Endothelium‐dependent responses in the microcirculation observed in vivo

Endothelium‐dependent responses were first demonstrated 40 years ago in the aorta. Since then, extensive research has been conducted in vitro using conductance vessels and materials derived from them. However, the microcirculation controls blood flow to vital organs and has been the focus of in vivo studies of endothelium‐dependent dilation beginning immediately after the first in vitro report. Initial in vivo studies employed a light/dye technique for selectively damaging the endothelium to unequivocally prove, in vivo, the existence of endothelium‐dependent dilation and in the microvasculature. Endothelium‐dependent constriction was similarly proven. Endothelium‐dependent agonists include acetylcholine (ACh), bradykinin, arachidonic acid, calcium ionophore A‐23187, calcitonin gene‐related peptide (CGRP), serotonin, histamine and endothelin‐1. Normal and disease states have been studied. Endothelial nitric oxide synthase, cyclooxygenase and cytochrome P450 have been shown to generate the mediators of the responses. Some of the key enzyme systems generate reactive oxygen species (ROS) like superoxide which may prevent EDR. However, one ROS, namely H₂O₂, is one of a number of hyperpolarizing factors that cause dilation initiated by endothelium. Depending upon microvascular bed, a single agonist may use different pathways to elicit an endothelium‐dependent response. Interpretation of studies using inhibitors of eNOS is complicated by the fact that these inhibitors may also inhibit ATP‐sensitive potassium channels. Other in vivo observations of brain arterioles failed to establish nitric oxide as the mediator of responses elicited by CGRP or by ACh and suggest that a nitrosothiol may be a better fit for the latter.     

Trophic effects of hypophyseal hormones on resistance vessels and the heart in normotensive and renal hypertensive rats

Folkow , B., Isaksson , O. G. P., Karlström , G., Lever , A. F. & Nordlander , M. 1992. Trophic effects of hypophyseal hormones on resistance vessels and the heart in normotensive and renal hypertensive rats. Acta Physiol Scand 144 , 291–306. Received 20 June 1991, accepted 7 October 1991. ISSN 0001–6772. Department of Physiology, University of Goteborg, Sweden, and MRC Blood Pressure Unit, Western Infirmary, Glasgow, Scotland, UK. Arterial pressure is an important determinant of cardiovascular structure and relates positively to it. The purpose of this study was to determine whether pituitary hormones influence the relation of pressure and structure. Male Sprague-Dawley rats, aged 5 weeks, were studied in three groups: the first underwent hypophysectomy; the second was hypophysectomized but received replacement therapy with growth hormone and thyroxine; the third served as controls. Four days later half of each group underwent unilateral renal artery clipping, the other half serving as normotensive controls. For 5 weeks estimates were made of systolic blood pressure, heart rate, body weight and plasma renin activity. Rats were then killed; left ventricular, kidney and adrenal weights were determined and, using hindquarter perfusion, estimates were made of resistance at maximal dilatation (reflecting inner radius), and of maximal pressor response (reflecting wall thickness). Results: (1) Hypophysectomy in non-clipped rats reduced growth rate, systolic blood pressure and heart rate while plasma renin activity rose. As related to pressure and to body weight, resistance at maximal dilatation, maximal pressor response, left ventricular weight remained at the juvenile values of a 5-week-old rat. Hormone replacement restored values to those of control rats aged 11 weeks. (2) Clipping in the control group rats increased systolic blood pressure more than in hypophysectomized and growth hormone and thyroxine receiving hypophysectomized groups even though plasma renin activity remained higher in hypophysectomized than in control rats. Plasma renin activity was highest in hypophysectomized rats with highest pressure. (3) Systolic blood pressure related positively to left ventricle weight, resistance at maximal dilatation, maximal pressor response and calculated wall thickness to inner radius ratios in all groups. However, these regressions were all, like renal structural adaptation, considerably depressed in the hypophysectomized group. Hormone replacement restored the relation of structure and pressure towards that of control group rats. Thus, growth hormone and thyroxine influence maturation of the normal cardiovascular system and greatly enhance its structural upward resetting in hypertension.     

Mechanisms of K(+) induced renal vasodilation in normo- and hypertensive rats in vivo

Aim: We investigated the mechanisms behind K⁺‐induced renal vasodilation in vivo in normotensive Sprague–Dawley (SD) rats and spontaneously hypertensive rats (SHR). Methods: Renal blood flow (RBF) was measured utilizing an ultrasonic Doppler flow probe. Renal vascular resistance (RVR) was calculated as the ratio of mean arterial pressure (MAP) and RBF (RVR = MAP/RBF). Test drugs were introduced directly into the renal artery. Inward rectifier K⁺ (K_ir) channels and Na⁺,K⁺‐ATPase were blocked by Ba²⁺ and ouabain (estimated plasma concentrations ～20 and ～7 μm ) respectively. Results: Confocal immunofluorescence microscopy demonstrated K_ir 2.1 channels in pre‐glomerular vessels of SD and SHR. Ba²⁺ caused a transient (6–13%) increase in baseline RVR in both SD and SHR. Ouabain had a similar effect. Elevated renal plasma [K⁺] (～12 mm ) caused a small and sustained decrease (5–13%) in RVR in both strains. This decrease was significantly larger in SHR than in SD. The K⁺‐induced vasodilation was attenuated by Ba²⁺ in control SD and SHR and by ouabain in SD. Nitric oxide (NO) blockade using l ‐NAME treatment increased MAP and decreased RBF in both rat strains, but did not affect the K⁺‐induced renal vasodilation. Conclusion: K⁺‐induced renal vasodilation is larger in SHR, mediated by K_ir channels in SD and SHR, and in addition, by Na⁺,K⁺‐ATPase in SD. In addition, NO is not essential for K⁺‐induced renal vasodilation.     

Structure of renal glomeruli in hypertensive NISAG rats reared by normotensive females

Morphometry of ultrastructural components of renal glomeruli in 3-week- and 6-month-old rats with hereditary stress-induced arterial hypertension (NISAG rats) reared by normotensive Wistar females showed deceleration of the development of arterial hypertension in NISAG rats and attenuated its negative consequences for kidneys as the target organs.     

Effect of alveolar hypoxia on reactivity of pial vessels in normotensive and spontaneously hypertensive rats

Intravital television microscopy revealed reduced reactivity of pial arterioles in response to alveolar hypoxia in spontaneously hypertensive rats (SHR) compared to normotensive (WKY) rats, which manifested in decreased number of dilatory reactions and in less prononced vascular dilation.     

Defective renal dopamine D1‐like receptor signal transduction in obese hypertensive rats

It is reported that dopamine promotes renal sodium excretion via activation of D₁‐like dopamine receptors located on the proximal tubules. In spontaneously hypertensive rats the natriuretic and diuretic response to exogenously administered and endogenously produced dopamine is reduced, which results from a diminished dopamine‐induced inhibition of the enzyme, Na⁺,K⁺‐ATPase. The present study was designed to examine dopamine‐receptor mediated inhibition of Na⁺,K⁺‐ATPase and its associated signal transduction pathway in the proximal tubules of Zucker obese and lean control rats. The obese animals were hypertensive, hyperinsulinaemic and hyperglycaemic compared with the lean rats. While dopamine caused inhibition of Na⁺,K⁺‐ATPase activity in lean rats, this effect was significantly attenuated in the obese animals. There was significant reduction in D₁‐like receptor numbers in the basolateral membranes of obese rats compared with lean rats with no change in the affinity to the ligand [³H]SCH 23390 between the two groups of rats. Dopamine failed to stimulate G proteins as measured by [³⁵S]GTPγS binding in the obese rats. Also, dopamine was unable to cause phospholipase‐C activation in obese rats, but it did activate phospholipase‐C in lean rats. These results show that reduction in D₁‐like receptor numbers and a defect in receptor–G protein coupling may account for the inability of dopamine to activate the D₁‐like receptor‐coupled signal transduction pathway and cause inhibition of Na⁺,K⁺‐ATPase in the obese hypertensive rats.     

Trophic effects of hypophyseal hormones on resistance vessels and the heart in normotensive and renal hypertensive rats.

Arterial pressure is an important determinant of cardiovascular structure and relates positively to it. The purpose of this study was to determine whether pituitary hormones influence the relation of pressure and structure. Male Sprague-Dawley rats, aged 5 weeks, were studied in three groups: the first underwent hypophysectomy; the second was hypophysectomized but received replacement therapy with growth hormone and thyroxine; the third served as controls. Four days later half of each group underwent unilateral renal artery clipping, the other half serving as normotensive controls. For 5 weeks estimates were made of systolic blood pressure, heart rate, body weight and plasma renin activity. Rats were then killed; left ventricular, kidney and adrenal weights were determined and, using hindquarter perfusion, estimates were made of resistance at maximal dilatation (reflecting inner radius), and of maximal pressor response (reflecting wall thickness). Results: (1) Hypophysectomy in non-clipped rats reduced growth rate, systolic blood pressure and heart rate while plasma renin activity rose. As related to pressure and to body weight, resistance at maximal dilatation, maximal pressor response, left ventricular weight remained at the juvenile values of a 5-week-old rat. Hormone replacement restored values to those of control rats aged 11 weeks. (2) Clipping in the control group rats increased systolic blood pressure more than in hypophysectomized and growth hormone and thyroxine receiving hypophysectomized groups even though plasma renin activity remained higher in hypophysectomized than in control rats. Plasma renin activity was highest in hypophysectomized rats with highest pressure. (3) Systolic blood pressure related positively to left ventricle weight, resistance at maximal dilatation, maximal pressor response and calculated wall thickness to inner radius ratios in all groups. However, these regressions were all, like renal structural adaptation, considerably depressed in the hypophysectomized group. Hormone replacement restored the relation of structure and pressure towards that of control group rats. Thus, growth hormone and thyroxine influence maturation of the normal cardiovascular system and greatly enhance its structural upward resetting in hypertension.     

Acquisition of conditioned suppression and responsivity to thermal stimulation in spontaneously hypertensive,renal hypertensive and normotensive rats

The acquisition of conditioned suppression of instrumental responding and responsivity to thermal stimulation were assessed in spontaneously hypertensive rats of the Okamoto-Aoki strain (SHRs), renal hypertensive rats of the Wistar-Kyoto strain (WKYs), and normotensive WKY rats. Spontaneously hypertensive rats showed significantly faster acquisition of conditioned suppression than age-matched WKY normotensive rats. This acquisition difference between SHRs and normotensive WKYs was maintained even following chronic preexposure to shock alone; a treatment that retarded normal acquisition of conditioned suppression. In contrast, renal hypertensive WKYs acquired conditioned suppression at the same rate as age-matched normotensive WKYs. Spontaneously hypertensive rats and renal hypertensive WKYs both showed significantly longer latencies than normotensive WKYs to respond to thermal stimulation using a hot-plate assay. These outcomes suggest that the relationship between blood pressure and the acquisition of conditioned suppression in SHRs is either pleiotrophic or a consequence of random fixation, and that acquisition of conditioned suppression may serve as a valuable marker variable of hypertension in the SHR. The dissociation between acquisition of conditioned suppression and responsivity to thermal stimulation suggests that different mechanisms may mediate nociceptive responses to shock and thermal stimulation in these models of hypertension.     

Selective distribution of medial thickening in the renal vessels in experimental hypertension

Direct effects, especially the distribution of medial thickening due to chronic hypertension on the renal arterial tree, were examined pathologically and morphometrically in cats with one-kidney DOCA-salt hypertension. Twenty-one adult male cats were divided into three groups: (i) an experimental group of one-kidney DOCA-salt hypertension (12 cats): (ii) a uninephrectomized group without administration of DOCA and salt (four cats); and (iii) a group without uninephrectomy and administration of DOCA and salt (five cats). The duration of hypertension varied from 3 weeks to 4 months and the aortic blood pressure was monitored every 5 min. The increase of mean blood pressure (MBP) of the experimental group was 15–44 mmHg. A significant medial thickening of the right renal arterial tree was evaluated by a comparison of the right and left regression lines of each case between the midwall radius (R) and medial thickness (D) of arteries in a distended state. The evaluation was made separately for arteries with R values above or below 40 μm, because a regression line between R and D showed a maximum bend at a level of R from 30 to 40 μm. The arteries of the right kidney with a diameter less than 40 μm, corresponding to the interlobular arteries and afferent arterioles, showed significant medial thickening in six cases. Medial thickening was absent in other cats of the experimental group and cats of a control group given uninephrectomy alone. Increase of MBP during the hypertensive phase of these six cases was above 34.6 mmHG and all values of other cats without medial thickening were below this level. The renal arteries of the right kidney with a diameter larger than 40 μm showed medial thickening in only one cat. This preferential distribution of medial thickening of the interlobular arteries and afferent arterioles due to one kidney DOCA-salt hypertension was considered to be a direct effect of hypertension and a result of the specific functional (autoregulation of the renal blood flow) and structural features of these arteries.     

The distensibility of the resistance vessels of skeletal muscle in hypertensive patients

The distensibility of the resistance vessels of skeletal muscle was evaluated in 23 patients with essential arterial hypertension (WSHO grade T-II), and in 14 normotensive control subjects. Five of the hypertensive patients were well treated for over 2 years and 18 were untreated. The ¹³³Xenon wash-out rate from the anterior tibial muscle during reactive hyperemia was recorded before and during an increase of the vascular transmural pressure, brought about by application of a subatmospheric pressure to the leg. At ambient pressure the ¹³³Xenon wash-out rate did not differ between the normotensive and hypertensive subjects, reflecting equal muscle blood flows. However, when the transmural pressure was augmented the ¹³³Xenon wash-out rate in the normotensive subjects increased about twice as much as in the hypertensive patients. This suggests a decreased distensibility of the resistance vessels in hypertensive patients as compared to normotensive subjects, and supports the concept that structural changes of these vessels take place in arterial hypertension. Since the vascular distensibility was equally reduced in the untreated and well treated hypertensive patients it is indicated that the structural changes in the resistance vessels of the leg muscles do not readily decline during antihypertensive treatment.     

Collagen‐induced arthritis increases inducible nitric oxide synthase not only in aorta but also in the cardiac and renal microcirculation of mice

Rheumatoid arthritis (RA) may promote endothelial dysfunction. This phenomenon requires further investigation, especially in collagen‐induced arthritis (CIA), as it is considered the experimental model most similar to RA. The objectives of this study were to identify CIA‐induced changes in noradrenaline (NE) and acetylcholine (ACh) responses in mice aortas that may suggest endothelial dysfunction in these animals. Moreover, we characterize CIA‐induced modifications in inducible nitric oxide synthase (iNOS) expression in the aortas and cardiac and renal tissues taken from these mice that may be related to possible endothelial dysfunction. Male DBA/1J mice were immunized with 100 μg of emulsified bovine collagen type II (CII) plus complete Freund's adjuvant. Twenty‐one days later, these animals received a boost of an additional 100 μg plus incomplete Freund's adjuvant. Fifteen days after the onset of the disease, aortic rings from CIA and control mice were challenged with NE and ACh in an organ bath. In these animals, iNOS was detected through immunohistochemical analysis of aorta, heart and kidneys. Plasma nitrite concentration was determined using the Griess reaction. CIA did not change NE or ACh responses in mice aorta but apparently increased the iNOS expression not only in aorta, but also in cardiac and renal microcirculation. In parallel, CIA reduced nitrite plasma concentration. In mice, CIA appears to increase the presence of iNOS in aorta, as well as in heart and in kidney microcirculation. This iNOS increase occurs apparently in parallel to a reduction of the bioavailability of NO. This phenomenon does not appear to change NE or ACh responses in aorta.     

Chronic increases in transmural pressure reduce NO‐mediated dilations in isolated resistance arteries of the hamster

It is unclear whether the impairment of NO‐mediated dilation in hypertension is the cause or the consequence of high blood pressure. We therefore studied in isolated resistance arteries whether elevated transmural pressure affects NO‐mediated dilation. Arteries (n=5–7) were perfused at hydrostatic pressures of either 45, 120 or 160 mmHg for 48 h. Subsequently, diameter and calcium responses (fura 2) were studied at a transmural pressure of 45 mmHg. Pre‐exposure to 120 and 160 mmHg reduced resting diameters and minimal diameters after stimulation with noradrenaline and significantly increased corresponding intracellular free calcium levels in vascular smooth muscle. Moreover, the NO‐mediated dilation in response to acetylcholine was significantly reduced although the increase in endothelial calcium was not altered. Dilations induced by the NO donor SNP were not affected. It is concluded that chronically elevated pressure per se impairs endothelial NO production by a mechanism distal to receptor‐dependent calcium increases.     

Influence of sympathetic and AT1‐receptor blockade on angiotensin II and adrenergic agonist‐induced renal vasoconstrictions in spontaneously hypertensive rats

Aim: This study investigated the influence of angiotensin II (Ang II) receptor and adrenergic blockade on the renal vasoconstrictions caused by Ang II and adrenergic agonists in spontaneously hypertensive rats (SHR). Methods: Forty‐eight SHR were subjected to 7 days of losartan (10 mg kg^?1 day^?1 p.o.), carvedilol (5 mg kg^?1 day^?1 p.o.) or losartan + carvedilol (10 mg kg^?1 day^?1 + 5 mg kg^?1 day^?1 p.o.). On day 8, the rats were anaesthetized and renal vasoconstrictor experiments performed. One group of rats underwent acute unilateral renal denervation. Results: There were significant (P < 0.05) reductions in the renal vasoconstrictor responses to noradrenaline, phenylephrine, methoxamine and Ang II after losartan and carvedilol treatments compared with that in untreated rats (all P < 0.05). However, in renally denervated SHR treated with carvedilol, the vasoconstrictor responses to all the vasoactive agents were enhanced compared with those in SHR with intact renal nerves treated with carvedilol. Intact SHR given both losartan and carvedilol showed greater renal vasoconstrictor responses to the vasoactive agents than when given either losartan or carvedilol alone (all P < 0.05). Conclusion: Carvedilol reduced the vasoconstrictor response to Ang II and all the adrenergic agonists in the presence of the renal nerves, but, following the removal of renal sympathetic activity, carvedilol enhanced the sensitivity of both renal α₁‐adrenoceptors and AT₁ receptors to the vasoactive agents. Co‐treatment with losartan and carvedilol reduced the renal vasoconstrictor responses to exogenously administered vasoactive agents but to a lesser extent than losartan or carvedilol alone. The results obtained demonstrate an interaction between Ang II receptors and adrenergic neurotransmission in the SHR.     

Simultaneous renal hypertension and type 2 diabetes exacerbate vascular endothelial dysfunction in rats

Despite the high rate of occurrence of both diabetes and hypertension in humans, the cardiovascular effects of the two conditions have not been investigated when they occur simultaneously. Thus this study examined the vascular effects of simultaneous type 2 diabetes and renal hypertension on endothelial function. Serum malondialdehyde and systolic blood pressure (SBP) were measured, glucose tolerance test (GTT) was performed, and concentration-response to phenylephrine (PE) in the absence and presence of nitro-l-arginine methyl ester (l-NAME), acetylcholine and sodium nitroprusside were conducted on aortic rings from diabetic control, type 2 diabetes, sham-operated, renal hypertensive, and simultaneous type 2 diabetes plus hypertension rats respectively. Hypertension, diabetes, and simultaneous diabetes and hypertension were associated with either increased or decreased maximal responses (E(max)) of PE dependent on in the presence or absence of l-NAME. There was also increased serum malondialdehyde and decreased E(max) of acetylcholine. Thus simultaneous hypertension and diabetes caused a greater decrease in E(max) of acetylcholine compared to that seen with either diabetes or hypertension alone higher than that seen in hypertension. The blood glucose during GTT was lower than that seen in diabetes groups. Thus simultaneous type 2 diabetes and the SBP was renal hypertension is associated with improved glucose tolerance, but with further deterioration of endothelial dysfunction compared with either condition alone.