine dinucleotide) were investigated. Vascular volume (CVV), mean transit time of blood flow (tm), blood flow (CBF), and NADH fluorescence of the cat brain cortex were measured through a cranial window with a microscope fluororeflectometer. The reference values of CVV, tm, and CBF, measured in the artificial cerebrospinal fluid (mock CSF) which superfused brain cortex, were regarded as 100%. Adenosine and 2-chloroadenosine, in the concentration range of 10(-6) - 10(-3) M, resulted in concentration-dependent increases in CBF and NAD reduction. 10(-5) M adenosine and 2-chloroadenosine increased CBF by 49.6 +/- 5.6% and 80.4 +/- 10.3%, respectively. At a pharmacologically high concentration (10(-3) M), ADO increased CBF by 164.6 +/- 13.5%, CADO by 333 +/- 44%. At the same time, 10(-3) M ADO and CADO shifted the cortical NAD/NADH redox state by 7.9 +/- 0.4% and 12.4 +/- 0.7%, respectively toward a more reduced state. Our results, concerning the vasodilator potency of adenosine and 2-chloroadenosine, accord with available data in the literature. However, the pronounced NAD reduction obtained with these adenosine nucleosides suggests that, besides an action on vascular adenosine receptors, some other changes, such as increased substrate mobilization and possibly cAMP production, may contribute to the vasodilator effect of adenosine and 2-chloroadenosine.

A cellular mechanism for myogenic regulation of cat cerebral arteries

Autoregulation of cerebral blood flow is accomplished through integration of metabolic, neurogenic and myogenic mechanisms. Myogenic mechanisms involve activation of cerebral arterial muscle cells as transmural pressure increases, providing a means through which vessel caliber can be regulated to maintain blood flow constant. The cellular mechanisms involved in this myogenic response may involve changes in the electrical potential across the plasma membrane. When isolated cat middle cerebral arteries are cannulated and prepared in a manner allowing manipulation of transmural pressure, the muscle cell membrane depolarizes as pressure increases. The degree of membrane depolarization in response to an elevated pressure is dependent upon extracellular Ca2+ [( Ca]o), increasing as [Ca]o is elevated and markedly decreasing as [Ca]o is reduced to low levels. When these arterial preparations are maintained at a physiological pressure of around 100 mm Hg, spontaneous action potentials can be recorded which increase in frequency upon further elevation in pressure. Vessels exhibiting such electrical activity can be observed to decrease in diameter as pressure is increased. Such finding suggest a membrane electrical mechanism for myogenic autoregulation of cerebral arteries.

Effects of bradykinin on permeability and diameter of pial vessels in vivo

The effect of bradykinin on the permeability and vasomotor response of pial vessels has been studied to enhance our understanding of the pathophysiological role of the kallikrein-kinin system in cerebral tissue. Intravital fluorescence microscopy of the pia arachnoidea was conducted using Na+-fluorescein, FITC-dextran, and FITC-albumin as low and high molecular weight blood-brain barrier indicators. Massive arterial dilatation evolved immediately upon administration of bradykinin by superfusion of the exposed cerebral surface. An increase of the arterial diameter by 40% was the maximal response found at bradykinin concentrations of 4 x 10(-5) M. Arterial dilatation became attenuated with continuous superfusion of the preparation with bradykinin. In pial veins, a moderate reduction of the vessel diameter was observed, however, only after prolonged superfusion of the preparation. Bradykinin led to selective opening of the blood-brain barrier for Na+-fluorescein at superfusate concentrations of greater than or equal to 4 x 10(-7) M, but not for FITC-dextran or FITC-albumin. Topical administration of l-isoproterenol (10(-4) M) was found to prevent extravasation of Na+-fluorescein in the presence of bradykinin concentrations of 4 x 10(-6) M. Protection of the blood-brain barrier by isoproterenol was not observed when higher concentrations of bradykinin were employed. Intracarotid infusion of bradykinin were employed. Intracarotid infusion of bradykinin led also to a selective opening of the blood-brain barrier for Na+-fluorescein, but not for FITC-dextran or FITC-albumin. In contrast to superfusion, this route of administration did not induce changes of the vasomotor behavior of the arteries or veins. Additional experiments with B1-agonists and -antagonists suggest that bradykinin causes the openings of the blood-brain barrier th rough an interaction with B2-receptors on endothelial cells, and arterial dilatation via interaction with B2-receptors on vascular smooth muscle cells. Our findings support the concept that the release of kinins in the brain during an acute cerebral lesion mediates secondary damaging processes by the enhancement of blood-brain barrier dysfunction.

Pressure-dependent membrane depolarization in cat middle cerebral artery

This study was undertaken to examine the effect of increasing transmural pressure on membrane electrical properties of cat middle cerebral arterial muscle. Middle cerebral arteries were removed from the cat brain, cannulated, and prepared so that transmural pressure within a segment could be manipulated. Intracellular membrane potential was recorded with glass microelectrodes at various transmural pressures. There was a positive slope relating changes in intracellular membrane potential as a function of transmural pressure with a correlation coefficient of 0.79. Blockade of nerve excitation with tetrodotoxin and inhibition of alpha-adrenergic receptors with phentolamine not only did not block the pressure-induced depolarization, but increased the slope of the intracellular membrane potential vs. pressure relationship. This slope was increased upon elevation of extracellular calcium concentration from 2.5 to 4.0 mM and was significantly reduced upon reduction of extracellular calcium concentration to 0.5 mM. When arterial preparations were equilibrated at 0 mm Hg prior to pressurization, action potentials were recorded only when pressure was initially elevated, while a sustained depolarization was recorded during the pressure plateau. However, when arteries were equilibrated at a transmural pressure of 100 mm Hg for 90 minutes, spontaneous action potentials were recorded which increased in frequency as a function of pressure until they were inactivated when intracellular membrane potential approached -30 mV at high transmural pressures. Photomicrographs demonstrated that these vessels either maintained or decreased diameter upon pressurization. These findings provide a cellular mechanism for myogenic regulation of cerebral arterial diameter.

Mechanisms of regulation of cerebral microflow during bicuculline-induced seizures in anaesthetized cats

Before, during, and after bicuculline-induced seizures, changes in microflow, local tissue PO2, and extracellular H+ and K+ activities were continuously recorded in the suprasylvian gyrus of the cat in parallel with electrical activity. Additionally, the patterns of microflow during seizures after blockade of the beta-adrenergic and cholinergic receptors and after phentolamine application were studied. With the onset of discharges, microflow increased at all sites. The maximum increase was observed when the electrical activity was the strongest. During the period of alternating silent and nonsilent phases, microflow oscillated in parallel with functional activity. When the discharges ceased, microflow decreased to a new steady-state level. Tissue hypoxia was not responsible for the increase in flow because local tissue PO2 increased after the onset of seizures. H+ activity increased after a short delay and also oscillated during the period of oscillating functional activity. After the end of discharges, H+ activity decreased. K+ activity increased immediately with the onset of discharges and mirrored the electrical activity in the further course. The pattern of microflow was not changed by blockade of alpha- and beta-adrenergic and cholinergic receptors. We conclude that besides the increase in systemic blood pressure, K+ and H+ activities could be the main factors responsible for the increase in flow during seizures.

Cerebral circulation and metabolism

Recent developments in the field of cerebral circulation and metabolism are reviewed, with emphasis on circulatory and metabolic events that have a bearing on brain damage incurred in ischemia. The first part of the treatise reviews aspects of cerebral metabolism that provide a link to the coupling of metabolism and blood flow, notably those that lead to a perturbation of cellular energy state, ionic homeostasis, and phospholipid metabolism. In the second part, attention is focused on the derangement of energy metabolism and its effects on ion fluxes, acid-base homeostasis, and lipid metabolism. It is emphasized that gross brain damage, involving edema formation and infarction, is enhanced by tissue acidosis, and that neuronal damage, often showing a pronounced selectivity in localization, appears related to a disturbed Ca2+ homeostasis, and to Ca2+-triggered events such as lipolysis and proteolysis.

Effects of bradykinin on pial arteries and arterioles in vitro and in situ

The effect of bradykinin on cerebrovascular resistance vessels was investigated by the use of in vitro and in situ preparations. Bradykinin, in the range of 10(-10) to 10(-5) M, elicited a concentration-dependent vasodilatation on both feline and human pial arteries in vitro; the half-maximal response was found to be approximately at 2.8 X 10(-7) M and 1.3 X 10(-8) M (EC50), respectively. This dilatatory effect of bradykinin in vitro was found only in arteries preconstricted with prostaglandin F2 alpha or 5-hydroxytryptamine. In order to determine the effects of bradykinin on the diameter of cat pial arteries in situ, perivascular microapplication was employed. The dose-response curves obtained showed vasodilatation; the EC50 and the maximal response (EAm) were 4.4 X 10(-7) M and 45.5% at 10(-5) M, respectively. Statistically significant (p less than 0.01) reactions were observed at 10(-7) M and higher concentrations of bradykinin. The observed effects were independent of initial vessel size (80-260 microns). These in situ findings are very similar to those found in vitro. The isolated guinea pig ileum was used to check the stability of the bradykinin solutions. In this instance, a concentration-dependent contraction was found when "freshly prepared" or "5 hours stored" bradykinin was applied, indicating no measureable degradation of bradykinin. We conclude that bradykinin is a powerful vasodilator of both human and feline pial arteries.

Analysis of bradykinin receptor mediating relaxation of cat cerebral arteries in vivo and in vitro

Bradykinin (BK), methionyl-lysyl-BK (M-L-BK) and des-Arg9-BK produced, in decreasing potency, dose-related dilatations of the superficial pial arteries of the cat in vivo. The competitive, specific B1-receptor antagonist, des-Arg9-Leu8-BK was ineffective against BK-induced dilatations in this in vivo model. On the cat middle cerebral artery in vitro (but not the basilar artery), under resting tension and when contracted with 5-hydroxytryptamine (5-HT) or KCl, concentration related relaxations were produced by BK, M-L-BK, and des-Arg9-BK, this being the order of relative potency of the three kinins. There was no increase in the sensitivity of either the middle cerebral or the basilar artery in vitro under resting tension or when contracted with 5-HT or KCl to BK or des-Arg9-BK, concentration effect curves to which were produced at 2 h intervals over an 8 h period. The B1-receptor antagonist des-Arg9-Leu8-BK was ineffective against relaxations to BK or des-Arg9-BK of the middle cerebral artery under resting tension or when contracted with 5-HT. The receptor mediating dilatation of the superficial pial arteries of the cat in vivo and relaxation of the middle cerebral artery in vitro to kinins is of the B2-type. The cat basilar artery in vitro is relatively insensitive to the action of kinins and this is possibly due to an absence of receptors for kinins on this tissue.

Inhibition of the pial artery constriction induced by sympathetic stimulation by local microapplication of a cholinomimetic agent

We studied the effects of microapplications of carbachol plus atropine on cat pial artery diameter in vivo during resting conditions and during stimulation of the cervical sympathetic nerve. The cats were anesthetized with alpha-chloralose and artifically ventilated. The pial surface was exposed by trepanation and protected by a 1-2-cm layer of oil. Calibrated applications of solutions were made by micropipette into the subarachnoid space, while the pial artery diameter was measured by the television image-splitting method. Sympathetic stimulation during 100 s induced a constrictive response of about 10%, which was constant from 60 to 100 s and which remained so during application of inert mock spinal fluid from 65 to 100 s. Application of 10(-5) M carbachol plus 10(-7) M atropine (solution A) or 10(-4) M carbachol plus 10(-6) M atropine (solution B) did not produce any significant changes in diameter during resting conditions. During sympathetic stimulation, application of solution A from 65 to 100 s induced a small nonsignificant reduction of the constriction, whereas application of solution B induced a highly significant reduction of the constriction from 9.63 +/- 1.09% at 60 s to 1.20 +/- 2.40% at 100 s. These results are discussed in terms of the hypothesis that carbachol may act on the sympathetic fibers on the pial arteries by a nonmuscarinic mechanism to reduce the liberation of the transmitter.

Effect of acute arterial hypo- and hypertension on cerebrocortical NAD/NADH redox state and vascular volume

The effects of stepwise arterial hypotension (MABP: 80, 60, 40 mm Hg) and moderate arterial hypo- and hypertension (MABP: 80, 150-160 mm Hg) on cerebrocortical vascular volume and NAD/NADH redox state were studied in anaesthetized cats. The vascular volume and NADH fluorescence measurements were performed on closed skull preparations using a microscope fluororeflectometer. To determine the possible role of adrenergic alpha-receptors in the autoregulatory adjustment of cerebrocortical vascular volume, some of the animals were pretreated with intra-arterially infused phenoxybenzamine (1 mg/kg). It was found that longlasting stepwise arterial hypotension leads to a gradual increase in cerebrocortical vascular volume and NADH fluorescence. Though the cerebrocortical arteries dilatated considerably at 80 mm Hg, sustained for 30 min, the NAD/NADH redox state failed to be reoxidized but was shifted to a more reduced state. This finding suggests that some factor other than tissue hypoxia is responsible for the dilatation of cerebrocortical vessels during moderate arterial hypotension. When the arterial blood pressure was restored following stepwise arterial hypotension, the cerebrocortical vascular volume did not decrease and the NAD/NADH redox state remained reduced, showing that the autoregulatory capability of the vessels was lost and the tissue metabolism was irreversibly altered. During a 5-min duration of moderate arterial hypo- and hypertension, biphasic changes were obtained in cerebrocortical vascular volume while the NAD/NADH redox state was shifted to a more reduced and oxidized state. Since the dilatation and the constriction of the cerebrocortical vessels during arterial hypo- and hypertension lagged by 40-80 s behind the redox state alterations, it is suggested that the myogenic mechanism has a minor role in CBF autoregulation. Phenoxybenzamine (PBZ) dilatated the cerebrocortical vessels, indicating the existence of an active alpha-receptor-mediated vasoconstrictory tone. Since the extent of autoregulatory vascular volume changes was not affected by PBZ pretreatment, the involvement of adrenergic alpha-receptors in the autoregulation of CBF can be excluded, at least for cats.

Pial arterial response to systemic hypoxia in anaesthetised cats

Pial arterial responses to reduction in arterial oxygen tension were studied in anaesthetised cats. In 12 cats under chloralose, dilatation occurred in vessels of all sizes between 20 and 200 micrometers, at variable levels of PaO2, and to a very variable extent. At PaO2 25-35 mm Hg, dilatation ranged from negligible to 175% above initial diameter. The variations in response were largely dependent on associated blood pressure (BP) changes. Increase in BP counteracted dilatation; dilatation was greater during hypoxia when the BP change was prevented. At the induction of hypoxia, the first response of the vessels was a constriction, which occurred about 5 s after the chemoreflex increase in BP. Dilatation was delayed a further 30-90 s, and this delay was similar when BP was prevented from rising. Vessels of all sizes responded in the appropriate manner when only BP was transiently changed within the autoregulatory range. In 3 cats in which similar procedures were compared under pentobarbitone anaesthesia, there were smaller and less consistent changes in responses to BP changes alone and to hypoxia and its associated BP changes. The findings were compatible with a local effect of lowered PO2. From the time course of the changes there was no indication of a chemoreflex component in the responses of these vessels at the induction of hypoxia.

Cerebral circulation and histamine: 2. Responses of pial veins and arterioles to receptor agonists

H2-receptors predominantly mediate pial arteriolar dilatation in response to histamine, but the reaction of pial veins to histamine has not been clearly identified. In anesthetized cats, we examined responses of pial veins and arterioles to perivascular microapplication of histamine and specific histamine H1 and H2 receptor agonists. Arterioles were very sensitive to the H2-receptor agonist impromidine, with significant dilatation (+16%) occurring at concentrations as low as 10(-10) M. Arteriolar responses to H1 receptor stimulation by 2.2-pyridylethylamine were small, even at high concentrations. The order of potency and maximum dilatations found for the receptor agonists were: H2 (43%) greater than histamine (28%) greater than H1 (17%). By contrast, pial veins did not respond to histamine or the receptor agonists. The results indicate that pial venomotor activity to histamine is negligible, and suggest a sparse distribution of histamine receptors on the outer surfaces of pial veins.

Adenosine response on pial arteries, influence of CO2 and blood pressure

The television image-splitting technique was used to study the influence of arterial pCO2 and blood pressure on the dilatatory response of pial arterioles to topically applied adenosine in chloralose anaesthetised cats. At normocapnia (pCO2 congruent to 35 mm Hg) 10(-5) adenosine caused pial arteriole dilatation of 29.2 +/- 2.7% (S.E.M.). This was significantly reduced to 14.5 +/- 1.6% (P < 0.001) at pCO2 25 mm Hg and to 8.5 +/- 1.6% (P Ø 0.001) at pCO2 48 mm Hg. Lowering the blood pressure to 65--85 mm Hg had no significant effect on the adenosine response, but raising the blood pressure to 140--160 mm Hg significantly reduced the adenosine response to 22.1 +/- 1.8% (P < 0.005). The response was independent of vessel size except at hypertension where vessels < 150 micrometer were significantly more reactive than the larger vessels (P < 0.01). These results indicate that adenosine induced vasodilatation of pial arterioles shows little change in the face of alterations in vessel tone induced by altering blood pressure, but is markedly decreased by the combination of changing perivascular pH and vascular resistance through moderate changes in arterial pCO2. The importance of these results in assessing the role of adenosine as a cerebral vasodilator is discussed.