Ionic fluxes in rat uterine smooth muscle

The relaxant action of BRL 34915 in rat uterus

1 BRL 34915 (0.04-1.3 microM) caused concentration-dependent inhibition of spontaneous phasic spasms of the isolated uterus of the term pregnant rat and this effect was not antagonized by propranolol. Spasms evoked by low concentrations of KCl (less than 20 mM) were inhibited by BRL 34915 but those evoked by higher concentrations (greater than 40 mM) were unaffected. 2 In experiments using extracellular electrical recording, BRL 34915 (10 microM) selectively inhibited oxytocin-induced phasic spasms and the associated spike activity but had little effect on the tonic component of the spasms. BRL 34915, as an inhibitor of phasic spasms to oxytocin (0.2 nM), was antagonized by procaine (0.3 and 1 mM). 3 BRL 34915 (10 microM) did not inhibit Ca2+-induced spasm of saponin-skinned thin myometrial strips. 4 Intracellular microelectrode recording from myometrial strips showed that BRL 34915 (10 microM) inhibited action potentials and phasic spasms in the presence of oxytocin (0.2 nM) and produced a hyperpolarization of 5 mV. 5 In single myometrial cells under current or voltage clamp, BRL 34915 (10 microM) had no effect on action potentials and inward current in Ca2+- or Ba2+-containing media in the presence of tetraethylammonium, 4-aminopyridine and caesium chloride. In the absence of these K+-channel inhibitors, BRL 34915 had no effect on resting membrane potential, membrane resistance, action potential, inward current or outward current. 6 BRL 34915 (1 or 10 microM) had no effect on 86Rb efflux from myometrial strips. 86Rb efflux was increased by oxytocin (0.2 and 20 nM). 7 The relaxant profile of BRL 34915 in the rat uterus is similar to that described for other smooth muscles where an action to open membrane K+-channels has been proposed. BRL 34915 inhibited spike production but produced only a small hyperpolarization without a detectable increase in 86Rb efflux. Membrane resistance and transmembrane currents were unaffected. These results suggest that in the uterus the effects of BRL 34915 may be restricted to K+-channels involved in the production of pacemaker activity.

Prevention by calcitonin of the pathological modifications of the rabbit arterial wall induced by immunization with elastin peptides: effect on vascular smooth muscle permeability to ions

Immunization of rabbits with elastin peptides prepared from purified bovine ligamentum nuchae elastin produces calcified arteriosclerotic lesions and fragmentation of elastic lamellae. Simultaneous administration of porcine calcitonin largely prevents the development of lesions. Experiments were carried out to clarify the mechanisms involved in the development of lesions as well as those involved in the preventive effect of calcitonin. Control experiments were carried out using bovine serum albumin (BSA) as antigen. Circulating antibodies and soluble immune complexes increased steadily in the sera of animals immunized with elastin peptides or BSA. The cellular immune reaction was weak as assessed by [3H]thymidine incorporation into lymphocytes in the presence of antigen or phytohemagglutinin. Arterial lesions appeared only in the animals immunized with elastin peptides, not in those immunized with BSA. Ion flux measurements were also carried out on strips of aorta obtained from immunized and control animals. Immunization with elastin peptides significantly increased the ouabain-insensitive 22Na+ efflux, the 86Rb efflux (indicator of K+ efflux), and the 45Ca2+ influx. Simultaneous calcitonin administration prevented the increase in Ca2+ influx but did enhance passive permeability to Na+ and K+ as well as the sodium pump. When calcitonin was administered without immunization, it decreased arterial smooth muscle permeability to Na+ and K+ and also decreased the basal Ca2+ influx. It is concluded that the pathological modifications of the arterial wall triggered by immunization with elastin peptides is at least partly mediated by the effect of antielastin antibodies and immune complexes on the ion permeability of arterial smooth muscle. Prevention of the increased Ca2+ influx by calcitonin is probably a key effect in the prevention of the development of lesions. The fact that calcitonin alone can modify the ion permeability of arterial smooth muscle suggests that this hormone may play a role in the regulation of vascular homeostasis.

Investigation of factors affecting the intracellular sodium activity in the smooth muscle of guinea-pig ureter

1. The intracellular Na+ activity (aNai) of the smooth muscle cells from guinea-pig ureter has been measured using double-barrelled Na+-sensitive micro-electrodes. aiNa in modified Krebs solution at 35 degrees C was of a mean 7.4 +/- 2.9 mM (n = 32, S.D. of an observation), equivalent to a Na+ equilibrium potential (ENa) of +66.7 mV. Membrane potential (Em) was of a mean -50.8 +/- 4.6 mV. 2. Inhibition of the Na+ pump by application of ouabain or removal of external K+ (K+o) resulted in a restricted rise of aNai. The rate of rise was faster in the presence of ouabain (10(-4) M) but the stabilized aNai was not significantly different from that observed after the prolonged absence of K+o. The mean aiNa recorded after prolonged Na+ pump inhibition was 20.6 +/- 5.5 mM (n = 28), equivalent to an ENa of +39.6 mV. Neither removal of K+o after aNai had stabilized in the presence of ouabain nor application of ouabain after aNai had stabilized in K+-free solution caused a rise in aiNa, suggesting that the Na+ pump was fully inhibited by either procedure. 3. Reduction of Na+o resulted in a rapid fall in aiNa against the electrochemical gradient, both before and after Na+ pump inhibition. At each level of Na+o, aNai stabilized such that ENa remained approximately constant in either condition. Readdition of Na+o resulted in a rapid recovery of aNai. 4. Elevation of Ca2+o (at constant Na+o) caused a fall in aNai of much the same time course as that observed on reduction of Na+o, both before and after Na+ pump inhibition. The extent of the fall was dependent upon the initial aNai. Reduction of Ca2+o resulted in a rise in aNai. 5. Elevation of the external divalent cation concentration with Mn2+ or, to a lesser extent, Mg2+ reduced aiNa in the presence of a functional Na+ pump. But after prolonged exposure to ouabain or K+-free solution, elevation of Mg2+o had no effect on aiNa while application of Mn2+o caused a slow rise. These results suggest that Ca2+o affects aiNa in two ways. One is mimicked by Mg2+ and Mn2+ and is probably due to alteration of the Na+ leak. The other is a specific effect, revealed by Na+ pump inhibition. 6. It is concluded that aiNa can be maintained far from equilibrium in the absence of a functional Na+ pump. Several lines of evidence are discussed which indicate the participation of Na+-Ca2+ exchange in Na+ extrusion in this condition.

Sodium 22+ washout from cultured rat cells

The washout of Na+ isotopes from tissues and cells is quite complex and not well defined. To further gain insight into this process, we have studied 22Na+ washout from cultured Wistar rat skin fibroblasts and vascular smooth muscle cells (VSMCs). In these preparations, 22Na+ washout is described by a general three-exponential function. The exponential factor of the fastest component (k1) and the initial exchange rate constant (kie) of cultured fibroblasts decrease in magnitude in response to incubation in K+-deficient medium or in the presence of ouabain and increase in magnitude when the cells are incubated in a Ca++-deficient medium. As the magnitude of the kie declines (in the presence of ouabain) to the level of the exponential factor of the middle component (k2), 22Na+ washout is adequately described by a two-exponential function. When the kie is further diminished (in the presence of both ouabain and phloretin) to the range of the exponential factor of the slowest component (k3), the washout of 22Na+ is apparently monoexponential. Calculations of the cellular Na+ concentrations, based on the 22Na+ activity in the cells at the initiation of the washout experiments, and the medium specific activity agree with atomic absorption spectrometry measurements of the cellular concentration of this ion. Thus, all three components of 22Na+ washout from cultured rat cells are of cellular origin. Using the exponential parameters, compartmental analyses of two models (in parallel and in series) with three cellular Na+ pools were performed. The results indicate that, independent of the model chosen, the relative size of the largest Na+ pool is 92-93% in fibroblasts and approximately 96% in VSMCs. This pool is most likely to represent the cytosol.

Ion movements in membrane vesicles: a new fluorescence method and application to smooth muscle

A method is described for studying ion permeabilities of membrane vesicles based on the principle that when membrane permeability to H+ is very high, the H+ movement is determined by the membrane potential generated by the H+ movement. The rate of H+ movement under these conditions thus gives a measure of the rate of dissipation of this membrane potential by comovement of anions or countermovement of cations present. Thus, by studying the H+ efflux using an impermeant cation and different anions, the membrane permeability to the anions can be assessed. Similarly, the use of an impermeant anion allows the study of the permeation of various cations. H+ movement was followed across the membranes by monitoring a change in the fluorescence intensity of the pH-sensitive dye pyranine trapped inside the membranes. This method when tested using phosphatidylcholine liposomes yielded the expected results, i.e., permeability of the liposomal membrane was: Cl- greater than SO2-4 and K+ greater than Na+. A plasma membrane-enriched fraction loaded with pyranine was isolated from estrogen-dominant rat myometrium. The anion permeability characteristics of this membrane were studied using tetramethylammonium (TMA+) as the poorly permeant cation, and the cation permeability was studied using L-glutamate- as the poorly permeant anion. The anion permeabilities were D-glutamate- less than L-glutamate- less than glutarate2- less than Cl- less than or equal to SO2-4, and the cation permeabilities were TMA+ less than K+ less than Na+. It is hypothesized that the observed anomalously higher Na+ and SO2-4 movements may involve special mechanisms.

Direct action of aldosterone on transmembrane 22Na efflux from arterial smooth muscle. Rapid and delayed effects

Acute subcutaneous (s.c.) administration of aldosterone increases ex vivo 22Na efflux from rat tail artery smooth muscle, which appears to be due to a specific action on mineralocorticoid receptors. Indeed, this effect is blocked by the antimineralocorticoid compounds RU 28318 [17 beta-hydroxy-3-oxo,7 alpha-propyl(17 alpha)-pregn 4-ene, 21 potassium carboxylate] and spironolactone. The specific glucocorticoid receptor agonist RU 26988 [11 beta,17 beta-dihydroxy-17-(1-propynyl) androesta-1,4,6 trien-3-one] does not modify 22Na efflux. We show here that aldosterone has, at physiological concentrations, a mineralocorticoid specific stimulating effect on passive and sodium pump dependent transmembrane movements of sodium from the rat tail artery smooth muscle. Aldosterone exerts two types of action on sodium transport: 1) a delayed stimulation of ouabain-dependent 22Na efflux and ouabain-independent 22Na efflux, which are completely blocked by actinomycin D; and 2) a very rapid increase of passive 22Na efflux, which is insensitive to actinomycin D and therefore does not seem to depend on transcription of genomic information.

Different effects of phenylephrine and clonidine on 86Rb efflux and on contraction in the rat caudal artery

A large transitory efflux of 86Rb was induced by concentrations of phenylephrine (10(-6) and 10(-4)M) or clonidine (10(-4)M) which were able to produce phasic contraction. By contrast, a lower concentration of clonidine (10(-6) M), which could only induce a slower (tonic) contraction, only produced a weak but sustained 86Rb efflux. These results show qualitative and quantitative differences in the effects of the two alpha-agonists. They further support the view that phasic and tonic alpha-adrenergic responses may be related to different mechanisms.

Mode of action of cyclothiazide and triamterene. Ex vivo effect on 22Na and 86Rb efflux from arterial smooth muscle

Acute oral cyclothiazide treatment of conscious rats increased ex vivo 86Rb efflux from tail artery smooth muscle. This effect was blocked by oral triamterene. Identical results were obtained in binephrectomized rats, suggesting that the two drugs had a direct effect on smooth muscle K+ (86Rb) permeability. Decreased ex vivo smooth muscle 22Na efflux induced by oral cyclothiazide and triamterene is probably secondary to their renal actions, since there was no effect in binephrectomized rats.

Improved electrical coupling in uterine smooth muscle is associated with increased numbers of gap junctions at parturition

We have studied some passive electrical properties of uterine smooth muscle to determine whether a change in electrical parameters accompanies gap junction formation at delivery. The length constant of the longitudinal myometrium increased from 2.6 +/- 0.8 mm (X +/- SD) before term to 3.7 +/- 1 mm in tissues from delivering animals. The basis of the change was a 33% decrease in internal resistance and a 46% increase in membrane resistance. Axial current flow in an electrical syncytium such as myometrium is impeded by the cytoplasm of individual cells plus the junctions between cells. Measurement of the longitudinal impedance indicated that the specific resistance of the myoplasmic component was constant at 319 +/- 113 omega . cm before term and 340 +/- 93 omega . cm at delivery. However, a decrease in junctional resistance was apparent from 323 +/- 161 omega . cm to 134 +/- 64 omega . cm at delivery. 1.5-2 d after delivery, the junctional resistance was increased, as was the myoplasmic resistance. Thin-section electron microscopy of some of the same muscle samples showed that gap junctions were present in significantly greater numbers in the delivering tissues. Therefore, our results support the hypothesis that gap junction formation at delivery is associated with improved electrical coupling of uterine smooth muscle.

Techniques and applications of extracellular space determination in mammalian tissues

This review summarizes the ways in which the extracellular space (ECS) may be estimated in mammalian tissues, and briefly describes some of the uses to which the EC confinement of certain molecules (markers or tracers) may be put in the elucidation of physiological functions. The introductory section is followed by a description of the more commonly used marker molecules and their functional characteristics, and of factors likely to lead to the spurious over- or under-estimation of the ECS. Certain alternative methods are also described, in particular those based on morphological and electrical criteria which seek to demonstrate small, functionally important, changes in the size of specialized regions of the ECS (e.g. lateral cellular interspaces) without necessarily being required to provide a quantitatively precise estimate of their size. Section III describes the results of measurements of ECS in various mammalian tissues (muscle, gastro-intestinal tract, nervous tissue, crystalline lens, placenta, lung and kidney) and some applications of EC markers to investigation of cellular function (e.g. uptake of metabolic substrates and epithelial transport) and, in outline, characterization of capillary permeability. The available literature in this field is very extensive, and in the interests of brevity the reader is, where appropriate, referred to previous reviews covering specialized aspects of ECS determination and related topics.

Multiexponential and multicompartmental approaches for analysis of tracer washout data from animal

A comparison is made between multiexponential and multicompartmental analyses of tracer washout data from biological tissues. Various analytical and computer methods are used to explore the relations between kinetic parameters of three-compartment models and parameters of three-exponential functions fitted to typical observed tracer washout records and also to examine relations between kinetic parameters of the same three-compartment models determined analytically and numerically. It is concluded that exponential factors and coefficients cannot always be equated respectively with conpartmental transport rate constants or compartment sizes. Sufficient analytical and computer methodologies exist so that selection of meaningful biological models should not be hampered by a lack of appropriate solution procedures.

Electrophysiological study of the action of angiotensin II on the rat myometrium

Angiotensin II (A II) produces a contraction of visceral and vascular smooth muscles of different species. The accompanying electrophysiological changes were measured on strips of rat myometrium at 35 degrees C using the double sucrose-gap technique. A II at concentrations from 5 x 10(-10) to 10(-6)M produces a depolarization and an increase in membrane conductance. This increase in membrane conductance is not membrane potential dependent since it is observed even when the membrane potential is maintained at the resting level. When all Na + in the test solution is replaced by either Li + or Mg (2+), the depolarizing effect of 10(-6)M A II is either markedly reduced or abolished. Under these conditions, A II produces a small initial hyperpolarization, which is modified by external potassium concentration changes and abolished by tetraethylammonium chloride. When all Cl- is replaced by either NO (-3) or cyclohexanesulfamate, A II (10(-6) M) still produces a 20-m V depolarization. The removal of extracellular Ca (2+) or K+ does not have any effect on the depolarizing action of A II, which also is not changed by 10(-3) M ouabain. In conclusion, A II produces a depolarization of the uterine smooth muscle membrane through an increase in the membrane conductance to Na+. The membrane conductance to potassium is increased simultaneously. The contraction induced by A II shows two components: a phasic component triggered by the Ca (2+) entry associated with spike production and a tonic component due to the release of Ca(2+) from intracellular stores.

Identification of different sodium compartments from smooth muscle cells, fibroblasts and endothelial cells, in arteries and tissue culture

1. The (22)Na efflux curve from the rat tail artery, at 35 degrees C, can be analysed as the sum of three distinct components, from 0 to 90 min of washout. After an initial diffusional component the two late exponential components Be(-kBt) and Ce(-kCt) have the following values: B = 3.03 +/- 0.15 m-mole/kg wet wt. and C = 0.56 +/- 0.04; k(B) = 0.145 +/- 0.005 min(-1) and k(C) = 0.015 +/- 0.007.2. In order to identify the cellular origin of the different compartments we compared the (22)Na efflux curve from the rat tail artery with the curves obtained from whole rabbit aortal strips, rabbit aortal medial or adventitial strips; and primary cultures from rabbit aorta medial smooth muscle cells, cultures of a non-fusing muscle cell line (BC(3)H1), fibroblasts and endothelial cells.3. It is possible to identify under these experimental conditions the cellular compartments from which the different exponential components of the efflux from the whole arteries originate. Fibroblasts and endothelial cultures, as well as adventitial strips exchange (22)Na slowly with exponential constants resembling k(C). Their efflux rate constants are: fibroblast cultures 0.010 +/- 0.002 min(-1), endothelial cells 0.015 +/- 0.003 min(-1) and adventitia 0.019 +/- 0.007 min(-1). Smooth muscle cells are exclusively responsible for the intermediate component Be(-kBt), but they present also a slow component, indistinguishible from the slow exponential component from the other types of cells in the artery. The rate constants for muscle cells are: rabbit aortic media k(B) 0.25 +/- 0.09 min(-1) and k(C) = 0.013 +/- 0.004 min(-1); medial cultures k(B) = 0.202 +/- 0.005 min(-1) and k(C1) = 0.020 +/- 0.003 min(-1); and BC(3)H1 cell culture k(B) = 0.205 +/- 0.083 min(-1) and k(C) = 0.016 +/- 0.003 min(-1).4. The efflux from compartment B of smooth muscle cells is inhibited by ouabain and in the absence of extracellular K(+). The efflux from compartment C is inhibited only by ouabain but not by the suppression of extracellular K(+).5. We propose a distribution of Na(+) in smooth muscle cells in two intracellular compartments: (1) Na(+) freely dissolved in the sarcoplasm, exchanging with the kinetics of compartment B and (2) a second cellular compartment which could be contained in the sarcoplasmic reticulum exchanging with the kinetics of compartment C.6. On the basis of the previous model of Na(+) distribution, considering our values, and without any correction, the estimated sarcoplasmic concentration of Na(+) is 9.6 mM, compatible with the direct measurements obtained in skeletal and heart muscle. The Na(+) concentration in the sarcoplasmic reticulum would be 4-10 times higher than in the cytoplasm. In order to increase the accuracy of our calculations it would be necessary to account for the interdiffusion and back diffusion of Na(+) between compartments. It is not possible to attain this goal at the present time.

Pharmacology of the adrenoceptors and cholinoceptors of the BC3H1 nonfusing muscle cell line

1. We have studied the action of different transmitters on the transmembrane 42K or 86Rb efflux from tissue cultures of the BC3H1 muscle cell line. 2. The effect of catecholamines and carbachol (CCh) on the isotope efflux rate was measured by addition of the drugs at different times during the washout. 3. Noradrenaline (NA), phenylephrine (Phe), isoprenaline (Iso) and CCh increased 42K and 86Rb efflux rate in a dose-dependent manner. 4. The action of NA seems to be due exclusively to the stimulation of alpha-receptors, since its effect was blocked by phentolamine but not by propranolol. The effects of Iso on the 86Rb efflux were inhibited by propranolol. The beta-receptors in the BC3H1 cells seem to be the beta2-type since they are stimulated by Iso and insensitive to NA. 5. The effect of CCh was blocked (+)-tubocurarine but not by atropine. This result confirms the presence of nicotinic receptors in BC3H1 cells.