Contraction and membrane activation in several mammalian vascular muscles

Smooth muscle contractile diversity in the control of regional circulations

Each regional circulation has unique requirements for blood flow and thus unique mechanisms by which it is regulated. In this review we consider the role of smooth muscle contractile diversity in determining the unique properties of selected regional circulations and its potential influence on drug targeting in disease. Functionally smooth muscle diversity can be dichotomized into fast versus slow contractile gene programs, giving rise to phasic versus tonic smooth muscle phenotypes, respectively. Large conduit vessel smooth muscle is of the tonic phenotype; in contrast, there is great smooth muscle contractile diversity in the other parts of the vascular system. In the renal circulation, afferent and efferent arterioles are arranged in series and determine glomerular filtration rate. The afferent arteriole has features of phasic smooth muscle, whereas the efferent arteriole has features of tonic smooth muscle. In the splanchnic circulation, the portal vein and hepatic artery are arranged in parallel and supply blood for detoxification and metabolism to the liver. Unique features of this circulation include the hepatic-arterial buffer response to regulate blood flow and the phasic contractile properties of the portal vein. Unique features of the pulmonary circulation include the low vascular resistance and hypoxic pulmonary vasoconstriction, the latter attribute inherent to the smooth muscle cells but the mechanism uncertain. We consider how these unique properties may allow for selective drug targeting of regional circulations for therapeutic benefit and point out gaps in our knowledge and areas in need of further investigation.

Alteration of endothelium-dependent hyperpolarizations in porcine coronary arteries with regenerated endothelium

The present study was designed to test the ability of regenerated endothelium to evoke endothelium-dependent hyperpolarizations. Hyperpolarizations induced by serotonin and bradykinin were compared in isolated porcine coronary arteries with native or regenerated endothelium, 4 weeks after balloon endothelial denudation. The experiments were performed in the presence of inhibitors of nitric oxide synthase (Nomega-nitro-L-arginine) and cyclooxygenase (indomethacin). The transmembrane potential was measured using conventional glass microelectrodes. Smooth muscle cells from coronary arteries with regenerated endothelium were depolarized in comparison with control coronary arteries from the same hearts. Spontaneous membrane potential oscillations of small amplitude or spikes were observed in some of these arteries but never in arteries with native endothelium. In coronary arteries from control pigs, both serotonin and bradykinin induced concentration-dependent hyperpolarizations. In the presence of ketanserin, 10 micromol/L serotonin induced a transient hyperpolarization in control coronary arteries. Four weeks after balloon denudation, the response to serotonin was normal in arteries with native endothelium, but the hyperpolarization was significantly lower in coronary arteries with regenerated endothelium. In control arteries, the endothelium-dependent hyperpolarization obtained with bradykinin (30 nmol/L) was reproducible. Four weeks after balloon denudation, comparable hyperpolarizations were obtained in coronary arteries with native endothelium. By contrast, in arteries with regenerated endothelium, the hyperpolarization to bradykinin became voltage-dependent. In the most depolarized cells, the hyperpolarization to bradykinin was augmented. The changes in resting membrane potential and the alteration in endothelium-dependent hyperpolarizations observed in the coronary arteries with regenerated endothelium may contribute to the reduced response to serotonin and the unchanged relaxation to bradykinin described previously.

Simultaneous oscillations in the membrane potential of pig coronary artery endothelial and smooth muscle cells

1. The effects of tetrabutylammonium (TBA) on the mechanical tension and on the electrical behaviour of endothelial and smooth muscle cells were studied in intact porcine coronary artery strips. 2. Superfusion of strips with TBA (2-20 mM) induced mechanical oscillations, leading to an increase in tonic isometric tension. 3. TBA-induced mechanical oscillations were correlated with fluctuations of the membrane potential of endothelial cells, which were identified by iontophoretic injection of Lucifer Yellow. 4. The endothelial cell membrane potential fluctuations appeared as action potentials or smaller amplitude slow waves, and were synchronized with electrical membrane potential fluctuations of the underlying coronary smooth muscle cells. 5. Oscillations induced by TBA in smooth muscle cells were not affected by removal of the endothelium, and depended on the presence of calcium in the external medium. 6. To our knowledge, this is the first description of action potential-like fluctuations in the endothelium. It is concluded that the oscillations were generated in the smooth muscle and that they propagate to the endothelium. The question of the mode of propagation of the signal is discussed.

Involvement of a cholinergic mechanism in the sustained depolarization and contraction of the lower oesophageal sphincter muscle cells in the cat

Membrane potentials were recorded in vitro with intracellular electrodes from the circular muscle cells of the cat lower oesophageal sphincter and oesophageal body. In addition, the tension of lower oesophageal sphincter and oesophageal body strips was recorded isotonically. Under the experimental conditions, no spontaneous electrical activity or variation in the tension of the strips occurred. The resting membrane potential of the circular muscle cells was significantly lower in the lower oesophageal sphincter (-51.0 +/- 0.3 mV) than in the circular muscle cells of the oesophageal body (-57.1 +/- 0.4 mV). These values were not affected by infusion of tetrodotoxin 3.1 x 10(-6) M. In the presence of atropine (3.5 x 10(-7) M), the resting membrane potential of the circular muscle cells of the lower oesophageal sphincter increased significantly (-57.6 +/- 0.4 mV), whereas the resting membrane potential of the circular muscle cells of the oesophageal body was not significantly affected (-57.8 +/- 0.6 mV). In the presence of atropine, no significant difference in the values of the resting membrane potential of the circular muscle cells was observed between the lower oesophageal sphincter and the oesophageal body. Hyoscine (2.9 x 10(-7) M) significantly increased the resting membrane potential of the circular muscle cells of the lower oesophageal sphincter, whereas eserine (3.6 x 10(-6) M) significantly decreased it. Atropine induced a significant decrease in the membrane resistance of the circular muscle cells of the lower oesophageal sphincter. Atropine decreased the resting tension of lower oesophageal sphincter strips whereas eserine increased it, but no such effects were recorded on oesophageal body strips.(ABSTRACT TRUNCATED AT 250 WORDS)

Electrical and mechanical properties of spontaneous contraction in hypertensive rat portal vein

Contractile and electrical activities of longitudinal smooth muscle of portal vein from normotensive Wistar Kyoto rats (WKY) and stroke-prone spontaneously hypertensive rats (SHRSP) were compared. Amplitude and duration of spontaneous contraction of SHRSP portal vein were greater than those of WKY portal vein. No significant differences were observed in the resting membrane potentials between these preparations. Spontaneous spike activity appeared as a form of bursts. Duration of the burst and the number of spikes in each burst was greater in the portal vein of SHRSP than that of WKY. The amplitude of phasic and tonic components of K-contracture was also greater in SHRSP portal vein. Adrenergic and cholinergic nerves were not involved in the differences in contractions of the portal vein of these animal strains. Cross-sectional area of the longitudinal muscle layer was greater in SHRSP portal vein. These results suggest that the differences in spontaneous electrical activity are the cause of the differences in force and duration of the spontaneous contraction of portal vein from WKY and SHRSP, although the difference in excitation-contraction coupling of smooth muscle may be involved in much less extent.

ATP causes postjunctional potentiation of noradrenergic contractions in the portal vein of guinea-pig and rat

Superfusion of the portal vein of rat and guinea-pig with Krebs' solution maintained at 25 degrees C greatly inhibits spontaneous contractions of the preparations and allows contractile responses to ATP and noradrenaline to be measured accurately. Under these conditions, continuous superfusion with ATP (10(-5) M), a concentration which had no effect on either basal tension or spontaneous activity, caused a significant shift to the left of the concentration-response curve to exogenous noradrenaline in both tissues. The mechanism of this potentiation induced by ATP may differ in the two tissues since in the rat portal vein potentiation appeared to be rapidly reversed by superfusing with ATP-free solution, whereas in the guinea-pig portal vein a further concentration-response curve to noradrenaline, in the absence of ATP, was still significantly shifted to the left compared with the control curve. However, potentiation in the rat portal vein may have had a longer duration than is suggested by the results since control concentration-response curves to noradrenaline in this tissue showed a progressive shift to the right which, although not significant, is likely to have affected the apparent time course of potentiation. It is concluded that ATP can potentiate contractions to exogenous noradrenaline in the portal vein of rat and guinea-pig via an, as yet, unidentified postjunctional mechanism.

Effects of diltiazem on phasic and tonic activity in rat portal vein

The effects of the Ca2+ entry blocking drug, diltiazem, have been evaluated in the rat isolated portal vein, against phasic or tonic responses induced by a range of agonists. Diltiazem was a potent antagonist of phasic responses induced by low concentrations of K+, tetraethylammonium (TEA), the selective alpha 2-adrenoreceptor agonists UK 14304 or TL99 and angiotensin II (AII). Diltiazem was significantly less potent as an antagonist of phasic responses induced by the selective alpha 1-adrenoreceptor agonists phenylephrine (PE) or methoxamine (ME) or the non-selective alpha-adrenoreceptor agonist (NA), or of tonic responses evoked by high concentrations of K+, or PE. The non-stimulated phasic activity of the portal vein was antagonised by diltiazem at higher concentrations only. It is concluded that in the rat portal vein, phasic or tonic activity are associated with different Ca2+-gating mechanisms. It is considered that these differences could represent different Ca2+-channels, different rates of activation or deactivation of the channels, or involve other sources of activator Ca2+ than extracellular Ca2+. The alpha 2-adrenoreceptor subtype may be functionally linked with a voltage dependent Ca2+-channel to cause phasic responses in this preparation.

Mechanism of action of angiotensin II on excitation-contraction coupling in the rat portal vein

1 The action of angiotensin II (At II) has been studied on the electrical and mechanical activity of the vascular smooth muscle of the rat portal vein.2 At low concentrations (between 5 x 10(-10) and 10(-9) M) At II induces an acceleration of spontaneous action potential (AP) discharge without change in the resting membrane potential. The frequency and size of the associated contractions are simultaneously augmented. Under these conditions the size of the spikes is not affected, thus suggesting that At II triggers the release of Ca(2+) from internal stores.3 The increase in AP discharge rate produced by low concentrations of At II results from an acceleration of the pacemaker potential. Furthermore, in the presence of 10 mM tetraethylammonium (TEA), there is an acceleration of the repolarizing phase of AP.4 Ouabain (10(-3) M) inhibits the increase in rhythmic activity induced by low concentrations of At II (in the presence of 10 mM TEA), thus suggesting that the Na-K pump is directly or indirectly involved in this action of the peptide.5 At higher concentrations, At II produces a concentration-dependent depolarization with an EC(50) of 1.2 x 10(-8) M and a maximum of 10(-7) M. The associated contraction has an EC(50) of 3.3 x 10(-8) M and a maximum of 3 x 10(-7) M.6 Ouabain (3 x 10(-3) M) depolarizes the cell membrane. Under these conditions, At II (10(-7) M) has a slight depolarizing effect, but it still produces a large tonic contraction.7 It is concluded, that At II acts on different steps of excitation-contraction coupling, depending on the concentration. At low levels, the peptide mainly accelerates spike discharge, through a mechanism involving the Na-K pump. At higher concentrations, At II depolarizes the cell membrane. The contraction is then activated by the influx of Ca(2+) due to secondary AP discharge and the release of Ca(2+) from intracellular stores. Pharmacomechanical coupling has an important role in the triggering of contractions both at high and at low concentrations of At II.

Membrane electrical mechanism of basilar artery constriction and pial artery dilation by norepinephrine

To study the mechanism by which norepinephrine acts on vascular muscle cell membrane, we recorded membrane potential with intracellular microelectrodes in isolated cat basilar and pial arteries. On addition of norepinephrine concentrations less than 1 microM, pial arteries hyperpolarized and relaxed while basilar arteries depolarized and contracted. Relaxation and hyperpolarization of the pial arteries occurred without the need for addition of any other drug, which indicates the relaxation of spontaneous tone. The relaxation and hyperpolarization could be completely blocked by addition of propranolol before exposure to norepinephrine. The depolarization and contraction of both basilar and pial arteries was blocked by the previous exposure to phentolamine. Electrical spikes were not found spontaneously, but could be induced in both arteries by tetraethylammonium and subsequent addition of norepinephrine, blockable by phentolamine. We conclude that membrane property differences between basilar and pial arteries result in qualitatively different effects of norepinephrine.

A comparative study of neuromuscular transmission in several mammalian muscular arteries

Intracellular recordings were made from the smooth muscle of isolated segments of the rabbit ear artery, rabbit saphenous artery, rat tail artery, and guinea pig mesenteric artery. Resting membrane potentials recorded from cells in all these arteries were the same (-65 to -75 mV) and perivascular nerve stimulation evoked excitatory junction potentials (EJPs). At stimulation frequencies > 0.2 Hz facilitation of the EJPs was observed in all but the rat tail artery; in this artery the amplitude of the second EJP in a train was less than the first for stimulation frequencies between 0.3 and 2.0 Hz. Spontaneous excitatory junction potentials (SEJPs) at frequencies up to 20/min were recorded during all impalements in the rat caudal artery. In rabbit ear, saphenous and guinea pig measenteric arteries single, supramaximal intensity stimuli evoked EJPs which were not associated with any detectable contraction. Facilitation of EJPs during trains of stimuli > 1 Hz gave rise to graded active responses of up to 50 mV in the saphenous and tail arteries whereas all-or-nothing action potentials of 42--50 mV were recorded in both the rabbit ear and guinea pig mesenteric arteries. Such responses were always associated with contraction. Intraluminal distension of the arteries had no effect on the RMP or response to nerve stimulation.

High shortening velocity of isolated single arterial muscle cells

Surprisingly high shortening velocities (less than 200 msec contraction-relaxation cycles) were found in isolated vascular muscle cells cultured from rat or chick aorta. All of the small fraction cells with such quick contractions had membrane excitation by short duration spikes, rather than the slower graded depolarization of the other cells which produced 20-fold slower contractions.

On the nature of basal vascular tone in cat skeletal muscle and its dependence on transmural pressure stimuli

The aim of the present study was to elucidate in some detail the characteristics of the intrinsic basal vascular tone in the adrenergically blocked skeletal muscle with regard to its extent and site along the vascular bed, its dependence on arterial pressure via static and dynamic transmural pressure stimuli, and its sensitivity to local metabolic influence. Basal tone, which apparently is of myogenic nature, was pronounced in 'proximal arterial vessels' (greater than 25 mmicrometer i.d.) and in the 'microvessels' (less than 25 micrometers), but low in 'large veins'. Its functional characteristics, however, were different in the 'proximal arterial vessels' and the 'microvessels'. Normal basal tone in the 'microvessels' thus seemed to be intimately dependent on the arterial blood pressure level and, at least partly, initiated by its static mean pressure distension effect as well as by its dynamic pulse pressure oscillations. It could be virtually abolished by a transmural pressure decrease applied at fast rate ('strong inhibitory dynamic transmural pressure stimulus'). Basal tone in the 'proximal arterial vessels', on the other hand, was little affected by arterial pressure and almost irresponsive to transmural pressure stimuli. Basal tone in the 'microvessels' was much more sensitive to metabolic stimuli than that in the 'proximal arterial vessels'. The present results, viewed in the light of some recent electrophysiological studies on vascular smooth muscle, suggest that smooth muscle in the 'microvessels' is mainly of the spike-generating type, whereas that in the 'proximal arterial vessels' seems to be of different nature, possibly of the non-spike-generating type.

Electrogenesis of increased norepinephrine sensitivity of arterial vascular muscle in hypertension

The possibility that the vascular muscle cell might contribute to the development of essential hypertension by being more responsive to norepinephrine because of an inherently lower membrane potential (Em) was investigated. Experiments were designed to test the hypothesis that Em of arterial vascular muscle cells from spontaneously hypertensive rats (SHR) are less negative than those from matched Kyoto normotensive rats (KNR). The caudal artery, a muscular, densely innervated regulating artery 300-400 mum in outside diameter, which is activated by graded (nonspiking) depolarization to produce a maintained contraction, was studied. Vascular muscle cells from SHR always had less negative Em than those from KNR at 16 degrees C, but not at 36 degrees C, over a range of K+ concentrations from 2.7 mM to 150 mM. From the relationship between Em and K+ concentration, intracellular K+ concentration ([K+]i) was estimated to be 150 mM for SHR and 170 mM for KNR. The caudal artery undergoes a large depolarization when K+ is removed from the superfusing solution and a transient hyperpolarization that exceeds the calculated EK (potassium equilibrium potential) when K+ is replaced. The magnitude of the hyper-polarization on returning to 30 mM or 50 mM K+ always was greater for vascular muscle of SHR than KNR. The apparently lower [K+]i and more active (compensating) electrogenic ion transport in the SHR vascular muscle cells thus result in an unaltered Em at body temperature in the physiological range of K+ concentrations. However, depolarization by norepinephrine was greater over the middle of the dose-response curve, and this greater depolarization caused the contractions of SHR arteries to be greater. The altered electrogenesis of the SHR vascular muscle cells is postulated to provide a mechanism for the increased reactivity of arteries to norepinephrein in hypertension.

Vascular muscle series elastic element stiffness during isometric contraction

Series elastic element stiffness was studied during isometric muscle contraction in cylindrical segments of dog common carotid artery. These vessel segments were mounted in vitro and restored to in situ length. Vessel diameter was constantly monitored with a linear displacement transducer. The arteries were subjected to isometric contraction by adding norepinephrine to the bath and elevating the transmural pressure enough to maintain constant vessel diameter. Small, rapid quick release-quick stretch cycles were performed repeatedly during isometric contractions to evaluate the stiffness of the undamped series elastic element. After contraction was complete, repeated quick release-quick stretch cycles were also performed during inactivation of the muscle caused by the administration of potassium cyanide. At strains up to the peak in the length-active tension curve, series elastic element stiffness was a linear function of stress thoughout both isometric contraction and isometric relaxation for both quick releases and quick stretches. Vessels excited at strains less than that associated with the peak in the length-tension curve (100 mm Hg) exhibited similar stress-series elastic element stiffness slopes, but vessels excited at larger strains exhibited decreased or negative stress-series elastic stiffness slopes. Studies on potassium cyanide-poisoned vessels failed to indicate any alteration in the quick release properties of the connective tissue at large strains. The concomitant decline in the stress-series elastic element stiffness slope and the active isometric stress at large strains suggests (1) that the series elastic element undergoes mechanical yielding at high stresses or (2) that the series elastic element is an integral part of the force-generating apparatus.

Series elastic and contractile elements in vascular smooth muscle

Segments of dog common carotid artery were excised, cannulated, and restored to in situ length. They were immersed in a Krebs-Ringer's bath and inflated with 100% O 2 under nonoscillating pressure. Diameter was continuously monitored with a linear displacement transducer. The vessel segments were relaxed and then treated isometrically with norepinephrine to excite the muscle. These continuously contracted vessels were subjected to 10- or 25-mm Hg quick release steps to 0 mm Hg. Then the muscle was inactivated with potassium cyanide, and the quick releases were repeated to study the parallel elastic elements. Finally, vessel wall volumes were determined radiographically. Computations were performed to compute series elastic element stiffness for both the Maxwell and the Voigt model of the arterial wall. These experiments indicated that the series elastic element stiffness was dependent on the applied stress and independent of muscle length provided that the vessels were excited at initial strains less than 0.70. Vessels excited isometrically at various initial pressures up to 150 mm Hg yielded identical stress-series elastic element stiffness curves. The computed series elastic element extension at an applied stress of 1.0 x 10 6 dynes/cm was 18.9% for the Hill model and 14.7-17.9% for the Voigt model; these values are percents of the vessel diameter at 0 mm Hg after application of potassium cyanide. The values were reduced to 11.2% for the Maxwell model and 8.6-10.5% for the Voigt model when they were expressed as a percent of the circumferential length associated with the peak in the length-tension curve. Vessels excited isometrically at pressures higher than 150 mm Hg exhibited greater series elastic element extensibility.

Multiple pacemaker sites in spontaneously active vascular muscle

Multiple-electrode extracellular recording was used to determine the degree to which membrane electrical activity spreads through the rat anterior mesenteric-hepatic portal vein. Glass-pore electrodes filled with physiological salt solution were placed against the isolated intact vein or longitudinal strips of vein at 1-mm intervals to record spontaneous spiking. No pressure electrode effect was found in the rat portal vein. Patterns and amplitudes of spiking indicated that spike activity was conducted for only a few millimeters. Multiple pacemaker sites and decremental conduction were found in both normal physiological solution and solution in which calcium concentration was halved or doubled and potassium concentration was doubled or tripled. An average of three or four pacemaker sites was found in all solutions. Approximate equipotential contours were constructed from a grid map of average spike amplitudes, and they showed the multiple regions of high-amplitude spiking, the nonuniform spatial decay of spike amplitude, and the shift of high-amplitude sites with time.