Hypertension reduces the number of beta-adrenergic receptors in rat brain microvessels

[Regulation of brain microvessel function]

The brain microvessels are formed by a specialized endothelium and regulate the movement of solutes between blood and brain. The endothelial cells are sealed together by tight junctions and play a role as the blood-brain barrier. The brain microvessels express GLUT1 as the major form of glucose transporter, aquaporin-4 as a water channel, and p-glycoprotein as a xenobiotic transporter. Occludin and claudin-5 have been identified as the components of tight junction. Increasing evidence suggests that the activities of the transporters are regulated by adrenergic nerve activity as well as by bioactive peptides such as adrenomedullin. The regulation of the activity as well as expression of these transporters may become a strategy for prophylaxis and treatment of not only cerebral vascular diseases but also neurodegenerative disorders, developmental abnormalities and aging of the brain.

Aquaporin subtypes in rat cerebral microvessels

We investigated the expression of aquaporin (AQP) subtypes in the rat cerebral microvessels by reverse transcription-polymerase chain reaction, immunoblotting and immunohistochemistry. mRNA for AQP4, but not for AQP1, 2, 3 or 5, was detected in the microvessels. Immunoblot analysis showed that AQP4 protein was detected as a 30 kDa band with higher molecular weight bands. Immunohistochemical staining showed that AQP4 was located on cell surface of the cerebral microvessels. These results suggest that AQP4 in the cerebral microvessels is involved in the regulation of water transport between blood and brain.

C-type natriuretic peptide increases cyclic GMP in rat cerebral microvessels in primary culture

Effect of CNP on cGMP level in cultured rat cerebral microvessels was investigated. The cerebral microvessels were prepared from rat cerebral cortex by dispase and collagenase digestion and Percoll gradient centrifugation, and cultured. CNP increased cGMP level in a dose-dependent manner suggesting that CNP has a regulatory role in the cerebral microvessel function.

Pituitary adenylate cyclase-activating polypeptides (PACAPs) increase cAMP in rat cerebral microvessels

Effect of PACAP on cAMP level in the rat cerebral microvessels was investigated. The cerebral microvessels were prepared from rat cerebral cortex by albumin flotation and glass beads filtration technique. When the microvessels were incubated with PACAP 27, PACAP 38 and VIP, cAMP in the microvessels was increased rapidly reaching a plateau value within 60 s. PACAP 27, PACAP 38 and VIP increased cAMP level in a dose-dependent manner with EC50 values of 4.7, 7.0 and 34 nM, respectively. These results suggest that PACAPs play a role in the regulation of the cerebral microvessel function.

Quantitative receptor autoradiographic analysis for angiotensin II receptors in bovine retinal microvessels: quantitation with radioluminography

1. Specific 125I-Sar1, Ile8-Angiotensin II (125I-Sar1, Ile8-AII) binding sites in bovine retinal microvessels were investigated using the quantitative receptor autoradiographic method with pellet sections. 2. A quantitation was made with the computerized radioluminographic imaging plate system, a newly developed and highly sensitive method. Binding characteristics of the retinal microvessels were compared with those of the cerebral microvessels and the retinal macrovessels. 3. We isolated microvessels from the bovine retina and bovine cerebral cortex using the method composed of two-size sievings and high-speed homogenization with a Polytron. The isolated microvessels were composed of capillaries, and the retinal macrovessels contained vessels with smooth muscle. 4. There were specific binding sites for 125I-Sar1, Ile8-AII which were single and of a high affinity, in both the cerebral and the retinal microvessels and the retinal macrovessels. There were no differences in affinity between the vessels, but the retinal microvessels did have a higher density of binding sites than the cerebral microvessels. 5. The method we used is simple and sensitive for detecting and characterizing 125I-Sar1, Ile8-AII binding sites in retinal capillaries. Knowledge of the existence of large numbers of specific binding sites, candidates of physiologically active angiotensin II receptors, aids with understanding the regulatory roles of angiotensin II in the blood-retinal barrier.

Noradrenergic facilitation of motor neurons: localization of adrenergic receptors in neurons and nonneuronal cells in the trigeminal motor nucleus

Both alpha- and beta-adrenergic receptors (ARs) are involved in the facilitation of the monosynaptic jaw-closing reflex in the trigeminal motor nucleus (MoV) caused by norepinephrine (NE). The amplitude of muscle spindle afferent-evoked EPSPs in masseter motor neurons is 65% greater when noradrenergic axons to the motor nucleus are concomitantly activated and seems to be due to a presynaptic mechanism (Vornov, J. J., and J. Sutin. 1986. J. Neurosci. 6: 30-37). To determine the subtypes of ARs located on motor neurons and other cells, the cytotoxic lectin Ricin communis was injected into the masseter nerve of the trigeminal motor root to eliminate motor neurons in the masseter subnucleus of MoV. Autoradiography following incubation of tissue sections in the alpha 1 ligand 125IBE 2254 (125I-HEAT) or the nonselective beta ligand [125I]iodocyanopindolol (125ICYP) showed a decrease in alpha 1-AR binding related to the motor neuron degeneration and an increase in beta-AR binding associated with the glial reaction. To determine the extent to which glial proliferation was responsible for the increase in beta-ARs, cytosine arabinofuranoside (AraC) was administered to inhibit mitosis. Following AraC treatment, the total number of glial cells in the ricin-treated MoV was similar to that in normal MoV. Both beta-AR density and GFAP immunoreactivity remain increased, but to a lesser degree than following the ricin treatment alone. AraC also partially prevented the increase of immunolabeled or histochemically visualized microglia and capillary endothelial cells. The coincidence of the increases in beta-AR binding and GFAP in a region devoid of neurons argues that reactive astrocytes and other nonneuronal cells express beta-ARs in vivo. To determine whether the increase in astroglial beta-ARs was due to an up-regulation resulting from transynaptic degeneration of NE terminals, NE content was measured in MoV tissue punches, and NE terminals were visualized by immunocytochemical labeling of dopamine-beta-hydroxylase. NE content and NE terminal density remained unchanged following ricin-induced motor neuron degeneration.

Atrial natriuretic peptide receptors in cerebral microvessels and choroid plexus of spontaneously hypertensive rats

Binding sites of atrial natriuretic peptide in the cerebral microvessels and choroid plexus of spontaneously hypertensive rats (SHR) and Wistar Kyoto control rats (WKY) were measured. In the microvessels, the number of ANP binding sites was lower in SHR than WKY, but there was no difference in affinity of binding sites for the ligand between SHR and WKY. In the choroid plexus, the number of ANP binding sites was lower and the affinity for the ligand was higher in SHR than in WKY. These results suggest that the physiological function regulated by the ANP receptors in the cerebral microvessels and choroid plexus were altered in hypertension and that ANP receptors in the cerebral microvessels and choroid plexus were differentially regulated.

Acute effects of antihypertensive agents on cerebral blood flow in hypertensive rats

The acute effects of various antihypertensive agents on cerebral blood flow and mean arterial pressure (MAP) were studied in anesthetized (amobarbital 100 mg/kg) spontaneously hypertensive rats. Cerebral blood flow in the cortex and thalamus was measured by the hydrogen clearance method before and during a 60-min i.v. infusion of calcium antagonist (nifedipine), angiotensin converting enzyme inhibitor (captopril) or beta-blocker (propranolol). Nifedipine, 30 or 150 micrograms/kg per h, decreased dose dependently the MAP by 20 or 31%, and concomitantly increased cortical blood flow by 28 or 74%, and thalamic blood flow by 51 or 64%, respectively. Captopril, 10 or 100 mg/kg per h, decreased MAP by 7 or 14%, but changed cerebral blood flow minimally. In contrast, propranolol, 1.0 or 5.0 mg/kg per h, decreased MAP by 13 or 11%, with a concomitant reduction of cortical and thalamic blood flow by 20 or 15 and 33 or 37%, respectively. It is concluded that the changes in cerebral blood flow in response to hypotension are varied by antihypertensive drugs depending on the direct or indirect effect of the drugs (dilatation or constriction) on cerebral vessels. Nifedipine seems to dilate while propranolol constricts cerebral vessels.

Effects of epinephrine and norepinephrine on cerebral oxygen delivery and consumption during open-chest CPR

The effect of epinephrine and norepinephrine on cerebral oxygen delivery and consumption after five minutes of cardiopulmonary arrest and three minutes of open-chest cardiac massage was studied in 21 pigs. Norepinephrine, like epinephrine, has a marked alpha- and beta 1-sympathomimetic activity, but compared with epinephrine, the degree of beta 2-stimulation is weak. Epinephrine probably stimulates cerebral oxygen and glucose consumption by its beta 2-adrenergic effect. After three minutes of CPR, three groups of seven animals each blindly received either placebo (control group), 45 micrograms/kg epinephrine, or 45 micrograms/kg norepinephrine. During CPR but before drug administration, cerebral blood flow was 23 +/- 14 mL/min/100 g in the control group, 30 +/- 7 mL/min/100 g in the epinephrine group, and 30 +/- 11 mL/min/100 g in the norepinephrine group. At 90 seconds after epinephrine, cerebral blood flow increased to 54 +/- 14 mL/min/100 g and after norepinephrine, to 58 +/- 22 mL/min/100 g (P less than .05). Cerebral perfusion pressure for both drugs was significantly higher than the control group. Compared with mechanical measures alone, cerebral oxygen delivery rose from 4.3 +/- 1.2 to 7.4 +/- 1.7 mL/min/100 g after epinephrine and from 3.7 +/- 1.4 to 7.3 +/- 2.7 mL/min/100 g after norepinephrine (P less than .05). There was no increase in cerebral oxygen consumption after both catecholamines, and cerebral oxygen extraction ratio decreased. Cerebral glucose delivery increased in relation to glucose consumption, and extraction ratio did not change significantly after both catecholamines.(ABSTRACT TRUNCATED AT 250 WORDS)

Atrial natriuretic peptide and angiotensin II binding sites in cerebral capillaries of spontaneously hypertensive rats

1. We carried out investigations on specific atrial natriuretic peptide (ANP) and angiotensin II (ANG) binding sites in capillaries isolated from the cerebral cortex of spontaneously hypertensive rats (SHR), an animal model of human essential hypertension, and also from Wistar Kyoto rats (WKY). 2. In an equilibrium binding study done in the presence of increasing concentrations of the radiolabeled ligands, the binding of 125I-rat alpha-ANP (1-28) [ANF-(99-126)] (125I-rANP) and 125I-ANG (5-L-isoleucine) (125I-ANG) to the cerebral capillaries was single and of a high affinity. 3. The maximum binding capacity (Bmax) and dissociation constant (Kd) in the 125I-rANP binding of 20-week-old, hypertensive SHR was significantly lower than in age-matched, normotensive WKY. Conversely, a significant increase in the Bmax of 125I-ANG binding of adult SHR was observed, with a significant decrease in the Kd. 4. There was no differences in the Bmax of 125I-rANP and 125I-ANG binding between 4-week-old, prehypertensive SHR and age-matched WKY. However, there was a significant decrease in the Kd of 125I-rANP binding of SHR. 5. As a dramatic change in the binding kinetics of 125I-rANP and 125I-ANG was noted in the cerebral capillaries of adult sustained-hypertensive SHR, the possibility that ANP and ANG play a role in the etiology of dysfunction of the blood-brain barrier complicated with hypertension, by interacting with specific receptors, would have to be considered.

Neurochemical coupled actions of transmitters in the microvasculature of the brain

The discovery that monoamine nerves end on the central microvessels of the choroid plexus, pia-arachnoid and parenchyma has prompted an intense investigation as to their physiological and neuropathological roles. The source of the monoamine fibers to the pial vessels and choroid plexus was shown to be the superior cervical ganglion. Ganglionic stimulation causes vasoconstriction or vasodilation of pial vessels, an event depending upon the functional ratio of alpha to beta adrenergic receptors. Moreover, stimulation of the superior cervical ganglion evokes an inhibition of cerebrospinal fluid formation in choroid plexus. The locus coeruleus is the site of adrenergic nerve supply to the parenchymal capillaries and stimulation of this nucleus increases capillary permeability to small molecules and water. Neurotransmitter receptors (adrenergic, histamine, adenosine, dopamine, prostacyclin, prostaglandins and specific amino acids or neuropeptides) have been identified on microvessels and in many instances these transmitter actions are coupled to cyclic AMP synthesis. Moreover, cyclic AMP has been shown to increase the rate of capillary endothelial pinocytosis and produce brain edema. In small vessels containing smooth muscle cells cyclic AMP production improves cerebral blood flow via an initiation of vasodilatory processes. The presence of receptors for serotonin and acetylcholine have likewise been demonstrated to occur on cerebral microvessels. Limited information is available as to the receptor coupled actions of these two transmitters, but cholinergic mechanisms may act to restrict catecholamine-induced formation of cyclic AMP. Altered sensitivity of microvessels to neurotransmitters has been demonstrated following conditions of stroke, hypertension, aging, diabetes and X-irradiation.

Neuronal control of brain microvessel function

Cerebral capillary endothelium forms a barrier limiting and controlling the movement of ions and solutes between blood and brain. Recent anatomical, physiological and biochemical studies have suggested the possibility that capillary function may be directly controlled by neuronal structures. Alterations in neuronal systems involved in the regulation of microcirculation may account for microvascular dysfunctions which occur in different pathologic conditions.

Cerebral blood flow autoregulation and hypertension

Hypertension and antihypertensive therapy have clinically important effects on cerebral blood flow. The autoregulatory changes that occur with chronic arterial hypertension should influence the clinician's choice of antihypertensive agents and the rapidity with which the blood pressure is lowered in order to avoid symptoms of focal or global cerebral hypoperfusion.

Angiotensin-converting enzyme activity is reduced in brain microvessels of spontaneously hypertensive rats

Angiotensin -converting enzyme (ACE) activity in brain microvessels of spontaneously hypertensive rats (SHR) and Wistar Kyoto (WKY) controls was measured. Cerebral microvessels, prepared from the cerebral cortices by the albumin flotation and glass bead filtration technique, were free of neuronal and glial elements. ACE activity in brain microvessels of SHR was lower than that of WKY. A Woolf - Augustinsson -Hofstee plot showed that the reduction of the enzyme activity in SHR was due to a 30% decrease in Vmax, without any change in Km for substrate. The decrease of ACE activity in brain microvessels of SHR may indicate an impairment of the central renin-angiotensin system and may be related to cerebral microvascular dysfunctions occurring in hypertension.

Beta 2-adrenoceptor density on membranes and on intact mononuclear cells in essential hypertension

Alterations in the status or in the regulation of adrenoceptors may contribute to essential hypertension. This could be studied using the recently introduced radio-ligand binding techniques to characterize the adrenoceptors on human peripheral blood cells. The present study shows that patients with essential hypertension have a twofold increase of beta 2-adrenoceptor density on intact mononuclear cells as compared to normotensive controls: 859 +/- 260 (n = 10) vs. 420 +/- 119 (n = 10) maximal binding sites for (+/-) 125-Iodocyanopindolol expressed as molecules per cell (P less than 0.001). Furthermore, there is a highly significant correlation (r = 0.86) between the calculated mean arterial blood pressure and the beta 2-adrenoceptor density over a wide range of normal and increased blood pressure. These findings could only be demonstrated with intact mononuclear cells but not with membrane fractions. No difference was found in receptor affinity between patients with essential hypertension and normotensive controls. Thus, essential hypertension is combined with a higher beta 2-adrenoceptor density on intact mononuclear cells which might represent, for example, an increased density of prejunctional beta 2-adrenoceptors. Mean arterial blood pressure is positively correlated with beta 2-adrenoceptor density over a wide range of blood pressure in normotensives and hypertensives. The expression of beta 2-binding sites on the cell surface is possibly altered in essential hypertension resulting in a disparity between intracellular and extracellular binding sites as compared with normotensives.