Blood-flow sensing by anionic biopolymers.

Using 23Na-NMR techniques we could show that the polyanion proteoheparan sulfate integrated into the membrane of endothelial cells may serve as "flow sensor'. Based on its viscoelastic properties, heparan sulfate proteoglycan is present as a random coil under "no flow' conditions, whereby most of its polyanionic sites undergo intramolecular hydrogen bonding. With increasing flow the macromolecule becomes unfolded into a filamentous structure. Additional anionic binding sites to which Na+ ions from the blood bind are released by this shear stress-dependent conformational change. The Na+ binding triggers the signal transduction chain for a vasodilatory vessel reaction. Decrease in flow effects, for reasons of the intramolecular elastic recoil forces of the macromolecules, an entropic coiling, the release of Na+ ions and thus an interruption of the signal chain. Proteoheparan sulfate adsorbed onto a hydrophobic surface in physiological Krebs solution at pH 7.3 demonstrated clearly its characteristic as a Na+ sensor. While Ca2+ ions modulated the adsorption (promotion with increasing Ca2+ concentrations) by changing the conformation of the sensor molecule, the adsorbed amount was determined preferably by the Na+ concentration. K+ and Mg2+ ions showed slightly desorbing properties with increasing concentrations. Thus, it may be concluded that Na+ ions play the role as "first messenger' in flow-dependent vasodilation.

Endothelial dysfunction in human atherosclerotic coronary arteries.

Human coronary arteries were taken from heart transplant patients. Arteriosclerotic arteries were more depolarized and constricted over the whole PO2 range between 535 and 0 mmHg. During oxygen deficiency, control preparations showed a maximal hyperpolarization of delta V = 10.9 mV and a maximal relaxation of delta T = 0.466 g. Arteriosclerotic arteries, however, became hyperpolarized by merely delta V = 7.1 mV and relaxed by delta T = 0.258 g. In normal coronary arteries, indomethacin reduced the hypoxic hyperpolarization and dilatation at 30 mmHg PO2 by about 51%. The reduction was 27% in arteriosclerotic vessels. The complete removal of the endothelium caused a 49% (73% in arteriosclerotic coronaries) restriction of dilatory vascular reactivity. The relationship was quite similar for a carbogen Krebs solution. The hyperpolarizing and dilatory contribution of prostacyclin was 32% in normal and 12% in arteriosclerotic coronary arteries. The remainder could be attributed to the basal release of endothelium-derived hyperpolarizing factor (EDHF). Thus, it may be concluded that in arteriosclerotic blood vessels, prostacyclin (PGI2) synthesis and release are predominantly diminished. Finally, we found that the ratio PGI2/EDHF in the voltage and tension changes strongly shifted to the PGI2 side with a declining oxygen concentration. This is true for normal and arteriosclerotic vessels. In addition, a disturbed transmembrane cation distribution in the arteriosclerotic coronary vessels may be an additional explanation for the depolarized membrane potential and increased muscle tone. [Na+]i of normal arteries amounted to 16.6, of arteriosclerotic arteries to 60.9 mmol.l-1; [K+]i values were 134.6 in normal and 95.0 mmol.l-1 in arteriosclerotic coronaries, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)

Membrane physiological reactions of human arteriosclerotic coronary arteries to hypoxia.

Human coronary arteries were taken from heart transplant patients. Arteriosclerotic arteries were more depolarized and constricted over the whole PO2 range between 535 and 0 mm Hg. During oxygen deficiency, control preparations showed a maximal hyperpolarization of delta V = 10.9 mV and a maximal relaxation of delta T = 0.466 g. Arteriosclerotic arteries, however, became hyperpolarized by merely delta V = 7.1 mV and relaxed by delta T = 0.258 g. In normal coronary arteries, indomethacin reduced the hypoxic hyperpolarization and dilation at 30 mm Hg PO2 by about 51%. The reduction was 26% in arteriosclerotic vessels. The complete removal of the endothelium caused a 49% (74%) restriction of dilatory vascular reactivity. The relationship was quite similar for a carbogen Krebs solution. The hyperpolarizing and dilatory contribution by prostacyclin was 32% in normal and 12% in arteriosclerotic coronary arteries. The remainder could be attributed to the basal release of the endothelial dilator endothelium-derived hyperpolarizing factor (EDHF). Thus, it may be concluded that in arteriosclerotic blood vessels, prostacyclin (PGI2) synthesis and release are predominantly diminished. Finally, we found that the ratio PGI2/EDHF in the voltage and tension changes strongly shifted to the PGI2 side with a declining oxygen concentration. This is true for normal and arteriosclerotic vessels. In accordance with the activation curve for vascular smooth muscle, the hyperpolarization leads to relaxation via a closure of Ca2+ channels. Hyperpolarization of 2.5 mV reduces the tension developed by one-half.

Role of prostacyclin in normal and arteriosclerotic human coronary arteries during hypoxia.

Human coronary arteries were taken from heart transplant patients. Arteriosclerotic arteries were more depolarized and constricted over the whole PO2 range between 535 and 0 mm Hg. During oxygen deficiency, control preparations showed a maximal hyperpolarization of delta V = 10.9 mV and a maximal relaxation of delta T = 0.466 g. Arteriosclerotic arteries, however, became hyperpolarized by merely delta V = 7.1 mV and relaxed by delta T = 0.258 g. The isometric pretension was 2 g in all investigations. Two series of experiments, one with an application of indomethacin and another one with deendothelialized blood vessels, confirmed the hypothesis that the endothelium of arteriosclerotic coronary arteries arteries, indomethacin reduced the hypoxic hyperpolarization and dilatation at 30 mm Hg PO2 but about 51%. The reduction was 26% in arteriosclerotic vessels. The complete removal of the endothelium caused a 49% (74%) restriction of dilatory vascular reactivity. The relation was quite similar for a carbogen Krebs solution (resting, control conditions). The hyperpolarizing and dilatory contribution by prostacyclin was 32% in normal and 12% in arteriosclerotic coronaries. The rest can be attributed to the basal release of the endothelial dilator EDHR. Thus, it may be concluded that, in arteriosclerotic blood vessels, PGI2 synthesis and release are predominantly diminished or its effectivity is impaired. Finally we found the ratio PGI2/EDHF in the voltage and tension changes strongly shifted to the PGI2 side with a declining oxygen concentration. This is true for normal and arteriosclerotic vessels. In accordance with the activation curve for vascular smooth muscle, the hyperpolarization leads to relaxation via a closure of Ca2+ channels. 2.5 mV hyperpolarization reduces the tension developed by half.

Potassium channel activation, hyperpolarization, and vascular relaxation.

1) Numerous compounds and changes in physical state functions shift the membrane potential of vascular smooth muscle to more negative values. The consequence is a vasodilatation because Ca2+ channels are closed. K+ channel opening frequently causes the hyperpolarization. 2) Acidification of the blood substitute solution, a fall in O2 partial pressure, and an increase in blood flow dilate arterial vessels. Acidosis is associated with a rise in K+ permeability and a simultaneous fall in Na+ permeability. Prostacyclin has a 20-30% share, and EDHF a 70-80% share in hypoxic vasodilatation. Experiments with iloprost (PGI2 analogue) confirmed the K+ channel opening properties of this drug. A voltage-dependent K+ channel and a Ca(2+)-activated K+ channel, via the influence of cA-PK or cG-PK, are responsible for the hyperpolarization with iloprost and with oxygen deficiency. 3) With 23Na+ nuclear magnetic resonance techniques, it has been demonstrated that with flow-dependent vasodilatation, proteoheparan sulphate integrated in the membrane of endothelial cells possibly served as a "flow sensor". With an external strain, such a compound can go from a randomly coiled state to an oriented state. Based on these viscoelastic properties, heparan sulphate proteoglycan is present as a random coil under "no flow" conditions and as an unfurled filament structure with increasing flow. This conformational change produces additional anionic binding sites to which Na+ ions of the blood are bound. A membrane hyperpolarization could be directly initiated by this Na+ binding via the protein fraction within the macromolecule or via a change in zeta-potential. Therefore, these ions can trigger the signal transduction for a vasodilatory vessel reaction. Decrease in flow is followed by a structural change of the macromolecule towards coil conformation, a release of Na+ ions and, thus, an interruption of the signal chain. 4) Cicletanine, aqueous garlic extract, and ajoene cause a concentration-dependent membrane hyperpolarization and are potent vasodilators. A cicletanine concentration, which is attained by the dosage given to patients, is sufficient to produce these effects. Under noradrenaline, the cicletanine effect is amplified. Aqueous garlic extract and ajoene exert a hyperpolarizing and vasodilating influence even in a concentration which may occur in the extracellular space by the administration of a single garlic clove. 5) The stationary activation curve "developed force vs. membrane potential" satisfactorily explains the effects of K+ channel openers. The tight electromechanical coupling expressed by this curve comprises a 50% vasorelaxation for a 2.5 mV hyperpolarization. In the linear part of the curve, the coupling ratio is 5.1 mV/g.(ABSTRACT TRUNCATED AT 400 WORDS)

Vasodilatation evoked by K+ channel opening.

In arterial smooth muscle with normal tone or predepolarized and precontracted by noradrenaline, iloprost effects a dose-dependent hyperpolarization and relaxation. The hyperpolarization is due to K+ channel opening. In hypoxic vasodilatation, which is likewise induced by membrane hyperpolarization, a share of 20-30% falls to prostacyclin and 70-80% to an endothelium-derived hyperpolarizing factor.

Vasorelaxation in prostacyclin-hyperpolarized arterial smooth musculature.

In arterial smooth muscle with normal tone or predepolarized and precontracted by noradrenaline, prostacyclin (10(-9) to 10(-6) M) effects a dose-dependent hyperpolarization and relaxation. The hyperpolarization is due to K+ channel opening. In this hyperpolarized portion of the stationary, sigmoid activation curve, vasodilation can be explained by means of Ca2+ channels that are closed between -40 mV and -80 mV in a voltage-dependent manner.

Prostacyclin, endothelium-derived relaxing factor and vasodilatation.

In arterial smooth muscle with normal tone or predepolarized and precontracted by noradrenaline, prostacyclin (10(-9) to 10(-6) M) effects a dose-dependent hyperpolarization and relaxation. The hyperpolarization is due to K+ channel opening. In hypoxic vasodilatation, which is likewise induced by membrane hyperpolarization, a share of 20% falls to prostacyclin and 80% to an endothelium derived hyperpolarizing factor.

The significance of the endothelium for hypoxic vasodilatation.

In isolated segments of the A. carotis communis of the dog, the membrane potential and isometric tension were recorded in dependence on the oxygen partial pressure. In the range between 535 and 35 mmHg PO2, lowering of oxygen tension leads to a dose-dependent hyperpolarization of maximally 9.8 mV with a simultaneous decrease in tone of 0.746 g. Further reduction of PO2 below 35 mmHg causes depolarization and contraction. These effects can be abolished almost completely by alpha-receptor blockade. Application of 6-hydroxydopamine (1.8 mM) augments the hypoxic hyperpolarization and relaxation in the entire PO2 range between 535 and 0 mmHg, i.e., below 35 mmHg depolarization and contraction fail to appear. The increasing release of noradrenaline from terminal sympathetic nerve endings thus limits the hyperpolarization and relaxation occurring with reduction of PO2 and is mainly responsible for the depolarization and contraction below 35 mmHg. Application of indomethacin (10(-5) M) or BW 755 C (3.8 x 10(-5) M) reduces the hypoxic hyperpolarization and dilatation by circa 20-30%. Their maximum is shifted towards a higher PO2 of 65 mmHg. Only complete removal of the endothelium effects a 70-80% restriction of dilatory vascular reactivity. Identical quantitative statements hold also for control conditions, i.e. in carbogen Krebs solution. This means that the basal release of prostacyclin from endothelial and smooth muscle cells has a share of 20-30%, the basal release of the endothelial dilator EDHF of 70-80% in the membrane polarization and reduction in muscle tone.(ABSTRACT TRUNCATED AT 250 WORDS)