Current spread in the smooth muscle of the rabbit aorta

Cell-cell communication in the vessel wall

Intercellular communication between cells within the blood vessel wall plays an important role in the control of artery diameter. The endothelial cells lining the lumen of arteries can evoke smooth muscle hyperpolarization both by the release of a factor (EDHF) and by direct cell-cell coupling through gap junctions. Hyperpolarizing current can spread rapidly to cause widespread vasodilatation, and thus increase blood flow to that segment. In addition to the spread of current, small molecules, such as Ca2+, can also pass between cells, but at a much reduced rate. Instead of co-ordinating changes in diameter, intercellular Ca2+ signalling acts to amplify and, in special cases, modulate vascular responses. Together, direct cell-cell communication enables the blood vessel wall to act as a functional syncytium, which is influenced by surrounding tissues and nerves, and blood constituents.

Integration and Modulation of Intercellular Signaling Underlying Blood Flow Control

Vascular resistance networks control tissue blood flow in concert with regulating arterial perfusion pressure. In response to increased metabolic demand, vasodilation arising in arteriolar networks ascends to encompass proximal feed arteries. By reducing resistance upstream, ascending vasodilation (AVD) increases blood flow into the microcirculation. Once initiated, e.g. through local activation of K(+) channels in endothelial cells (ECs), hyperpolarization is conducted through gap junctions along the endothelium. Via EC projections through the internal elastic lamina, hyperpolarization spreads into the surrounding smooth-muscle cells (SMCs) through myoendothelial gap junctions (MEGJs) to promote their relaxation. Intercellular signaling through electrical signal transmission (i.e. cell-to-cell conduction) can thereby coordinate vasodilation along and among the branches of microvascular resistance networks. Perivascular sympathetic nerve fibers course through the adventitia and release norepinephrine to stimulate SMCs via α-adrenoreceptors to produce contraction. In turn, SMCs can signal ECs through MEGJs to activate K(+) channels and attenuate sympathetic vasoconstriction. Activation of K(+) channels along the endothelium will dissipate electrical signal transmission and inhibit AVD, thereby restricting blood flow into the microcirculation while maintaining peripheral resistance and perfusion pressure. This review explores the origins and nature of the intercellular signaling that governs blood flow control in skeletal muscle with respect to the interplay between AVD and sympathetic innervation. Whereas these interactions are integral to daily activity and athletic performance, determining the interplay between respective signaling events provides insight into how selective interventions can improve tissue perfusion and oxygen delivery during vascular disease.

Hydraulic conductivity of endothelial cell-initiated arterial cocultures

This study describes cocultures of arterial smooth muscle cells (SMCs) and endothelial cells (ECs) and the influences of their heterotypic interactions on hydraulic conductivity (L p ), an important transport property. A unique feature of these cocultures is that ECs were first grown to confluence and then SMCs were inoculated. Bovine aortic smooth muscle cells and bovine aortic endothelial cells (BAECs) were cocultured on Transwell Permeable Supports, and then exposed to a pressure-driven transmural flow. L p across each culture was measured using a bubble tracking apparatus that determined water flux (J v ). Our results indicate that arterial L p is significantly modulated by EC-SMC proximity, and serum content in culture. The L p of cocultures was also compared to the predictions of a resistances-in-series model to distinguish the contributions of heterotypic interactions between SMCs and ECs. Conditions that lead to significantly reduced coculture L p , compared to BAEC monoculture controls, have been uncovered and the lowest L p in the literature for an in vitro system are reported. In addition, VE-cadherin immunostaining of intact BAEC monolayers in each culture configuration reveals that EC-SMC proximity on a porous membrane has a dramatic influence on EC morphology patterns. The cocultures with the lowest L p have ECs with significantly elongated morphology. Confocal imaging indicates that there are no direct EC-SMC contacts in coculture.

Perivascular innervation: a multiplicity of roles in vasomotor control and myoendothelial signaling

The control of vascular resistance and tissue perfusion reflect coordinated changes in the diameter of feed arteries and the arteriolar networks they supply. Against a background of myogenic tone and metabolic demand, vasoactive signals originating from perivascular sympathetic and sensory nerves are integrated with endothelium-derived signals to produce vasodilation or vasoconstriction. PVNs release adrenergic, cholinergic, peptidergic, purinergic, and nitrergic neurotransmitters that lead to SMC contraction or relaxation via their actions on SMCs, ECs, or other PVNs. ECs release autacoids that can have opposing actions on SMCs. Respective cell layers are connected directly to each other through GJs at discrete sites via MEJs projecting through holes in the IEL. Whereas studies of intercellular communication in the vascular wall have centered on endothelium-derived signals that govern SMC relaxation, attention has increasingly focused on signaling from SMCs to ECs. Thus, via MEJs, neurotransmission from PVNs can evoke distinct responses from ECs subsequent to acting on SMCs. To integrate this emerging area of investigation in light of vasomotor control, the present review synthesizes current understanding of signaling events that originate within SMCs in response to perivascular neurotransmission in light of EC feedback. Although often ignored in studies of the resistance vasculature, PVNs are integral to blood flow control and can provide a physiological stimulus for myoendothelial communication. Greater understanding of these underlying signaling events and how they may be affected by aging and disease will provide new approaches for selective therapeutic interventions.

Autoregulation and conduction of vasomotor responses in a mathematical model of the rat afferent arteriole

We have formulated a mathematical model for the rat afferent arteriole (AA). Our model consists of a series of arteriolar smooth muscle cells and endothelial cells, each of which represents ion transport, cell membrane potential, and gap junction coupling. Cellular contraction and wall mechanics are also represented for the smooth muscle cells. Blood flow through the AA lumen is described by Poiseuille flow. The AA model's representation of the myogenic response is based on the hypothesis that changes in hydrostatic pressure induce changes in the activity of nonselective cation channels. The resulting changes in membrane potential then affect calcium influx through changes in the activity of the voltage-gated calcium channels, so that vessel diameter decreases with increasing pressure values. With this configuration, the model AA maintains roughly stable renal blood flow within a physiologic range of blood flow pressure. Model simulation of vasoconstriction initiated from local stimulation also agrees well with findings in the experimental literature, notably those of Steinhausen et al. (Steinhausen M, Endlich K, Nobiling R, Rarekh N, Schütt F. J Physiol 505: 493-501, 1997), which indicated that conduction of vasoconstrictive response decays more rapidly in the upstream flow direction than downstream. The model can be incorporated into models of integrated renal hemodynamic regulation.

Vasorelaxation of Rat Thoracic Aorta Caused by Two Ca2+-Channel Blockers, HA-22 and HA-23

The pharmacological effects of HA-22 (2-(4′methoxyphenylmethyl)-3,4-dimethylpyrano[2,3-c]pyrazol-6(2H)-one) and HA-23 (2-(2′-thienylmethyl)-3,4-dimethylpyrano[2,3-c]pyrazol-6(2H)-one) on rat isolated thoracic aorta have been examined. In high potassium medium (60 Mm), Ca2+ (0·03–3 Mm)-induced vasoconstriction was inhibited by HA-22 and HA-23 (10–100 μg mL−1). Cromakalim-relaxed aortic rings precontracted with 15 Mm but not 60 Mm K+. However, HA-22, HA-23 and verapamil produced a greater relaxation in 60 Mm than in 15 Mm K+-induced contraction. The tonic contractions elicited by KCl (60 Mm) and Bay K 8644 (10−7  m) were also relaxed by the addition of HA-22 and HA-23. The phenylephrine concentration-response curves displayed antagonism by HA-22 and HA-23 (10–100 μg mL−1) in a non-competitive manner. The caffeine (10 Mm)-induced contraction and cAMP or cGMP levels were not affected by HA-22 or HA-23. It is concluded that HA-22 and HA-23 relaxed the rat aorta by suppressing the Ca2+ influx through both voltage-dependent and receptor-operated Ca2+ channels.

Calcium channel blocking activity of calycosin, a major active component of Astragali Radix, on rat aorta

AIM

To investigate the vasoactivity of calycosin, a major active component of Astragali Radix.

METHODS

Experiments were performed on isolated rat thoracic aortic rings pre-contracted with phenylephrine (PHE) or KCl.

RESULTS

Calycosin produced a concentration-dependent relaxation on the tissue pre-contracted using PHE with 4.46+/-0.13 of pD(2) and 95.85%+/-2.67% of E(max); or using KCl with 4.27+/-0.05 of pD2 and 99.06%+/-2.15% of Emax, and displaced downwards the concentration-response curves of aortic rings to PHE or KCl. The relaxant effect of calycosin on denuded endothelium aortic rings was the same as on intact endothelium aortic rings, and its vasorelaxant effect was not influenced by L-NAME or indomethacin. In Ca(2+)-free solution, calycosin (30 micromol/L) did not have an effect on PHE (1 micromol/L)-induced aortic ring contraction. The effects of calycosin and nifedipine where somewhat different; calycosin decreased aortic ring contractions induced by the two agonists, but nifedipine displayed a more potent inhibitory effect on KCl-induced contractions than on PHE-induced contractions, and the vascular relaxing effects of calycosin and nifidipine were additive on PHE-induced contraction but not KCl-induced.

CONCLUSION

Calycosin is a vasorelaxant. Its action is endothelium-independent and is unrelated to intracellular Ca(2+) release. It is a noncompetitive Ca(2+) channel blocker. The effect of calycosin on Ca(2+) channel blockade may be different from that of dihydropyridines. This study demonstrated a novel pharmacological activity of calycosin, and supplied a theoretic foundation for Astragali Radix application.

Vasorelaxant effects of eugenol on rat thoracic aorta

Eugenol is a natural pungent substance and the main component of clove oil, with vasorelaxant action. To elucidate some of the possible mechanisms involved in this action isometric tension was measured in aortic rings from male Wistar rats precontracted with phenylephrine (PHE, 10(-7) M) or KCl (75 mM). Responses to increasing concentrations of eugenol (10(-6)-10(-2) M) were obtained in the presence and absence of endothelium. In the presence of eugenol, dose-response curves to PHE (10(-9) to 10(-4) M) and KCl (5-125 mM) were displaced downwards. Concentration-dependent relaxation was observed in rings precontracted with PHE (10(-7) M) and KCl (75 mM). The tension increment produced by increasing external calcium concentration (0.25-3 mM) was also reduced by eugenol (300 microM) treatment. The inhibitory effects of eugenol (300 microM) were compared to those induced by nifedipine (0.01 microM), a selective Ca(2+) channel blocker, producing similar relaxant effects. Two other protocols were performed. After precontraction with PHE (10(-7) M), increasing concentrations of eugenol (10(-6)-10(-2) M) were used before and after N(w)-nitro-L-arginine (L-NAME, 10(-4) M) and methylene blue (10(-5) M) treatment. Eugenol-induced relaxation was reduced by endothelial damage (rubbing), L-NAME and methylene blue treatments. Results suggested that eugenol produces smooth muscle relaxation resulting from the blockade of both voltage-sensitive and receptor-operated channels that are modulated by endothelial-generated nitric oxide.

Information Networks in the Arterial Wall

The main task of the arterial system is to secure an adequate supply of oxygen to organs. This fact implies the integration of multiple signals in the vascular wall. This review deals with the exchange of information between and among smooth muscle and endothelial cells through gap junctions in the vessel walls of arteries and arterioles.

Physiological features of visceral smooth muscle cells, with special reference to receptors and ion channels

Visceral smooth muscle cells (VSMC) play an essential role, through changes in their contraction-relaxation cycle, in the maintenance of homeostasis in biological systems. The features of these cells differ markedly by tissue and by species; moreover, there are often regional differences within a given tissue. The biophysical features used to investigate ion channels in VSMC have progressed from the original extracellular recording methods (large electrode, single or double sucrose gap methods), to the intracellular (microelectrode) recording method, and then to methods for recording from membrane fractions (patch-clamp, including cell-attached patch-clamp, methods). Remarkable advances are now being made thanks to the application of these more modern biophysical procedures and to the development of techniques in molecular biology. Even so, we still have much to learn about the physiological features of these channels and about their contribution to the activity of both cell and tissue. In this review, we take a detailed look at ion channels in VSMC and at receptor-operated ion channels in particular; we look at their interaction with the contraction-relaxation cycle in individual VSMC and especially at the way in which their activity is related to Ca2+ movements and Ca2+ homeostasis in the cell. In sections II and III, we discuss research findings mainly derived from the use of the microelectrode, although we also introduce work done using the patch-clamp procedure. These sections cover work on the electrical activity of VSMC membranes (sect. II) and on neuromuscular transmission (sect. III). In sections IV and V, we discuss work done, using the patch-clamp procedure, on individual ion channels (Na+, Ca2+, K+, and Cl-; sect. IV) and on various types of receptor-operated ion channels (with or without coupled GTP-binding proteins and voltage dependent and independent; sect. V). In sect. VI, we look at work done on the role of Ca2+ in VSMC using the patch-clamp procedure, biochemical procedures, measurements of Ca2+ transients, and Ca2+ sensitivity of contractile proteins of VSMC. We discuss the way in which Ca2+ mobilization occurs after membrane activation (Ca2+ influx and efflux through the surface membrane, Ca2+ release from and uptake into the sarcoplasmic reticulum, and dynamic changes in Ca2+ within the cytosol). In this article, we make only limited reference to vascular smooth muscle research, since we reviewed the features of ion channels in vascular tissues only recently.

Electrical conduction within the cerebrovasculature of stroke-prone spontaneously hypertensive rats

Alterations in electrical conductivity between smooth muscle cells (SMCs) can alter the spread and effectiveness of electromechanical SMC contraction. We attempted to determine whether alterations in pressure-dependent constriction (PDC) occurring in relation to stroke development within the middle cerebral arteries (MCAs) of Wistar-Kyoto stroke-prone hypertensive rats (SHRsp) were associated with changes in electrical conductivity between the SMCs. Current was injected into nonpressurized MCAs, using a suction electrode. The conducting distance along the length of the MCA where the amplitude of the membrane potential deflection (electrotonic potential) produced by current injection declined to 1/e (length constant) was used to measure conductivity. PDC to a 100 mmHg pressure step was measured with a pressure myograph. A loss of PDC in the MCAs of SHRsp preceded stroke development. Heptanol (4 mM), a gap junction communication inhibitor, reversibly inhibited conductivity and PDC in the MCA of prestroke SHRsp. The ability of heptanol to reversibly inhibit PDC was likely not related to it's ability to alter electrical conduction. The length constant of electrical conduction in the MCAs was about 0.75 mm and didn't differ between MCA sampled from pre- versus post-stroke SHRsp or Sprague-Dawley rats. It was concluded that alterations in electrical conductivity along the MCA could modify the spread of PDC, but such changes do not contribute to the loss of PDC within the MCA of poststroke SHRsp.Key words: membrane potential, electrotonic potential, middle cerebral arteries, myogenic response, gap junction, stroke-prone hypertensive rats.

Gap junctions in vascular tissues. Evaluating the role of intercellular communication in the modulation of vasomotor tone

Integration and coordination of responses among vascular wall cells are critical to the local modulation of vasomotor tone and to the maintenance of circulatory homeostasis. This article reviews the vast literature concerning the principles that govern the initiation and propagation of vasoactive stimuli among vascular smooth muscle cells, which are nominally the final effectors of vasomotor tone. In light of the abundance of new information concerning the distribution and function of gap junctions between vascular wall cells throughout the vascular tree, particular attention is paid to this integral aspect of vascular physiology. Evidence is provided for the important contribution of intercellular communication to vascular function at all levels of the circulation, from the largest elastic artery to the terminal arterioles. The thesis of this review is that the presence of gap junctions, in concert with the autonomic nervous system, pacemaker cells, myogenic mechanisms, and/or electrotonic current spread (both hyperpolarizing and depolarizing waves through gap junctions), confers a plasticity, adaptability, and flexibility to vasculature that may well account for the observed diversity in regulation and function of vascular tissues throughout the vascular tree. It is hoped that the summary information provided here will serve as a launching pad for a new discourse on the mechanistic basis of the integrative regulation and function of vasculature, which painstakingly accounts for the undoubtedly complex and manifold role of gap junctions in vascular physiology/dysfunction.

Control of skeletal muscle blood flow during dynamic exercise: contribution of endothelium-derived nitric oxide

Traditional explanations for the hyperaemia which accompanies exercise have invoked the 'metabolic theory' of vasodilation, whereby contractile activity in the active muscle gives rise to metabolic by-products which dilate vessels bathed in interstitial fluid. Whilst metabolites with vasodilator properties have been identified, this theory does not adequately explain the magnitude of hyperaemia observed in active skeletal muscle, principally because large increases in flow are dependent on dilation of 'feed' arteries which lie outside the tissue parenchyma and are not subjected to changes in the interstitial milieu. Coordinated resistance vessel dilation during exercise is therefore dependent on a signal which 'ascends' from the microvessels to the feed arteries located upstream. Recent studies of ascending vasodilation have concentrated on the possible contribution of the endothelium, a monolayer of flattened squamous cells which lie at the interface between the circulating blood and vascular wall. These cells are uniquely positioned to respond to changes in rheological and humoral conditions within the cardiovascular system, and to transduce these changes into vasoactive signals which regulate blood flow, vascular tone and arterial pressure. Endothelial cells produce nitric oxide (NO), a rapidly diffusing labile substance which relaxes adjacent vascular smooth muscle. NO is released basally and contributes to the regulation of vascular tone by acting as a functional antagonist to sympathetic neural constriction. In addition, NO is spontaneously released in response to deformation of the endothelial cell membrane, indicating that changes in pulsatile flow and wall shear stress are likely physiological stimuli. Since the dilation of microvessels in response to exercise increases blood flow through the upstream feed arteries, which subsequently dilate, one explanation for ascending vasodilation is that NO release is stimulated by flow-induced shear stress. Evidence that NO contributes to ascending vasodilation is reviewed, along with studies which indicate that NO mediates exercise hyperaemia, that physical conditioning upregulates NO production and that NO controls blood flow by modifying other physiological mechanisms.

The pharmacological properties of K+ currents from rabbit isolated aortic smooth muscle cells

1. Using the whole-cell patch-clamp technique, the effects of several K+ channel blocking drugs on K+ current recorded from rabbit isolated aortic smooth muscle cells were investigated. 2. Upon depolarization from -80 mV, outward K+ current composed of several distinct components were observed: a transient, 4-aminopyridine (4-AP)-sensitive component (I1) and a sustained component (Isus), comprising a 4-AP-sensitive delayed rectifier current (IK(V)), and a noisy current which was sensitive to tetraethylammonium (TEA), and probably due to Ca(2+)-activated K+ current (IK(Ca)). 3. Several drugs in clinical or experimental use have as part of their action an inhibitory effect on specific K+ channels. Because of their differential K+ channel blocking effects, these drugs were used in an attempt to characterize further the K+ channels in rabbit aortic smooth muscle cells. Imipramine, phencyclidine, sotalol and amitriptyline failed to block selectively any of the components of K+ current, and were thus of little value in isolating individual channel contributions. Clofilium showed selective block of IK(V) in the presence of TEA, but only at low stimulation frequencies (0.07 Hz). At higher frequencies (1 Hz) of depolarization, both I1 and IK(V) were suppressed to a similar extent. Thus, the blocking action of clofilium was use-dependent. 4. The voltage-dependent inactivation of I1 and the delayed rectifier were very similar although a brief (100 ms) pre-pulse to -30 mV could preferentially inactivate I1. Together with the non-selective blocking effects of the K+ channel blockers, similarities in the activation and inactivation of these two components suggest that they may not exist as separate ionic channels, but as distinct kinetic states within the same K+ channel population. 5. The effects of all of these drugs on tension were examined in strips of rabbit aorta. The non-specific K+ channel blockers caused only minor increases in basal tension. TEA and 4-AP by themselves caused significant increases in tension and were even more effective when applied together. There appeared to be no correlation between the effects of the drugs tested on tension and their actions on currents recorded from isolated myocytes. Thus tension studies are an inappropriate means of investigating the mechanism of action of these drugs, and studies on ionic currents in isolated myocytes cannot easily predict drug actions on intact tissues.

Cell-to-cell communication coordinates blood flow control

The control of tissue blood flow is a dynamic process exemplified by the interaction among physical, chemical, and electrical events occurring within the vessel wall and between the vasculature and tissue parenchyma. The range of blood flow control achieved in vivo is illustrated by functional hyperemia in exercising skeletal muscle: maximal flow can exceed resting values by more than 50-fold. Blood flow control is integrated among many vessel segments, beginning with resistance arteries external to the muscle and encompassing the arteriolar network within the muscle. As metabolic demand increases, the locus of blood flow control shifts from distal arterioles, which control capillary perfusion and blood flow distribution within the tissue, to the proximal arterioles and resistance arteries, which control the total volume of flow into the muscle. A fundamental question centers on how this vasomotor activity is actually coordinated throughout the resistance network. The interaction within and among vascular segments can be explained by chemical and electrical signals to smooth muscle cells (SMCs) and endothelial cells (ECs) in response to changes in transmural pressure as well as luminal shear stress. Increasing pressure results in SMC contraction via the myogenic response. Increasing flow stimulates ECs to release autacoids (eg, nitric oxide), which relax SMCs. Pressure and flow thereby provide opposing mechanical stimuli that interact in the maintenance of vasomotor tone throughout the resistance network. Vasomotor signals are also conducted along arterioles through cell-to-cell coupling between ECs and SMCs, thereby coordinating vasomotor activity of cells within a branch and among branches.(ABSTRACT TRUNCATED AT 250 WORDS)

Cinnamophilin, a novel thromboxane A2 receptor antagonist, isolated from Cinnamomum philippinense

The pharmacological activity of cinnamophilin ((8R,8'S)-4,4'-dihydroxy-3,3'-dimethoxy-7-oxo-8,8'-neolignan), isolated from Cinnamomum philippinense, was studied in isolated rat aorta, guinea-pig trachea and rabbit platelets. Cinnamophilin was found to be a thromboxane A2 receptor blocking agent in these tissues as revealed by its competitive antagonism of the U-46619 (9,11-dideoxymethanoepoxy-9 alpha,11 alpha-prostaglandin F2 alpha)-induced contraction of rat aorta and guinea-pig trachea and aggregation of rabbit platelets with pA2 values of 7.3 +/- 0.2, 5.2 +/- 0.1 and 6.3 +/- 0.3, respectively. Protection against the irreversible vasoconstriction of rat aorta caused by U-46619 (0.05 microM) was obtained by cinnamophilin (10 microM) but not by caffeine (25 mM). Cinnamophilin (1-15 microM) also possessed voltage-dependent Ca2+ channel blocking action, judging from its antagonism of the high K+ (60 mM)- and Bay K 8644 (0.1 microM)-induced contraction in rat thoracic aorta. Cinnamophilin (30 microM) produced a slight relaxation of noradrenaline (3 microM)-induced tonic contractions, and this relaxing effect was abolished in the presence of nifedipine (1 microM). Nifedipine (10 microM) sufficient to inhibit high K(+)-induced contractions failed to attenuate the contractile response to U-46619. A high concentration of cinnamophilin (100 microM) did not affect the aortic contraction induced by endothelin-1, angiotensin II, carbachol or serotonin. Neither cAMP nor cGMP in rat aorta was increased by cinnamophilin. These results indicate that cinnamophilin is a selective thromboxane A2 receptor antagonist especially in rat aorta, and also possesses voltage-dependent Ca2+ channel blocking properties.

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.

Electrophysiology of cerebral blood vessels

In spite of the relatively large amount of in vitro and in vivo data indicating that, in a number of ways, cerebral arteries are pharmacologically different from peripheral arteries, the mechanisms responsible for these differences are far from clear. An understanding of these mechanisms is particularly important for a rational approach to the treatment of disorders of the cerebral circulation including migraine, hypertension and the responses of cerebral vessels to subarachnoid haemorrhage. This review outlines electrophysiological data which are available from cerebrovascular smooth muscle cells, including the possibility that inwardly-rectifying potassium channels, active at potentials close to the resting membrane potential, are intimately involved in the changes in smooth muscle tone which couple blood flow to regional changes in nerve cell activity. The membrane potential changes in response to perivascular nerve stimulation, noradrenaline, 5-hydroxytryptamine and endothelium-derived hyperpolarizing factor are also described, together with the underlying membrane mechanisms and their relationship to smooth muscle contraction and relaxation.

Vasorelaxing effect in rat thoracic aorta caused by fraxinellone and dictamine isolated from the Chinese herb Dictamnus dasycarpus Turcz: comparison with cromakalim and Ca2+ channel blockers

The components of Dictamnus dasycarpus Turcz were tested for their vasorelaxing effect on the rat aorta, and fraxinellone and dictamine were shown to be effective vasorelaxants. In high K+ (60 mmol/l) medium, Ca2+ (0.03 to 3 mmol/l)-induced vasoconstriction was inhibited concentration-dependently by both agents. The IC50 for fraxinellone and dictamine were calculated to be about 25 mumol/l and 15 mumol/l (for Ca2+ concentration of 1 mmol/l), respectively. Cromakalim (0.2-10 mumol/l) relaxed aortic rings precontracted with 15 but not 60 mmol/l of K+. Fraxinellone and verapamil were more potent and effective in producing relaxation in 60 mmol/l than in 15 mmol/l K(+)-induced contraction. However, dictamine was more potent in producing relaxation in 15 mmol/l K(+)-induced contraction. Nifedipine (1 mumol/l), dictamine (100 mumol/l) and fraxinellone (100 mumol/l) relaxed the aortic contraction caused by KCl or Bay K 8644. The tonic contraction elicited by noradrenaline (NA, 3 mumol/l) was also relaxed by dictamine (500 mumol/l), but not by fraxinellone (500 mumol/l) in the nifedipine (1 mumol/l)-treated aorta. This relaxing effect of dictamine persisted in endothelium-denuded aorta. Glibenclamide (10 mumol/l) shifted the concentration-relaxation curve of cromakalim, but not that of dictamine, to the right in rat aortic rings precontracted with NA. Dictamine (500 mumol/l) did not affect tonic contraction of NA which are reduced by H-7 (1 mumol/l) in Ca(2+)-depleted medium. In conclusion, fraxinellone is a selective blocker of voltage-dependent Ca2+ channel, while dictamine relaxed the rat aorta by suppressing the Ca2+ influx through both voltage-dependent and receptor-operated Ca2+ channels.

Vasodilatory action mechanisms of apigenin isolated from Apium graveolens in rat thoracic aorta

The effect of apigenin, isolated from Apium graveolens, on the contraction of rat thoracic aorta was studied. Apigenin inhibited the contraction of aortic rings caused by cumulative concentrations of calcium (0.03-3 mM) in high potassium (60 mM) medium, with an IC50 of about 48 microM. After pretreatment it also inhibited norepinephrine (NE, 3 microM)-induced phasic and tonic contraction in a concentration (35-140 microM)-dependent manner with an IC50 of 63 microM. At the plateau of NE-induced tonic contraction, addition of apigenin caused relaxation. This relaxing effect of apigenin was not antagonized by indomethacin (20 microM) or methylene blue (50 microM), and still existed in endothelial denuded rat aorta or in the presence of nifedipine (2-100 microM). Neither cAMP nor cGMP levels were changed by apigenin. Both the formation of inositol monophosphate caused by NE and the phasic contraction induced by caffeine in the Ca(2+)-free solution were unaffected by apigenin. 45Ca2+ influx caused by either NE or K+ was inhibited by apigenin concentration-dependently. It is concluded that apigenin relaxes rat thoracic aorta mainly by suppressing the Ca2+ influx through both voltage- and receptor-operated calcium channels.

Electrical responses of coronary artery smooth muscle associated with the cardiac muscle action potential in the monkey

1. In cardiac strips of the monkey ventricle which included a section of coronary artery (cardiac preparation), a depolarizing response, which appeared to be an excitatory junction potential (EJP), and a hyperpolarizing response were observed from the coronary artery smooth muscles when an action potential was generated in cardiac muscle by the application of electrical stimulation to the cardiac muscle alone. 2. In an isolated preparation of the coronary artery dissected from the ventricle (arterial preparation), similar responses, an EJP and a hyperpolarizing response, could be evoked by electrical stimulation. 3. In the cardiac preparation, the threshold of electrical stimulation for generation of the cardiac action potential and for production of an electrical response in the vascular smooth muscle were the same. 4. The EJP and the hyperpolarizing responses of the smooth muscle of the coronary artery ceased or were weakened by elimination of the adventitial connective tissues and endothelium, respectively, in both the cardiac preparation and the coronary artery preparation. 5. These results indicate that the action potential of cardiac muscle generated by electrical stimulation activates the nerves and the vascular endothelium, which, in turn, produce an EJP and a hyperpolarizing response of the coronary artery smooth muscle, respectively.

Free radicals mediate actions of oxyhemoglobin on cerebrovascular smooth muscle cells

Single smooth muscle cells were isolated from the basilar artery of the rat by enzymatic dispersion. The membrane properties of the cells were assessed using the patch-electrode voltage-clamp technique, and cell viability was monitored using fluorescein diacetate uptake. Exposure of the cells to oxyhemoglobin (5 microM) resulted in 1) contraction, 2) the appearance of membrane blebs, 3) an increase in the outward potassium currents, 4) a decrease in the membrane resistance, and 5) cell death. In contrast, no effect of oxyhemoglobin on cultured murine neuroblastoma cells was observed. Methemoglobin (100 microM) had no effects on the smooth muscle cells. Catalase (300 units/ml) or dimethyl sulfoxide (0.5%) protected against the effects of oxyhemoglobin; superoxide dismutase (100-1,000 units/ml) provided only partial protection. Exposure of the cells to superoxide anions generated by xanthine (1 mM) plus xanthine oxidase (10 units/l) or to hydrogen peroxide (500 microM) caused an increase in the outward potassium currents without affecting membrane resistance. Generation of hydroxyl radicals by metal ions plus hydrogen peroxide caused the same effects as oxyhemoglobin, that is, an increase in the potassium currents, followed by a decrease in the membrane resistance and cell death. In conclusion, it appears that oxyhemoglobin exerts its effects on vascular smooth muscle cells by the generation of free radicals, chiefly hydroxyl radicals.

Vasorelaxation of rat thoracic aorta caused by norathyriol isolated from Gentianaceae

The pharmacological effects of norathyriol on isolated rat thoracic aorta were examined. In the high-K+ (60 mM) medium, Ca2+ (0.03 to 3 mM)-induced vasocontraction was inhibited concentration dependently by norathyriol. Given as pretreatment norathyriol (20 to 200 microM) also inhibited the norepinephrine (NE, 3 microM)-induced tonic contraction. However, the phasic contraction was inhibited only by high concentrations of norathyriol (200 and 400 microM). The tonic contraction elicited by NE was also relaxed by the addition of norathyriol. This relaxing effect of norathyriol was not antagonized by methylene blue (50 microM) or indomethacin (20 microM) and was still seen in denuded rat aorta. Although the cAMP level was not changed by norathyriol, the cGMP level was increased by a high concentration of norathyriol (400 microM). [3H]Inositol monophosphate formation caused by NE was not affected by norathyriol at concentration of either 100 or 400 microM. The 45Ca2+ influx caused by either NE or high K+ was inhibited by norathyriol in a concentration-dependent manner. It is concluded that norathyriol relaxed the rat thoracic aorta mainly by suppressing the Ca2+ influx through both voltage-dependent and receptor-operated calcium channels.

Properties of sympathetic neuromuscular transmission and smooth muscle cell membranes in vascular beds

In vascular smooth muscle tissues, the cycle of contraction-relaxation is mainly regulated by the cytosolic Ca, and many other factors, such as substances released from endothelial cells and perivascular nerve terminals (mainly sympathetic nerves). In this article, we introduce regional differences in specific features of ionic channels in vascular smooth muscle membranes (mainly on features of Ca, Na and K channels) in relation to mobilization of the cytosolic Ca. In many vascular tissues, neurotransmitters released from sympathetic nerve terminals activate post-junctional receptors, and subsequently modify ion channels (receptor-activated cation channel and voltage-dependent Ca channel), whereas in some tissues, ionic channels are not modified by receptor activations (pharmaco-mechanical coupling). However, activation of receptors, with or without modulation of ionic channels, regulates the cytosolic Ca through synthesis of second messengers. In addition, receptors distributed on prejunctional nerve terminals positively or negatively regulate the release of transmitters. Roles of neurotransmitters (mainly ATP and noradrenaline) are also discussed in relation to the generation of excitatory junction potentials.

Rabbit aorta: electrical properties and agonist-induced depolarization

The resting membrane potential of the smooth muscle cells of strips of rabbit aorta varied between -50 to -60 mV. No spontaneous spike discharges were observed. The membrane of the myocytes showed a marked outward rectifying property. The rabbit aorta had cable properties with a space constant (lambda) of 1.54 +/- 0.04 mm. Parallel with a progressive mechanical tension development, noradrenaline and the alpha 1-agonist methoxamine depolarized the membrane in a concentration-dependent manner up to -40 mV. Stimulation of aortic alpha 1-adrenoceptors by noradrenaline reduced the steepness of the current-voltage relationship and diminished the space constant from 1.54 to 0.8 mm, indicating a decrease in membrane resistance. No action potentials were evoked by noradrenaline. The alpha 2-agonist, B-HT 920, produced only a slight contraction and virtually no change in membrane potential. As compared to noradrenaline or methoxamine, angiotensin II was a partial agonist to induce contraction, with an intrinsic activity of 0.6-0.7. The octapeptide depolarized the membrane of the myocytes in a concentration-dependent manner and to a maximal extent similar to that observed for alpha 1-adrenoceptor stimulation. No action potentials appeared with angiotensin II. In contrast to earlier reports, depolarization did occur in the rabbit aorta in response to noradrenaline. The demonstration of depolarization raises the possibility that contraction of this blood vessel occurs through electromechanical and not or not solely through pharmacomechanical coupling when receptors for two important endogenous vasoconstrictors, noradrenaline and angiotensin II, are stimulated.

Longitudinal propagation of contraction in the isolated conduit coronary arteries of humans and pigs

We examined the longitudinal propagation of contraction in isolated human and pig coronary arteries. Epicardial coronary arteries of about 2 cm were cut open longitudinally, and the tension development of circular muscles was measured simultaneously at three sites (both cut ends and the midportion of the segment). Cyclic tension changes arising at one site of human coronary artery propagated smoothly to the other sites, and the whole length of segment worked as a single unit. Contraction induced by locally applied prostaglandin F2 alpha or histamine also propagated in four of seven preparations. The remaining three human and all seven pig coronary arteries showed propagation of such drug-induced contraction after treatment with 10 mM tetraethylammonium (TEA). In pig coronary arteries treated with TEA, electrical stimulation evoked a reproducible local contraction and its propagation. Propagation velocity was 9.0 +/- 0.7 at 0.8 mM calcium concentration and increased to 11.1 +/- 0.9 and 13.1 +/- 1.4 mm/sec as calcium concentration rose to 1.8 mM and 7.2 mM, respectively. Local contraction did not propagate at calcium concentrations of 0.2 mM or less. The calcium antagonist diltiazem decreased the propagation velocity dose dependently and blocked propagation of contraction at 0.3 microM without significant effects on the magnitude of local contraction. We conclude that smooth propagation of contraction develops in epicardial coronary arteries of humans and pigs and that the propagation may depend on calcium influx.

Calcium uptake studies of 1,4-dihydropyridine agonists into rabbit aortic smooth muscle cells in culture

The effects of the three dihydropyridine calcium channel agonists (+/-)BAY K 8644, (+)202-791 and (+/-)CGP 28392 on 45Ca++ uptake were studied in cultures of rabbit aortic smooth muscle cells. At 10(-7) M each agonist enhanced 45Ca++ uptake in 15-50 mM K+ but had no effect on the basal 45Ca++ uptake at 5 mM K+. At the uptake threshold of 15 mM K+ each agonist potentiated 45Ca++ uptake in a dose-dependent manner with half maximal effects at 2.4 nM for (+/-)BAY K 8644, 22 nM for (+)202-791 and 18 nM for (+/-)CGP 28392. The agonists showed no significant antagonistic activity. Responses were antagonized competitively by nifedipine and non-competitively by (+/-)D-600. The 45Ca++ uptake dose-response curves and the half maximal effects of the three agonists were over the same range of concentrations as their inhibition of [3H]nitrendipine binding to rat ventricular receptor membrane preparations. The data suggest that these cells mimic the calcium uptake by the intact aorta better than commercial vascular smooth muscle lines or cardiac cells.

Effects of BRL 34915, a putative K channel opener, on transmembrane 45Ca movements in rabbit aortic smooth muscle

The effects of BRL 34915, a putative K channel opener, on transmembrane Ca2+ movements were studied in isolated rabbit aortic strips, using the 45Ca flux techniques. BRL 34915 (1 and 10 microM) inhibited the contraction and Ca entry in response to norepinephrine (NE) and angiotensin II, each at 0.3 microM. BRL 34915 differed from Na nitroprusside in that it demonstrated little or no inhibition on the contraction and 45Ca efflux provoked by NE in Ca2+-free buffer, suggesting that the effect of BRL 34915 on intracellular Ca release plays a minor role in inhibiting the vasoconstrictor responses to receptor agonists. Moreover, BRL 34915 (100 microM) did not stimulate cyclic GMP formation in rabbit aortic slices. BRL 34915 (10 microM) failed to inhibit the vascular responses to KCl (20 and 40 mM). Tetraethylammonium (20 mM), a K channel blocker, abolished the vasorelaxant action of BRL 34915 against NE. However, Na nitroprusside inhibition of NE contraction was similarly affected. It is concluded that reduction of Ca entry appears to be the predominant mechanism mediating the inhibitory effect of BRL 34915 on the contractile responses to vasoconstrictor stimuli.

Electrophysiological and mechanical effects of substance P and acetylcholine on rabbit aorta

1. The mechanical and electrical properties of smooth muscle cells of the rabbit aorta were recorded simultaneously using respectively a force transducer and a 3 M-KCl-filled glass microelectrode. 2. Acetylcholine had two effects depending on concentration. At low concentration, it caused a persistent endothelium-dependent relaxation and hyperpolarization. At higher concentrations the acetylcholine endothelium-dependent relaxation summed with an endothelium-independent contraction. 3. Substance P caused a transient endothelium-dependent relaxation and hyperpolarization. 4. Acetylcholine and substance P depolarized and contracted de-endothelialized smooth muscle. When the de-endothelialized strip was pre-contracted by noradrenaline, acetylcholine depolarized the muscle but substance P did not. 5. In a 'cascade' experiment, the perfusate from an upstream intact aorta passed over a downstream de-endothelialized strip. Acetylcholine and substance P relaxed the downstream strip showing that they released an endothelial humoral factor which relaxes smooth muscle. 6. The results suggest a constant release of a factor from the endothelial cells which hyperpolarizes the smooth muscle cells in the media. Activation of acetylcholine and substance P receptors on the endothelium accelerates the release of this factor and causes vasodilatation.

Effect of adrenergic agonists on Ca2+-channel currents in single vascular smooth muscle cells

Ca2+-channel currents have been measured in enzymatically dispersed single smooth muscle cells of the rabbit ear artery using the whole-cell patch clamp technique. Inward currents were elicited by depolarizing test pulses from a holding potential of -50 mV. These currents were activated from -30 mV onward and reached full activation around 0 mV. alpha-Adrenergic agonists did not affect the background current measured at the holding potential, but markedly reduced the peak amplitude of the voltage-activated Ca2+-channel currents. This alpha-adrenergic inhibition also occurred in cells which were internally perfused with solutions containing either 10 microM cAMP, 10 microM cGMP or 0.1 mM GTP, but became irreversible when the pipette solution contained a non-hydrolyzable GTP-analog. The action of beta-agonists on the voltage-activated Ca2+-channel currents was variable, and ranged from no effect at all to a 50% reduction of the current. It is concluded that alpha-agonists do not open receptor-operated Ca2+-channels in these smooth muscle cells. The inhibition of the voltage-activated Ca2+-currents does not seem to be mediated through changes in cyclic nucleotide levels, but might be mediated through G-proteins. Its physiological relevance remains however unclear. The action of beta-agonists is consistent with their relaxing effect, but the reason for the non-uniform response has not been elucidated.

Functional characterization of cell-to-cell coupling in cultured rat aortic smooth muscle

Gap junction (GJ) occurrence and function was studied in cultured rat aortic smooth muscle cells, since cell-to-cell coupling is proposed to coordinate smooth muscle function but is difficult to study in the intact tissue. Cell proliferation in vitro formed a multilayered structure 10-15 cells thick. GJs connected cells to lateral and vertical neighbors, appearing in freeze fracture as P-face particles aggregated into circular plaques but also as linear arrays. The membrane potential was 58 +/- 3 mV. From quantification of the spread of electrotonic potentials according to a two-dimensional model, the intercellular resistivity was 900-1,400 omega X cm, whereas the nonjunctional membrane resistivity was 10(4) omega X cm2. Intercellular spread of 5(6)-carboxyfluorescein (CF; mol wt 376) in aortic cultures suggests that metabolic coupling is an important consequence of GJs in smooth muscle. CF transfer was not blocked by A23187 (10(-5) M), although rat fibroblasts became uncoupled by 10(-6) M. Ultimately uncoupled by the more potent ionophore ionomycin (10(-5) M), aortic cells seem more able to maintain GJ permeability during challenge from increased intracellular Ca than cells of noncontractile origin.

Membrane potentials and sodium channels: hypotheses for growth regulation and cancer formation based on changes in sodium channels and gap junctions

Based on several convergent lines of investigation, we make two hypotheses which are sufficient to explain many phenomena of growth regulation in both normal and cancer cells. 1. The first hypothesis is that there is a boundary or threshold of resting cell membrane potential that separates normal resting cells from normal proliferating cells and cancer cells. The basis for this in existing literature values of membrane potentials in resting and proliferating cells is established. A discussion of how these differences in potential can be explained focuses on changes in sodium permeability and internal sodium concentration. Of many sodium transfer mechanisms, the sodium channel is emphasized and how increased intracellular transfer may stimulate DNA synthesis. The effects of changing cell junctions, in particular gap junctions, on membrane potentials is also discussed, as well as the indications of altered junctions in tumor cells. The linking factor of the effects of growth factors on both cell junctions and sodium permeability leads to the second hypothesis. 2. Since growth initiation and inhibition involve sodium channels and gap junctions, several phenomena can be explained by postulating that they are one and the same entity. The basis for this hypothesis in existing descriptions of functional and structural similarities is outlined. The possible interchange of these elements in the cell cycle lead to several corollaries consequent to the conservation of their total number. The formation of gap junctions would consume sodium channels, decrease sodium permeability and stop DNA synthesis. Conversely, growth factors may competitively bind to channel-connexon elements, cleave gap junctions, liberate sodium channels to increase sodium permeability, and trigger DNA synthesis. Alterations in the structure of gap junction-channel elements in tumor cells would be sufficient to explain some carcinogenesis.

Flow control among microvessels coordinated by intercellular conduction

Optimal distribution of blood flow requires coordination of vasodilation among resistance vessels. During hyperemia, blood vessels dilate upstream from the initiating stimulus. Spreading vasodilation independent of flow changes has not been previously demonstrated. In the present study, iontophoresis of acetylcholine adjacent to single hamster cheek pouch arterioles in situ (diameter, 20 to 37 micrometers) induced a rapid bidirectional dilation that was not attenuated when blood flow was eliminated with vascular occlusion. This finding indicates that a vasodilatory stimulus is conducted along the arteriole and demonstrates the existence of a mechanism of intercellular communication that is capable of coordinating diameter changes among resistance vessels.

Effects of the calcium channel facilitator, CGP 28,392, on different modes of contraction in smooth muscle of rabbit and rat aortae and guinea-pig taenia caeci

Effects of a Ca2+ channel facilitator, CGP 28,392, on smooth muscle contractions were examined in order to delineate characteristics of Ca2+ channels in rabbit and rat aortae and guinea-pig taenia caeci. Application of increasing concentrations of KCl induced contractile responses in these smooth muscles and CGP 28,392 shifted the concentration-response curve for KCl to the left. The maximum response was also increased in rat aorta and guinea-pig taenia. CGP 28,392 also shifted the concentration-response curves for noradrenaline in rat aorta and for histamine in taenia to the left and increased the maximum response in rat aorta. However, the corresponding curve for noradrenaline in rabbit aorta was not affected by CGP 28,392. The sustained contractions induced by KCl were inhibited by cumulative application of verapamil in these smooth muscles. Pretreatment of the muscle with CGP 28,392 decreased the inhibitory effect of verapamil. The noradrenaline-induced contraction in rat aorta and the histamine-induced contraction in taenia were also inhibited by verapamil, and CGP 28,392 antagonized the effect of verapamil. The noradrenaline-induced contraction in rabbit aorta was only slightly inhibited by verapamil, and CGP 28,392 did not modify the effect of verapamil. In these smooth muscles, cumulative application of Ca2+ to the Ca2+-depleted, KCl-treated muscle induced contraction, and the concentration-response curve for Ca2+ was shifted to the left by CGP 28,392 and to the right by verapamil. The concentration-response curves for Ca2+ in Ca2+-depleted, noradrenaline-treated rabbit and rat aortae and in Ca2+-depleted, histamine-treated taenia were also shifted to the left by CGP 28,392 and to the right by verapamil. In some contractions, CGP 28,392 increased and verapamil decreased the maximum responses. CGP 28,392 antagonized the inhibitory effect of verapamil. 5 These results suggest that the Ca2 channel facilitator, CGP 28,392, has a relatively selective activating effect on voltage-dependent Ca2+ channels in rabbit aorta. However, it also activates receptor-linked Ca2+ channels in rabbit aorta when Ca2+ concentrations are low. In rat aorta and guinea-pig taenia this facilitator activates both types of Ca2+ channels.

The role of hyperpolarization in the relaxation of smooth muscle of monkey coronary artery

In monkey coronary arteries, outer and inner muscle had a similar resting potential (-39.5 and -40.0 mV). Both showed strong outward-going rectification, with no regenerative depolarization, on injection of depolarizing current. The depolarization spread electrotonically in all directions, particularly around the vessel wall. Hyperpolarization up to -45 mV by injection of constant current caused relaxation. Depolarization caused contraction. Pulses of field stimulation caused a brief depolarization which was reduced by tetrodotoxin or by stripping of the adventitia. They also caused a prolonged hyperpolarization which was not prevented by either, but was prevented by rubbing of the endothelium. The hyperpolarization in response to field stimulation therefore appears to result from electrically stimulated release of a substance from endothelial cells. Relaxation accompanied this hyperpolarization. It was twice the size of the relaxation produced by a similar hyperpolarization due to constant injection. Isoprenaline also produced hyperpolarization, and relaxation five times that seen with a similar hyperpolarization induced by direct current. Hyperpolarization appears to be an important, but not the only, mediator of relaxation induced in this artery both by endothelial cells and by beta-adrenergic stimulation.

Dihydropyridine sensitive calcium channels in a smooth muscle cell line

The pharmacological properties of voltage sensitive calcium channels (VSCC) were examined in a rat aortic smooth muscle cell line (A10). The inorganic VSCC blockers Co2+ and Cd2+ blocked 45Ca2+ uptake into these cells in both 5 mM K+ and 50 mM K+ (depolarizing) conditions. The organic VSCC antagonists nitrendipine, nimodipine, D-600 and diltiazem also blocked 45Ca2+ uptake at low concentrations. The relative potencies of blockade were similar to those found in intact vascular smooth muscle. The VSCC "agonist" BAY K8644 enhanced 45Ca2+ uptake and this effect could be reversed by nitrendipine. These results indicate that A10 cells possess VSCC and that these VSCC behave similarly to those in authentic smooth muscle.

Different electrical responses of outer and inner muscle of rabbit carotid artery to noradrenaline and nerves

Electrical responses of outer and inner muscle of the rabbit carotid artery to electrical stimulation and noradrenaline were investigated. Mean values of the resting potential and space constant in the direction of the long axis of the cells were -47.3 mV and 1.61 mm for the inner muscle and -45.9 mV and 0.92 mm for the outer muscle. Both muscles showed strong outward-going rectification with no evoked action potential on continuous injection of depolarizing current. Such current spread not only in the direction of the long axis of the smooth muscle cells but also to a lesser degree transverse to this axis (space constant approximately 0.55 mm). There was little or no spread of current from the outer muscle layer to the inner muscle layer. At high frequencies of nerve stimulation (higher than 20 Hz), slow depolarizations representing very slow excitatory junction potentials (e.j.p.s) were recorded from the outer (innervated) muscle. However, e.j.p.s were not evoked from the inner (non-innervated) muscle at any rate of field stimulation. At low frequencies of stimulation (less than 5 Hz) no e.j.p. was observed in either the inner or outer muscle, although the muscle contracted. Noradrenaline (10(-6) M) depolarized inner but not outer muscle. High concentrations of noradrenaline (10(-5) M to 2 X 10(-4) M) caused large depolarization of the inner muscle, and also smaller depolarization of the outer muscle.

Stimulus effects on protein and electrolyte concentrations in parotid saliva

Twelve subjects collected ten 1 min samples and then a 2.5 ml sample of parotid saliva at a constant flow rate on five separate days with citric acid, salt, sugar, quinine sulphate, and sour lemon drops as gustatory stimuli. The ten 1 min samples were analysed for protein and electrolyte content and the final 2.5 ml sample was used for electrophoretic separation of the different salivary proteins. In most subjects, salt elicited the secretion of saliva with a much higher protein concentration than did the other stimuli, but none of the stimuli differentially influenced the relative proportions of the different proteins secreted. There were several small but statistically significant effects of the nature of the stimulus on the concentrations of sodium, calcium and chloride, but not on potassium, magnesium or phosphate. Since the nature of the gustatory stimulus can influence the composition of saliva, salivary composition could be influenced by the nature of the diet.

Differential calcium dependence of contractile responses and 86Rb efflux from the rabbit aorta induced by vasoactive stimuli

86Rb was used to monitor potassium movements in strips of rabbit aorta simultaneously with measurements of tension. Histamine, noradrenaline, the prostaglandin endoperoxide analogue U46619, angiotensin II, and 144 mM K+ each induced an increase in 86Rb efflux concomitantly with contraction. For the first four agonists there was a rank-order correlation between the contractile response and 86Rb efflux, but 144 mM K+ induced a massive increase in 86Rb efflux although it was the weakest contractile stimulus. Contraction and increase in 86Rb efflux-induced K+ were both reduced by verapamil, which blocks voltage-sensitive calcium channels, implying that both effects of K+ were mediated mainly by a depolarisation-induced influx of calcium. Noradrenaline increased both tension and 86Rb efflux through an action on alpha-adrenoceptors, but its effect on efflux, unlike its effect on tension, was apparently totally dependent on the presence of extracellular calcium. Experiments performed in the presence of lanthanum, which blocks calcium influx, showed that the intracellular store of calcium released by noradrenaline apparently played no role in inducing 86Rb efflux, although it could trigger contraction. Lanthanum also blocked contraction induced by K+ but had less effect on the increase in 86Rb efflux induced by K+. Thus, agonist-induced vascular contraction and 86Rb efflux can be dissociated, but under normal conditions all the contractile stimuli tested induced 86Rb efflux.

Neuronal influence on the mechanical activity of the ciliary muscle

1 Neuronal effects and the pharmacological properties of the bovine ciliary muscle were investigated in vitro. The bovine ciliary muscle exhibited no spontaneous activity. 2 Electrical stimulation of an isolated short ciliary nerve produced distinct contractions. The minimal stimulus duration required to evoke a contraction was 0.2 ms and amplitude of the contraction was maximal at 2 ms. Twitch or incomplete tetanus reached a complete tetanus with 4 Hz stimulation. 3 Raising the external potassium concentration from 5.9 to 158.8 mM produced a contracture which consisted of an initial phasic and then tonic components. 4 The contractions generated by either electrical stimulation (0.2-100 ms) or high K were potentiated by physostigmine and completely inhibited by atropine. Neither adrenoceptor agonists nor blockers influenced these contractions. 5 Application of tetraethylammonium (TEA), potentiated the electrically-induced ciliary muscle contraction, and the effect of TEA was not completely inhibited by high concentrations of either atropine or tetrodotoxin. Thus, TEA presumably acts both pre-junctionally and post-junctionally to increase the contractile development of ciliary muscle. 6 The ciliary contractile response is primarily mediated by acetylcholine released from nerves, and this response is accompanied by a negligible contribution from the sympathetic nerves. Depolarization induced by electrical currents or by high K was ineffective in evoking contraction of the ciliary muscle. 7 The results suggest that excitation of the ciliary muscle is probably mediated via junction potentials or by a direct transmitter action without any very great change in the potential. Action potentials are probably generated in the presence of TEA.

Adrenergic receptors and increased reactivity of aortic smooth muscle in renal hypertensive rats

The contractile responses of aortic smooth muscle and the dissociation constant (Ka) of the norepinephrine (NE) acting on alpha-adrenergic receptors in isolated thoracic aorta from normotensive and renal hypertensive rats were studied. Male Wistar rats were made hypertensive by uninephrectomy with figure-of-eight ligatures around the contralateral kidney. Two to three weeks after operation, the arterial systolic pressure for sham-operated rats was 123 +/- 2, and that for hypertensive rats was 164 +/- 4 mm Hg. Spontaneous rhythmic contractions of aortic rings, which were potentiated by low [Na]o and abolished by nifedipine (10(-6) - 10(-5) M), were observed in 4 out of 18 hypertensive rats. Aortic rings from normotensive rats showed no spontaneous rhythmicity. Although rhythmic phasic contractions could be induced in normotensive tissues by NE or caffeine in the presence of NE, tissues of hypertensive rats responded more readily. Furthermore, the threshold and the ED50 for tonic contractile response to NE was lower in hypertensive aortic rings than in the rings obtained from normotensive aortas. The Ka of NE acting on alpha-adrenergic receptors was derived from concentration-response data for NE before and after irreversible blockade of a fraction of the receptors with dibenamine (1-2 X 10(-7) M) in the presence of cocaine and propranolol. Mean Kas obtained were 5.58 +/- 0.42 X 10(-7) and 2.12 +/- 0.28 X 10(-7) M for normotensive and hypertensive rats, respectively. The increased contractile responses of the hypertensive aorta may be explained, at least partially, by a higher resting [Ca]i and a greater affinity of NE acting on the alpha-adrenergic receptors.

General features of electrical and mechanical properties of smooth muscle cells in the guinea-pig abdominal aorta

The membrane potential in smooth muscle cells of the guinea-pig's abdominal aorta has a mean value of -57.2 mV. These cells are electrically connected and the space and time constant are 0.66 mm and 180 ms respectively. An increased K-concentration elicited a contraction at 20 mM and the maximum was reached at 77 mM. The maximum depolarization produced by a tenfold increase of [K]0 was 45 mV. Tetraethylammonium at concentrations exceeding 2 mM, depolarized the membrane, increased the membrane resistance and reduced the rectifying properties of the membrane. Only at 20 mM a small active response could be induced by outward current pulses. Low concentrations of noradrenaline (less than 10(-8) M) hyperpolarized the membrane, while higher concentrations (greater than or equal to 10(-7) M) depolarized. Isoprenaline at concentrations below 10(-7) M also hyperpolarized, but it depolarized from 10(-5) M onward. Acetylcholine at concentrations over 10(-8) M hyperpolarized the cells without exerting an effect on the resting tension, but it reduced a noradrenaline induced contraction. Low concentrations of caffeine (less than or equal to 2 mM) hyperpolarize the membrane, while higher concentration (greater than or equal to 5 mM) depolarize. Caffeine is found to be a more efficient releaser of cellular Ca than noradrenaline. This might be due to the weak beta-agonist action of noradrenaline appearing at high noradrenaline concentrations. The hypothesis is supported by the finding that a beta-stimulation increases the Ca-uptake in the intracellular store. The study of the electrophysiological effects of different stimuli do not suggest an important role for electromechanical coupling in this tissue.

Electrophysiology and innervation of the smooth muscle of dog inferior vena cava

1. Electrical properties of outer and inner muscles of three portions of dog inferior vena cava and their catecholaminergic innervation were investigated by microelectrode recording and an immunohistochemical technique. 2. There was no difference in the electrical properties of outer and inner muscles of the supradiaphragmatic (portion a) or the infrarenal (portion c) segments which had a quiescent membrane potential, delayed rectification, strong outward going rectification and no action potential. 3. In the longitudinal muscle which made up most of the segment (portion b) between the liver and renal veins, some of the outermost cells fired slow discharges and others action potentials in response to depolarizing current, but no cells from the innermost layer of longitudinal muscle of this portion did so. 4. All smooth muscle portions of the inferior vena cava showed a current spread in the direction of the long axis of the cell. Mean values of space constant of portions a, b and c were 2.25, 1.15 and 0.99 mm, respectively. 5. Noradrenergic nerve terminals were widely distributed in the longitudinal muscle layer of portion b and the circular muscle layer of portion c. Few nerve terminals were seen in any part of portion a. 6. The results suggest that a tendency to repetitive electrical activity was associated with outer smooth muscle aligned longitudinally, though a low space constant may have been associated with noradrenergic innervation.

Adrenergic transmissions in the guinea-pig mesenteric artery and their cholinergic modulations

1. Neuromuscular transmission in the guinea-pig mesenteric artery and the effect of acetylcholine (ACh) on transmission were investigated by the micro-electrode method. 2. The membrane potential, length and time constant of the smooth muscle of the mesenteric artery were found to be -69.6 +/- 1.9 mV, 0.81 +/- 0.15 mm and 129 +/- 42 msec, respectively. 3. Perivascular nerve stimulation produced an excitatory junction potential (e.j.p.) and repetitive stimulation produced facilitation. When the amplitude of an e.j.p. reached threshold, a spike was evoked. 4. Very rarely, miniature e.j.p.s. were recorded. The amplitude histogram showed a skew distribution. Increases in the stimulus intensity enlarged the amplitude of e.j.p.s. as a stepwise manner. These results indicate multiple innervation to the muscle cells. 5. The time constant of the falling phase of an e.j.p. was consistently larger than that of the muscle membrane. 6. ACh, in concentrations of less than 10(-8) M, suppressed the e.j.p. amplitude without change in the membrane potential and resistance of the muscle membrane, but accelerated facilitation. These ACh actions were suppressed by pre-treatment with atropine. 7. ACh suppressed neither the conduction velocity of excitation of adrenergic nerves nor the number of nerves contributing to the generation of an e.j.p. 8. These results suggest that pre- and post-junctional muscarinic receptors possess different sensitivities to ACh, and a low concentration of ACh (less than 10(-8)M solely suppressed the release of noradrenaline by activating the pre-junctional muscarinic receptors.

Electrical current-induced contraction in the smooth muscle of the rabbit aorta

1. In the smooth muscle of rabbit aorta, the relationship between the change in membrane potential and the concentration evoked by current application was studied. 2. In normal Krebs solution, outward current produced contraction when the membrane was depolarized to about -45 mV and the membrane resistance was decreased. Further increase in the outward current intensity produced an increase in tension development with very small additional depolarization but with a marked decrease in membrane resistance. 3. With arteries depolarized to about -20 mV, in excess K solution of concentration more than 60 mM, outward current failed to produce further contraction. However, contraction was produced when the inward current was switched off. This may be due to removal of an inactivation process by hyperpolarization of the membrane. 4. In excess K solution, no relaxation was observed with inward current application. However, when artificial stretch was applied to the preparation immersed in 98 mM-K solution, a prolonged hyperpolarizing current of more than 3 sec could cause relaxation. 5. Ca-free solution and Mn ion (0.5 mM) blocked the concentration induced by outward current. Phentolamine and tetrodotoxin had not effect. 6. It is suggested that increased membrane conductance associated with depolarization by outward current increased the Ca influx which causes contraction.