Relationship between the sympathetic nervous system and vascular smooth muscle: a morphometric study of adult and juvenile spontaneously hypertensive rat/Wistar-Kyoto rat caudal artery

Angiotensinergic innervation of the kidney: present knowledge and its significance

Intrarenal neurotransmission implies the co-release of neuropeptides at the neuro-effector junction with direct influence on parameters of kidney function. The presence of an angiotensin (Ang) II-containing phenotype in catecholaminergic postganglionic and sensory fibers of the kidney, based on immunocytological investigations, has only recently been reported. These angiotensinergic fibers display a distinct morphology and intrarenal distribution, suggesting anatomical and functional subspecialization linked to neuronal Ang II-expression. This review discusses the present knowledge concerning these fibers, and their significance for renal physiology and the pathogenesis of hypertension in light of established mechanisms. The data suggest a new role of Ang II as a co-transmitter stimulating renal target cells or modulating nerve traffic from or to the kidney. Neuronal Ang II is likely to be an independent source of intrarenal Ang II. Further physiological experimentation will have to explore the role of the angiotensinergic renal innervation and integrate it into existing concepts.

Paracrine control of vascular innervation in health and disease

Proper vascular regulation is of paramount importance for the control of blood flow to tissues. In particular, the regulation of peripheral resistance arteries is essential for several physiological processes, including control of blood pressure, thermoregulation and increase of blood flow to central nervous system and heart under stress conditions such as hypoxia. Arterial tone is regulated by the periarterial autonomic nervous plexus, as well as by endothelium-dependent, myogenic and humoral mechanisms. Underscoring the importance of proper vascular regulation, defects in these processes can lead to diseases such as hypertension, orthostatic hypotension, Raynaud's phenomenon, defective thermoregulation, hand-foot syndrome, migraine and congestive heart failure. Here, we review the molecular mechanisms controlling the development of the periarterial nerve plexus, retrograde and localized signalling at neuro-effector junctions, the molecular and cellular mechanisms of vascular regulation and adult plasticity and maintenance of periarterial innervation. We particularly highlight a newly discovered role for vascular endothelial growth factor in the structural and functional maintenance of arterial neuro-effector junctions. Finally, we discuss how defects in neuronal vascular regulation can lead to disease.

Hyperinnervation of mesenteric arteries in spontaneously hypertensive rats by sympathetic but not primary afferent axons

Hypertrophy of the perivascular plexus is thought to play a role in the development of hypertension in spontaneously hypertensive rats (SHR). However, it is not known whether the sympathetic varicosities are more numerous or larger, or form more neurovascular junctions. Further, a parallel hypertrophy of primary afferent terminals around the vessels might modulate any effects of hypertrophied sympathetic terminals. We have investigated the perivascular plexus around second-order mesenteric arteries of SHR and Wistar-Kyoto (WKY) rats by electron microscopy. Noradrenergic terminals were identified by the presence of small granular vesicles after chromaffin fixation, and substance P (SP+) afferent axons were identified by immunohistochemistry. The numbers of noradrenergic axon and varicosity profiles were higher (48 and 25%, respectively) in SHR than in WKY rats, and the majority lay closer to the medio-adventitial border. In contrast, there was no difference in the numbers of SP+ axons. Sympathetic and SP+ varicosities were indistinguishable in size, shape, vesicle content and mitochondrion content between each other and between the strains. However, both the number of neuromuscular junctions and the proportion of varicosities that formed them in SHR arteries were more than double those in WKY vessels. The data clearly show that hyperinnervation in SHR is specific for noradrenergic axons.

Different vascular responsiveness to angiotensin II in two normotensive rat strains

Rats of the Fischer 344 (F344) and Wistar Kyoto (WKY) strains are known to present differences in stimuli responses involving the renin-angiotensin system and in cardiovascular responses to an acoustic startle stimulus. Here we compared the vascular reactivity to angiotensin II (ANG II) of these normotensive, inbred rat strains. Blood pressure (BP) and heart rate (HR) were recorded in conscious rats, before and after a neurohumoral blockade obtained by successive administration of chlorisondamine, enalapril, a V1-vasopressinergic receptor antagonist (Manning compound) and atropine methyl nitrate. BP was restored by a constant infusion of noradrenaline. Boluses of ANG II ranging from 0.001 to 1280 ng/kg were injected randomly. Average dose-response curves were established. After neurohumoral blockade, the minimum mean BP (MBP) produced by hydralazine (3 mg/kg, i.v.) and the maximum MBP produced by noradrenaline (60 microg/mL and 800 microL/min, i.v.) were used to reflect arterial wall structure. The maximal systolic blood pressure (SBP) and pulse pressure (PP) responses to ANG II were higher in F344 compared with WKY (+86 +/- 3 mmHg vs. +71 +/- 3 mmHg, P < 0.01 for SBP, +31 +/- 2 mmHg vs. +18 +/- 1 mmHg, P < 0.001 for PP). After the ANG II type 1 (AT1) receptor blocker valsartan, ANG II had no significant effect on BP. F344 and WKY exhibited the same maximum MBP in response to noradrenaline. However, MBP level following hydralazine was higher in F344 (F344: 48 +/- 2 mmHg vs. WKY: 37 +/- 3 mmHg, P < 0.01). The amplification in F344 of the vasoconstrictive response to ANG II mediated by AT1 receptors is compatible with a high number of AT1 receptors in this strain. In F344, the exaggerated systolic and PP responses to ANG II and the higher MBP level after hydralazine most likely reflect a structural modification of the arterial wall such as hypertrophic remodelling in F344.

Involvement of gamma-aminobutyric acid (GABA) B receptors in the hypotensive effect of systemically administered GABA in spontaneously hypertensive rats

We investigated the effects of intraduodenally (i.d.) administered gamma-aminobutyric acid (GABA) on blood pressure (BP) in anesthetized spontaneously hypertensive rats (SHR) and the mechanism underlying this effect, especially the type of GABA receptor involved in the depressive effect of this amino acid. GABA (0.3 to 300 mg/kg, i.d.) caused a dose-related decrease in the BP of 9.20 +/- 3.96 to 35.0 +/- 5.34 mmHg (mean +/- S.E.M.) that lasted for 30 to 50 min. The minimum effective i.d. dose of GABA was 0.3 to 1.0 mg/kg. Results pertaining to the mechanism underlying the GABA-induced effects on BP were as follows: a) GABA did not alter the BP-related effects of exogenous noradrenaline and acetylcholine; b) pretreatment with hexamethonium decreased the GABA-induced fall in BP, and GABA tended to reduce the pressor response associated with injection of dimethyl phenylpiperazinium; and c) pretreatment with 2-hydroxysaclofen markedly reduced the GABA-induced drop in BP, whereas pretreatment with bicuculline did not. In conclusion, in SHR, low-dose (0.3 to 1.0 mg/kg, i.d.) GABA had a hypotensive effect, which may result from attenuation of sympathetic transmission through the activation of GABA(B) receptors at presynaptic or ganglionic sites.

Effects of spaceflight on human calf hemodynamics

Chronic microgravity may modify adaptations of the leg circulation to gravitational pressures. We measured resting calf compliance and blood flow with venous occlusion plethysmography, and arterial blood pressure with sphygmomanometry, in seven subjects before, during, and after spaceflight. Calf vascular resistance equaled mean arterial pressure divided by calf flow. Compliance equaled the slope of the calf volume change and venous occlusion pressure relationship for thigh cuff pressures of 20, 40, 60, and 80 mmHg held for 1, 2, 3, and 4 min, respectively, with 1-min breaks between occlusions. Calf blood flow decreased 41% in microgravity (to 1.15 +/- 0.16 ml x 100 ml(-1) x min(-1)) relative to 1-G supine conditions (1.94 +/- 0.19 ml x 100 ml(-1) x min(-1), P = 0.01), and arterial pressure tended to increase (P = 0.05), such that calf vascular resistance doubled in microgravity (preflight: 43 +/- 4 units; in-flight: 83 +/- 13 units; P < 0.001) yet returned to preflight levels after flight. Calf compliance remained unchanged in microgravity but tended to increase during the first week postflight (P > 0.2). Calf vasoconstriction in microgravity qualitatively agrees with the "upright set-point" hypothesis: the circulation seeks conditions approximating upright posture on Earth. No calf hemodynamic result exhibited obvious mechanistic implications for postflight orthostatic intolerance.

Ontogenetic aspects of hypertension development: analysis in the rat

In this review, we attempt to outline the age-dependent interactions of principal systems controlling the structure and function of the cardiovascular system in immature rats developing hypertension. We focus our attention on the cardiovascular effects of various pharmacological, nutritional, and behavioral interventions applied at different stages of ontogeny. Several distinct critical periods (developmental windows), in which particular stimuli affect the further development of the cardiovascular phenotype, are specified in the rat. It is evident that short-term transient treatment of genetically hypertensive rats with certain antihypertensive drugs in prepuberty and puberty (at the age of 4-10 wk) has long-term beneficial effects on further development of their cardiovascular apparatus. This juvenile critical period coincides with the period of high susceptibility to the hypertensive effects of increased salt intake. If the hypertensive process develops after this critical period (due to early antihypertensive treatment or late administration of certain hypertensive stimuli, e.g., high salt intake), blood pressure elevation, cardiovascular hypertrophy, connective tissue accumulation, and end-organ damage are considerably attenuated compared with rats developing hypertension during the juvenile critical period. As far as the role of various electrolytes in blood pressure modulation is concerned, prohypertensive effects of dietary Na+ and antihypertensive effects of dietary Ca2+ are enhanced in immature animals, whereas vascular protective and antihypertensive effects of dietary K+ are almost independent of age. At a given level of dietary electrolyte intake, the balance between dietary carbohydrate and fat intake can modify blood pressure even in rats with established hypertension, but dietary protein intake affects the blood pressure development in immature animals only. Dietary protein restriction during gestation, as well as altered mother-offspring interactions in the suckling period, might have important long-term hypertensive consequences. The critical periods (developmental windows) should be respected in the future pharmacological or gene therapy of human hypertension.

Ultrastructure of sympathetic axons and their structural relationship with vascular smooth muscle

This review focuses on the more recent findings of the structure of sympathetic postganglionic axons and the association of their varicose terminals with vascular smooth muscle. These studies have investigated the innervation of a wide range of vessels from different regions of the vasculature in the rat, guinea pig and rabbit and have predominantly used serial sections and computerised three-dimensional reconstructions of entire varicosities. They have shown, contrary to previous studies conducted in the 1960s and 1970s, that sympathetic axon varicosities commonly form structurally specialised neuromuscular junctions with vascular smooth muscle cells of most resistance arteries and some small veins. In addition, they have shown that most axon varicosities innervating small arterioles and small mesenteric veins form neuromuscular junctions, indicating that neurotransmitter is primarily released at such neuromuscular junctions. This review discusses the structure of sympathetic neuromuscular junctions, their development, structural diversity and distribution on vessels from different regions of the vasculature. These more recent structural findings and their possible significance for our understanding of mechanisms involved in neural transmission in blood vessels is discussed.

Noradrenergic transmission in the tail artery of hypertensive rats transgenic for the mouse renin gene Ren-2

1. The aim of the present study was to analyse the noradrenergic transmission in the tail artery of hypertensive rats transgenic for the mouse renin gene Ren-2 (TGR) in comparison with its control, the Sprague-Dawley (SD) rat. 2. Electrical field stimulation (EFS) of vascular segments produced frequency-dependent vasoconstrictions that were significantly greater in TGR arteries. 3. These contractions were abolished by tetrodotoxin (0.1 microM). Phentolamine (50 nM) and prazosin (1 - 10 nM) produced an inhibition of these responses that was significantly greater in SD arteries, whereas that produced by yohimbine (0.5-1 microM) was higher in TGR arteries. In both strains, propranolol (1 microM) potentiated the responses to EFS, and this increase was observed at lower frequencies in TGR arteries. 4. The EFS-evoked [3H]-noradrenaline (NA) release was significantly greater in TGR than in SD rats. However, NA (10 nM-10 microM) reduced and yohimbine and phentolamine (10 nM-10 microM) increased the tritium outflow to a similar degree in both strains. 5. Exogenous NA also induced greater vasoconstriction in TGR arteries. 6. These results suggest the existence in TGR tail artery of an increase in: (a) NA-release and alpha 2-adrenoceptor-mediated contractions, which could contribute to the elevated blood pressure in these rats; and (b) beta-adrenoceptor-mediated vasodilatations, which may be a mechanism to counteract high blood pressure.

Proportions and structure of contacting and non-contacting varicosities in the perivascular plexus of the rat tail artery

Most sympathetic postganglionic noradrenergic varicosities of the perivascular plexus of small muscular arteries in laboratory mammals make contact with the outer smooth muscle cells of the media at neuromuscular junctions. These neurovascular junctions have most of the characteristics of those in skeletal muscle. In the rat tail artery, which bears a particularly dense perivascular plexus, many studies indicate that both purinergic and noradrenergic mechanisms underlie neurally mediated vasoconstriction. We have examined the relationship of large axonal varicosities to the smooth muscle surface of proximal parts of this vessel using three-dimensional reconstructions from serial thin sections photographed in the electron microscope. Unlike in small arterioles, less than 50% of the large photographed in the electron microscope. Unlike in small arterioles, less than 50% of the large varicosities lying within 1 micron of the outer surface of this artery were found to make neuromuscular junctions. In some non-contacting varicosities, accumulations of synaptic vesicles were aggregated toward axonal membrane which was bare of Schwann cell toward the vessel surface. Prejunctional membrane specializations were detected at 20% of contacting and 12% of non-contacting varicosities. All of the latter lay close (< 350nm) to the smooth muscle. These anatomical data suggest that, in the rat tail artery, transmitter release by exocytosis may occur from both types of varicosity.