Christian Aalkjaer

Evidence for increased media thickness, increased neuronal amine uptake, and depressed excitation--contraction coupling in isolated resistance vessels from essential hypertensives.

The functional and morphologic characteristics of isolated subcutaneous resistance vessels (about 170 micron i.d.) from 15 untreated subjects with essential hypertension and 15 matched controls were examined. The vessels from the hypertensives had a 29% increase in the media-thickness-to-lumen-diameter ratio. The maximal force development to noradrenaline (NA) expressed as active pressure (an estimate of the pressure the vessels could have contracted against in vivo) was 30% higher in vessels from the hypertensives, while active media stress (force per square unit of smooth muscle) and sensitivity to NA was not significantly different. Increased active pressure, as well as unaltered active media stress and sensitivity, was seen for vasopressin, serotonin, angiotensin II, and K+. There was, however, an enhanced leftward shift of the NA sensitivity with cocaine (an inhibitor of the neuronal amine pump) in vessels from the hypertensives [pD2(+cocaine) and pD2(-cocaine) were 0.185 +/- 0.53) and 0.040 +/- 0.044, hypertensives and normotensives, respectively, p less than 0.05] suggesting an abnormality of presynaptic function in essential hypertension. Furthermore, the calcium sensitivity was depressed (pD2 was 4.197 +/- 0.050 and 4.381 +/- 0.068, hypertensives and normotensives, respectively, p less than 0.05), and the rate of relaxation was faster (p less than 0.05) in vessels from hypertensives, suggesting that excitation-contraction coupling might be depressed. The results suggest that the increased pressor response in essential hypertension can, to a large extent, be explained by altered vascular structure, while smooth muscle function is either unchanged or possibly depressed.

An electroneutral sodium/bicarbonate cotransporter NBCn1 and associated sodium channel

Two electroneutral, Na+-driven HCO-3 transporters, the Na+-driven Cl-/HCO-3 exchanger and the electroneutral Na+/ HCO-3 cotransporter, have crucial roles in regulating intracellular pH in a variety of cells, including cardiac myocytes, vascular smooth-muscle, neurons and fibroblasts; however, it is difficult to distinguish their Cl- dependence in mammalian cells. Here we report the cloning of three variants of an electroneutral Na+/HCO-3 cotransporter, NBCn1, from rat smooth muscle. They are 89–92% identical to a human skeletal muscle clone, 55–57% identical to the electrogenic NBCs and 33–43% identical to the anion exchangers. When expressed in Xenopus oocytes, NBCn1-B (which encodes 1,218 amino acids) is electroneutral, Na+-dependent and HCO-3-dependent, but not Cl--dependent. Oocytes injected with low levels of NBCn1-B complementary RNA exhibit a Na+ conductance that 4,4-diisothiocyanatostilbene-2,2′-disulphonate stimulates slowly and irreversibly.

Small artery structure is an independent predictor of cardiovascular events in essential hypertension.

Objective Structural abnormality of resistance arteries is a characteristic pathophysiological phenomenon in essential hypertension and can be assessed in vitro as an increase in the media: lumen ratio (M: L) of isolated small arteries. We have investigated whether M: L is a risk predictor in uncomplicated essential hypertensive patients. Recently, high M: L was demonstrated as a prognostic marker in patients at high cardiovascular risk, including normotensive type 2 diabetic patients. Since diabetes is associated with pressure-independent changes in M: L, the relevance of this finding to essential hypertension has been uncertain. Methods We conducted a follow-up survey of 159 essential hypertensive patients, who had previously been submitted to a M: L evaluation while participating in a clinical trial. They composed a homogeneous moderate-risk group, with no concomitant diseases, and represented 1661 years of follow-up. Results Thirty patients suffered a documented predefined cardiovascular event during follow-up. Increased relative risk (RR) was associated with M: L ≥ 0.083 (mean level of the hypertensive cohort), RR = 2.34 [95% confidence interval (CI) 1.11–4.95], and with M: L ≥ 0.098 (mean level of a normotensive control group + 2SD), RR = 2.49 (95% CI 1.21–5.11). Both results remained significant (RR = 2.19, 95% CI 1.04–4.64, and RR = 2.20, 95% CI 1.06–4.56, respectively) when adjusted for Heart Score level (10-year mortality risk-estimate, integrating age, gender, systolic blood pressure, cholesterol and smoking). Conclusion Abnormal resistance artery structure independently predicts cardiovascular events in essential hypertensive patients at moderate risk.

Vasomotion: cellular background for the oscillator and for the synchronization of smooth muscle cells

1 Vasomotion is the oscillation of vascular tone with frequencies in the range from 1 to 20 min−1 seen in most vascular beds. The oscillation originates in the vessel wall and is seen both in vivo and in vitro. 2 Recently, our ideas on the cellular mechanisms responsible for vasomotion have improved. Three different types of cellular oscillations have been suggested. One model has suggested that oscillatory release of Ca2+ from intracellular stores is important (the oscillation is based on a cytosolic oscillator). A second proposed mechanism is an oscillation originating in the sarcolemma (a membrane oscillator). A third mechanism is based on an oscillation of glycolysis (metabolic oscillator). For the two latter mechanisms, only limited experimental evidence is available. 3 To understand vasomotion, it is important to understand how the cells synchronize. For the cytosolic oscillators synchronization may occur via activation of Ca2+‐sensitive ion channels by oscillatory Ca2+ release. The ensuing membrane potential oscillation feeds back on the intracellular Ca2+ stores and causes synchronization of the Ca2+ release. While membrane oscillators in adjacent smooth muscle cells could be synchronized through the same mechanism that sets up the oscillation in the individual cells, a mechanism to synchronize the metabolic‐based oscillators has not been suggested. 4 The interpretation of the experimental observations is supported by theoretical modelling of smooth muscle cells behaviour, and the new insight into the mechanisms of vasomotion has the potential to provide tools to investigate the physiological role of vasomotion.

Changes in noradrenaline sensitivity and morphology of arterial resistance vessels during development of high blood pressure in spontaneously hypertensive rats

SUMMARY We have investigated whether differences seen in the pharmacological and morphological properties of mesenteric resistance vessels from spontaneously hypertensive rats (SHR) and normotensive Wistar-Kyoto (WKY) controls are also present in vessels from young SHR and WKY rats in which there is little difference in blood pressure (BP). Segments of small arteries (lumen diameter 150

Vasomotion – what is currently thought?

m) were taken from a specific location in the mesenteric bed of 6-, 12-, and 24-week-old SHR and WKY rats, and mounted on a myograph capable of directly measuring their tension. Vessels were set to internal circumference L, = 0.8 Lin where Ll00 was an estimate of the internal circumference the vessels would have had when relaxed in situ and under a transmural pressure of 100 mm Hg. At all ages, compared with WKY vessels, the effective lumen diameter, I, = L,/x, was smaller in the SHR vessels. However, media hypertrophy was seen only in vessels from 12- and 24-week-old SHRs. In physiological salt solution the noradrenaline sensitivity of all vessels was similar (EDM * 2.4 nM). However, inhibition of neuronal uptake with cocaine revealed that at all ages the noradrenaline sensitivity of the vascular smooth muscle cells in the SHR vessels was greater than that of the cells in the WKY vessels. The results also suggested that the neuronal noradrenaline uptake was greater in the SHR vessels at all ages. The main increase in BP in the SHRs occurred between the ages of 6 week and 12 weeks. The results are therefore consistent with the hypothesis that differences in the structure of the resistance vessels are among the factors responsible for the development and maintenance of genetic hypertension. However, they point also to the possible involvement of differences in the noradrenaline sensitivity of the smooth muscle cells in the resistance vessel wails.

Abnormal structure and function of isolated subcutaneous resistance vessels from essential hypertensive patients despite antihypertensive treatment

This minireview discusses vasomotion, which is the oscillation in tone of blood vessels leading to flowmotion. We will briefly discuss the prevalence of vasomotion and its potential physiological and pathophysiological relevance. We will also discuss the models that have been suggested to explain how a coordinated oscillatory activity of the smooth muscle tone can occur and emphasize the role of the endothelium, the handling of intracellular Ca2+ and the role of smooth muscle cell ion conductances. It is concluded that vasomotion is likely to enhance tissue dialysis, although this concept still requires more experimental verification, and that an understanding at the molecular level for the pathways leading to vasomotion is beginning to emerge.

Junctional and nonjunctional effects of heptanol and glycyrrhetinic acid derivates in rat mesenteric small arteries

The morphological and functional characteristics of isolated subcutaneous resistance vessels (about 230 microns internal diameter) from 13 patients treated for essential hypertension for a median period of 14 months and from 15 matched normotensive controls were examined. The blood pressure of the patients and the controls were not significantly different at the time of examination. However, although compared with the controls, the lumen diameter of the vessels from the patients was not significantly different, the media thickness to lumen diameter ratio was 19% greater. Furthermore, although there was no difference in the active pressure response of the vessels from the two groups, the vessels from the patients had a lower sensitivity to calcium, relaxed faster after a contraction and the sensitivity to exogenous noradrenaline shifted more to the left with cocaine. Since the abnormalities found here have previously also been found in vessels from patients with untreated essential hypertension, the study suggests that despite antihypertensive treatment to normotensive levels for about 1 year, some morphological as well as functional characteristics of the resistance arteries are not fully normalized. This could have consequences for the prognosis of essential hypertension.

Effects of pH on Vascular Tension: Which Are the Important Mechanisms?

Heptanol, 18α‐glycyrrhetinic acid (18αGA) and 18β‐glycyrrhetinic acid (18βGA) are known blockers of gap junctions, and are often used in vascular studies. However, actions unrelated to gap junction block have been repeatedly suggested in the literature for these compounds. We report here the findings from a comprehensive study of these compounds in the arterial wall. Rat isolated mesenteric small arteries were studied with respect to isometric tension (myography), [Ca2+]i (Ca2+‐sensitive dyes), membrane potential and – as a measure of intercellular coupling – input resistance (sharp intracellular glass electrodes). Also, membrane currents (patch‐clamp) were measured in isolated smooth muscle cells (SMCs). Confocal imaging was used for visualisation of [Ca2+]i events in single SMCs in the arterial wall. Heptanol (150 μM) activated potassium currents, hyperpolarised the membrane, inhibited the Ca2+ current, and reduced [Ca2+]i and tension, but had little effect on input resistance. Only at concentrations above 200 μM did heptanol elevate input resistance, desynchronise SMCs and abolish vasomotion. 18βGA (30 μM) not only increased input resistance and desynchronised SMCs but also had nonjunctional effects on membrane currents. 18αGA (100 μM) had no significant effects on tension, [Ca2+]i, total membrane current and synchronisation in vascular smooth muscle. We conclude that in mesenteric small arteries, heptanol and 18βGA have important nonjunctional effects at concentrations where they have little or no effect on intercellular communication. Thus, the effects of heptanol and 18βGA on vascular function cannot be interpreted as being caused only by effects on gap junctions. 18αGA apparently does not block communication between SMCs in these arteries, although an effect on myoendothelial gap junctions cannot be excluded.

Role of Extracellular and Intracellular Acidosis for Hypercapnia-Induced Inhibition of Tension of Isolated Rat Cerebral Arteries

The pH has marked effects on the blood flow in several vascular beds but the underlying mechanisms remain incompletely understood. It is still not agreed, for example, whether it is the fall in extracellular pH or intracellular pH that is responsible for changes in tone resulting from hypercapnic acidosis. The issue has been further complicated by the recent discovery that nitric oxide (NO) may also be involved in vasodilator responses to hypercapnia with the result that, in some laboratories, attention has been focused away from vascular smooth muscle. The recent availability of fluorescent dyes sensitive to pH has enabled some of the uncertainties in this field to be addressed. In light of these new observations, we have attempted to put older viewpoints in perspective. We conclude that, whilst a fall in smooth muscle intracellular pH is likely to be responsible for immediate responses to acidosis, the extracellular pH probably plays the predominant role in the steady state. The role of NO is best investigated in the cerebral circulation where it plays an important modulating role in the response to acidosis, and is probably of extravascular origin.