Reorientation of myofilaments during contraction of a vertebrate smooth muscle

Structural limits on force production and shortening of smooth muscle

This study determined the factors that limit force production and shortening in two smooth muscles having very different relationships between active and passive force as a function of muscle length. The rat anococcygeus muscle develops active force over the range of lengths 0.2-2.0× the optimum length for force production (Lo). Passive tension due to extension of the resting muscle occurs only at lengths exceeding Lo. In contrast, the rabbit taenia coli develops force in the range of lengths 0.4-1.1 Lo, and passive force which is detectable at 0.56 Lo, increases to ~0.45 maximum active force at Lo, and increases sharply with further extension. The anococcygeus muscle can shorten to 0.2 Lo and the taenia coli to 0.4 Lo. Dynamic stiffness and energy usage at short muscle lengths suggest that the limit of shortening in the taenia coli, in contrast to the anococcygeus muscle, is not due to a failure of cross bridge interaction. Phosphorylation of the regulatory myosin light chains in intact muscles decreased to a small extent at short lengths compared to the decrease in force production. The differences in force production and the extent of shortening in the two muscles was maintained even when, following permeabilization, the myosin light chains were irreversibly phosphorylated with ATPγS, indicating that differences in activation played little, if any role. Ultrastructural studies on resting and activated muscles show that the taenia coli, which is rich in connective tissue (unlike the anococcygeus muscle) undergoes marked cellular twisting and contractile filament misalignment at short lengths with compression of the extracellular matrix. As a result, force is not transmitted in the longitudinal axis of the muscle, but is dissipated against an internal load provided by the compressed extracellular matrix. These observations on two very different normal smooth muscles reveal how differences in the relative contribution of active and passive structural elements determine their mechanical behavior, and how this is potentially modified by remodeling that occurs in disease and in response to changes in functional demand.

Contractile filament architecture and force transmission in swine airway smooth muscle

It is well known that the cyclic interaction of myosin cross bridges with actin filaments is responsible for force and shortening generation in smooth muscle. The intracellular organization of contractile filaments, however, is still poorly understood. Here, we show electron microscopic and functional evidence that contractile filaments in airway smooth muscle lie parallel to the longitudinal axis of the cell bundle, in contrast to the obliquely arranged filaments depicted in conventional models. The parallel arrangement of contractile filaments is maintained despite the fact that individual cells are spindle-shaped. This is accomplished through filament attachment to membrane-associated dense plaques that are in turn connected to similar structures on neighboring cells. Intracellularly, the parallel arrangement is maintained despite the centrally located nucleus. This is accomplished by attachment of actin filaments to the nuclear envelope and making the nucleus a force transmitting structure. The results suggest that smooth muscle cells in tissue form a mechanical syncytium and are able to function properly only as a group.

Hormone replacement therapy and the risk of incident congestive heart failure: the Cardiovascular Health Study

BACKGROUND

The development of congestive heart failure (CHF) in older persons is related to a variety of mechanisms. Hormone replacement therapy (HRT) affects several of the pathways that may be important in the development of CHF. We hypothesized that HRT would be associated with a decreased risk of incident CHF.

METHODS

Using Cox proportional-hazards regression, we assessed the risk of incident CHF (n = 304) associated with time-dependent past and current use of HRT compared to never use. The Cardiovascular Health Study is a prospective cohort study of community-dwelling adults aged 65 years and older. This analysis included female participants without a history of CHF at baseline (n = 3223).

RESULTS

At baseline, 62% were never users, 26% were past users, and 12% were current users of HRT. Compared with never users, the multivariable relative risk (RR) of CHF was 1.01 (95% confidence interval [95% CI] 0.76,1.34) for past users and 1.34 (0.93,1.94) for current users. Results were similar among most treatment and clinical subgroups, except that the association of current HRT with CHF appeared to depend on body mass index (BMI) or osteoporosis status. The RR was 0.82 (0.43,1.60) for normal weight women, 1.65 (0.95,2.88) for overweight women, and 2.22 (1.06,4.67) for obese women (p = 0.01 for interaction). Similarly, the RR was 0.15 (0.04,0.65) for women with osteoporosis and 1.82 (1.25,2.65) for women without osteoporosis (p = 0.001 for interaction).

CONCLUSIONS

Overall, HRT was not associated with the risk of incident CHF, although BMI and osteoporosis appeared to modify the association of HRT with CHF. The risk of CHF was lower in patients with lower BMI or osteoporosis.

Hyperinsulinemia and uterine perfusion in patients with polycystic ovary syndrome

OBJECTIVE

To determine whether hyperinsulinemia has a negative effect on uterine blood supply in patients with polycystic ovary syndrome (PCOS).

METHODS

Sixty-three patients with normal body mass index were included prospectively in the study: 48 had clinical and hormonal features of PCOS and 15 were normo-ovulatory. All patients underwent Doppler flow measurement of the uterine artery, and determination of serum concentrations of luteinizing hormone, follicle stimulating hormone, prolactin, estradiol, androgens, insulin and C-peptide during the early follicular phase of the menstrual cycle. The 48 PCOS-patients were divided into two groups according to the pulsatility index (PI) value of the uterine artery: Group 1, PI < 3; Group 2, PI >or= 3 and the groups were compared.

RESULTS

The mean PI of the uterine artery (3.01 +/- 1.0 vs. 1.93 +/- 0.3, respectively) and fasting levels of insulin (50.9 +/- 9.3 vs. 40.3 +/- 10.9) and C-peptide (366.9 +/- 118.4 vs. 243.6 +/- 120.3) of PCOS-patients were significantly higher than those of the control group. No correlation was found between insulinemia and C-peptide and PI of the uterine artery and no significant difference was found in insulin and C-peptide levels among the two groups of PCOS-affected patients. Only the serum level of dehydroepiandrosterone sulfate was significantly higher in Group 2, and a direct correlation was found between PI values of the uterine artery and DHEAS plasma levels.

CONCLUSION

Insulin and C-peptide do not seem to interfere with uterine perfusion in PCOS-affected patients.

Influence of vaginal danazol on uterine and brain perfusion during hormonal replacement therapy

OBJECTIVE

To evaluate the effectiveness of vaginal danazol as progestin supplement to estrogen replacement therapy, and its interference with uterine and carotid artery flow compared with medroxyprogesterone-acetate (MPA), estrogen alone, and placebo.

METHODS

Forty healthy women at least 12 months after natural menopause were randomly divided into four treatment groups: Group 1 (n=10), continuous transdermal estradiol (TE) (50 microg/day), plus a monthly 10-day course of MPA (10 mg/day); Group 2 (n=10), continuous TE plus a monthly 10-day course of vaginal danazol (200 mg/day); Group 3 (n=10), TE alone; Group 4 (n=10), placebo. At baseline and during the first, third, and sixth month of treatment, the endometrial thickness was assessed by transvaginal ultrasonography, while the pulsatility index (PI) of the carotid and uterine arteries was assessed by color Doppler. An endometrial biopsy was also performed before and after the treatment.

RESULTS

At baseline, no significant differences between ages and other evaluated parameters were present in the four groups. In groups 1, 2, and 3, the values of carotid and uterine PI decreased significantly and similarly during the treatment, while in group 4 they were unchanged. In group 3 only, the endometrium was significantly thicker during treatment than before. No endometrial hyperplasia was present in the four groups at the end of the treatment.

CONCLUSIONS

Vaginal danazol seems to be capable of counteracting the mitogenic effect of estrogen on the endometrium without reducing the effectiveness of estrogens to improve peripheral arterial perfusion.

Menopause and the characteristics of the large arteries in a population study

In previous cross-sectional and longitudinal population studies, we found that the slope of systolic pressure on age was steeper in postmenopausal than in premenopausal women. We hypothesised that this observation could be due to a specific effect of menopause on the elasticity of the large arteries. We investigated 315 randomly selected women, aged 30 to 70 years. Based on 5.2 years of follow-up, 166 women were premenopausal and 149 menopausal (44 reaching menopause and 105 postmenopausal). These women were matched on age and body mass index with 315 men. We used a wall-tracking ultrasound system to measure the diameter, compliance and distensibility of the brachial and the common carotid and femoral arteries as well as carotid-femoral pulse wave velocity. Pulse pressure was determined from 24-h blood pressure recordings. Both in menopausal women (r = 0.37; P < 0.001) and in matching male controls (r = 0.16; P = 0.04), pulse pressure widened with increasing age. The slope of the 24-h pulse pressure on age was steeper in menopausal women than in their premenopausal counterparts (0.428 vs -0.066 mm Hg per year; P = 0.003) and than in the male controls (0.428 vs 0.188 mm Hg per year; P = 0.06). After adjustment for age, 24-h mean pressure, body mass index, antihypertensive drug treatment, smoking and the use of oral contraceptives or hormonal replacement therapy, postmenopausal women showed a higher carotid-femoral pulse wave velocity (7.77 vs 6.71 m/s; P = 0.02) and had a slightly greater diameter of the common carotid artery (7.09 vs 6.79 mm; P = 0.07) than their premenopausal counterparts. After similar adjustments, menopausal class was not significantly associated with other vascular measurements in women or with any vascular measurement in control men. In conclusion, menopause per se may increase aortic stiffness. We hypothesise that this phenomenon may contribute to the rise in systolic pressure and pulse pressure in women beyond age 50 and, in turn, may lead to a slight dilatation of the common carotid artery.

Contraction in the smooth muscle cell

This paper advances the hypothesis that the rearrangement of the actin cytoskeleton that takes place during contraction in the SMC is a mechanical reflection of the spatiotemporal pattern of the cell's polarized stimulus. In that sense the cell is responding more like a motile non-muscle cell than like a skeletal muscle cell. The paper reviews how diffusion patterns are generated and modified and suggests how the patterns are detected by the cell and transduced into cytoskeletal movement. Evidence is presented suggesting the actin cytoskeleton is composed of conical-shaped myofibrils (contractile units) measuring half a cell in length and containing filament-free spaces at their centres filled with cell inclusions. It is argued that the SMC contracts by involving variable combinations of the myofibrils in sequence and that the cell takes advantage of that fact to translocate various contractile elements between the myofibrils during contraction, thus economizing on its needs for those elements. Among the elements translocated are thought to be myosin, SR and mitochondria.

Mechanism of biphasic response of renal nerve activity during acute cardiac tamponade in conscious rabbits

Renal sympathetic nerve activity (RSNA) responses to acute cardiac tamponade were studied in conscious rabbits with all reflexes intact (Int) or after either surgical sinoaortic denervation (SAD) or administration of intrapericardial procaine (ip-Pro) or intravenous procaine (iv-Pro). In Int rabbits, the mean arterial pressure (MAP) remained relatively constant until the pericardial volume reached 7. 7 ml, whereas the RSNA increased to 226% [compensated cardiac tamponade (CCT)], then, at a pericardial volume of 9.3 ml, the MAP fell sharply and RSNA decreased to 34% [decompensated cardiac tamponade (DCT)]; 1 min after cessation of pericardial infusion, an intravenous injection of naloxone resulted in increases in both MAP and RSNA. In SAD rabbits, RSNA did not alter throughout CCT and DCT, but increased on injection of naloxone. In ip-Pro rabbits, RSNA increased during CCT but did not decrease during DCT, whereas, in iv-Pro rabbits, the RSNA response was similar to that in Int rabbits. These results indicate that RSNA responses to cardiac tamponade are biphasic, with an increase during CCT and a decrease during DCT. Sinoaortic baroreceptors are involved in mediating the increase in RSNA, whereas cardiac receptors may be involved in mediating the decrease in RSNA. An endogenous opioid may be responsible for the decrease in RSNA seen during DCT.

A method for isolating adult and neonatal airway smooth muscle cells and measuring shortening velocity

Methods are described for isolating smooth muscle cells from the tracheae of adult and neonatal sheep and measuring the single-cell shortening velocity. Isolated cells were elongated, Ca2+ tolerant, and contracted rapidly and substantially when exposed to cholinergic agonists, KCl, serotonin, or caffeine. Adult cells were longer and wider than preterm cells. Mean cell length in 1.6 mM CaCl2 was 194 +/- 57 (SD) microm (n = 66) for adult cells and 93 +/- 32 microm (n = 20) for preterm cells (P < 0.05). Mean cell width at the widest point of the adult cells was 8.2 +/- 1.8 microm (n = 66) and 5.2 +/- 1.5 microm (n = 20) for preterm cells (P < 0.05). Cells were loaded into a perfusion dish maintained at 35 degreesC and exposed to agonists, and contractions were videotaped. Cell lengths were measured from 30 video frames and plotted as a function of time. Nonlinear fitting of cell length to an exponential model gave shortening velocities faster than most of those reported for airway smooth muscle tissues. For a sample of 10 adult and 10 preterm cells stimulated with 100 microM carbachol, mean (+/- SD) shortening velocity of the preterm cells was not different from that of the adult cells (0.64 +/- 0.30 vs. 0.54 +/- 0.27 s-1, respectively), but preterm cells shortened more than adult cells (68 +/- 12 vs. 55 +/- 11% of starting length, respectively; P < 0.05). The preparative and analytic methods described here are widely applicable to other smooth muscles and will allow contraction to be studied quantitatively at the single-cell level.

Responses of hepatic TNF-alpha mRNA to repeated hemorrhage in the conscious rat

Trauma victims may suffer from repeated hemorrhage, but responses of cytokines to it have not been described. To study this question, we first detected the time course of changes in serum tumor necrosis factor (TNF) activity and hepatic TNF-alpha mRNA by cytotoxicity against L929 cells and by reverse transcription (RT) and polymerase chain reaction (PCR), respectively, after different sizes of hemorrhage (10-20 ml/kg) with chronically cannulated rats. Then we examined the changes in TNF-alpha mRNA when two sequential 10 ml/kg hemorrhages were performed. TNF activity showed no significant increases after either size of hemorrhage. At mRNA level, both 15 ml/kg and 20 ml/kg hemorrhages induced significant increases after hemorrhage, whereas a 10 ml/kg hemorrhage did not. When the 10 ml/kg hemorrhage was repeated 24 h later, however, TNF-alpha mRNA showed a significant increase. There were no significant differences in blood pressure and heart rate after single and repeated 10 ml/kg hemorrhage. This potentiation persisted for >/=48 h. These results show that responses of hepatic TNF-alpha mRNA are augmented when moderate hemorrhage is repeated.

Effect of gender and sex steroids on the contractile response of canine coronary and renal blood vessels

The effect of gender, gonadal steroids, and antiandrogen/antiestrogen-treatment on the isotonic response of isolated preparations of the left anterior descending coronary artery (LAD), left circumflex coronary artery, and renal artery and vein of sexually mature dogs was investigated. The maximum isotonic response of the coronary and renal vasculature to the thromboxane A2 (TXA2)-mimetic U46619 was significantly greater, and the EC50 value was significantly lower in males as compared with females. Moreover, similar gender differences in the contractile response of the coronary vasculature to norepinephrine were observed. Pretreatment of male dogs with the antiandrogens flutamide or cyproterone acetate reduced the maximum contractile response of the LAD to the TXA2-mimetic. Pretreatment of female dogs with testosterone resulted in an increase in both the maximum contractile response and EC50 value to U46619. Antiestrogen treatment of female dogs with tamoxifen was associated with an increase in the maximum contractile response of the LAD to U46619. Estrogen pretreatment of male dogs decreased both the maximum contractile response and the EC50 value to U46619. Therefore, there is a sex difference in LAD and LCX contractile responses to both U46619 and norepinephrine. These results suggest that smooth muscle reactivity of dog coronary artery to the TXA2-mimetic U46619 may be susceptible to regulation by both androgens and estrogens. The observed gender differences in the catecholamine response may be similarly altered by changes in the hormonal milieu.

Actin isoform compartments in chicken gizzard smooth muscle cells

Differentiated smooth muscle cells typically contain a mixture of muscle (alpha and gamma) and cytoplasmic (beta and gamma) actin isoforms. Of the cytoplasmic actins the beta-isoform is the more dominant, making up from 10% to 30% of the total actin complement. Employing an antibody raised against the N-terminal peptide specific to beta-actin, which labels only the beta-isoform on two-dimensional gel immunoblots, we have shown that this isoform has a restricted localisation in smooth muscle. Using double-label immunofluorescence and immunoelectron microscopy of ultrathin sections of chicken gizzard, beta-actin was localised in the dense bodies and in longitudinal channels linking consecutive dense bodies that were also occupied by desmin. It was additionally found in the membrane-associated dense plaques, but was excluded from the actomyosin-containing regions of the contractile apparatus. Taken together with earlier results these findings identify a cytoskeletal compartment containing intermediate filaments, cytoplasmic actin and the actin cross-linking protein filamin. Using an antibody specific only for muscle actin, labelling was found generally around the myosin filaments of the contractile apparatus, but was absent from the core of the dense bodies that contained beta-actin. Thus, if dense bodies act as dual-purpose anchorage sites, for the cytoskeletal actin and the contractile actin, the thin filaments of the contractile apparatus must be anchored at the periphery of the dense bodies. A model of the structural organisation of the cell is presented and the possible roles of the cytoskeleton are discussed.

Corticotrope responsiveness to glucocorticoids is modulated via rapid CRF-mediated induction of the proto-oncogene c-fos

Although corticotropin releasing factor (CRF) and glucocorticoid hormones (GC) act directly at the level on the anterior pituitary corticotrope cell to stimulate (CRF) or inhibit (GC) pro-opiomelanocortin (POMC) expression, the actions of GC on POMC have been shown to be impaired if corticotrope cells are coincubated or preincubated with CRF. In the present study we have measured secreted beta-endorphin (beta EP) and changes in the level of nuclear POMC hnRNA as an indirect measure of gene transcription to characterize the molecular mechanisms involved in the CRF-mediated inhibition of glucocorticoid action. In primary cultures of rat anterior pituitary cells either co-treated or pretreated with CRF, acute dexamethasone (DEX)-mediated inhibition of POMC hnRNA levels was impaired. In contrast, the ability of CRF to block glucocorticoid action was abolished if the cells were pretreated with the protein synthesis inhibitor puromycin. Since previous studies have demonstrated that components of the AP1 transcription factor can modulate glucocorticoid receptor activity in other systems, we examined the regulation of the proto-oncogenes c-fos and c-jun in response to CRF. Treatment of the corticotrope cell line (AtT-20) with CRF rapidly activated c-fos mRNA to levels 11-12-fold above control by 30 and 60 min, with no apparent elevation of c-jun mRNA levels. Pretreatment of AtT-20 cells with antisense c-fos oligonucleotides prevented CRF from blocking glucocorticoid inhibition of POMC hnRNA levels and beta EP release.(ABSTRACT TRUNCATED AT 250 WORDS)

A method for isolating smooth muscle cells from pig urinary bladder with low concentrations of collagenase and papain: the relation between calcium concentration and isolated cell length

The present study describes a method for isolating single smooth muscle cells from pig urinary bladder using a continuous resuspension device. Low concentrations of collagenase and papain were sufficient to obtain a high yield of viable smooth muscle cells, which remained viable for about 3-4 h as tested with fluorescein diacetate. Addition of fetal calf serum increased the lifespan of the isolated cells and the percentage of contractile smooth muscle cells, but caused spontaneous shortening of the cells. The length and volume of the isolated smooth muscle cells depended on the calcium concentration used in the isolation buffer solution. The isolated muscle cells were apparently relaxed if a calcium concentration less than 1.0 mmol/l was used in the isolation medium. In higher calcium concentrations the isolated cells were significantly shorter, probably as a result of a contraction caused by mechanical stimulation of the cells during the isolation procedure.

Ordered distribution of membrane-associated dense plaques in intact quail gizzard smooth muscle cells revealed by freeze-fracture following treatment with cholesterol probes

The surface distribution of membrane-associated dense plaques in intact quail gizzard smooth muscle cells was investigated by freeze-fracture. Replicas of fractured smooth muscle cell plasma membrane showed caveola-free regions with few intramembrane particles, interspersed with caveola-populated areas with a higher intramembrane particle density. Electron microscopy of thin sections of quail gizzard smooth muscle revealed the regions free of caveolae to be occupied by membrane-associated dense plaques; anchoring sites for the contractile filaments of the cell. Demarcation between the caveola-populated and caveola-free regions on the relicated intramembrane surface was not clear and thus provided little information concerning the distribution of dense plaque sites. However, treatment of the smooth muscle tissue with the cholesterol-binding agents filipin or tomatin prior to freeze-fracture allowed the dense plaque sites to be easily observed as the sites remained free of the membrane deformations characteristic of these agents. The dense plaque sites consist of caveola-free oval areas juxtaposed in regular bands that traverse the long axis of the cell. The dense plaque sites on the freeze-fracture replica were confirmed by electron microscopy of thin sections of filipin-treated quail gizzard smooth muscle cells, which showed the plasma membrane associated with the dense plaques to be unaffected by the actions of filipin, whereas that of the caveola-populated region was severely deformed. The observations presented in this study provide evidence for a highly ordered distribution of dense plaques at the cell surface of intact quail gizzard smooth muscle cells and thus corroborate existing evidence for an organized substructure of smooth muscle cells.

Patterns of the functioning of the hypophyseoadrenocortical system in the presence of repeated stressful stimulations

The relationship of the magnitude of the response of the hypophyseoadrenocortical system (HACS) to the strength of the stimulus in the presence of isolated or repeated stressor influences was determined in experiments on awake rats. Electrical stimulation of 0.1-0.9 mA of the skin of the paws for 3 min served as the stimulus. The reaction of HACS was judged on the basis of the determination of blood corticosteroids. Changes take place in the physiological characteristics of the system 30 mins after the termination of the stressor activation of the HACS. These changes do not depend on the strength of the initial stressor, and consist of a decrease in the threshold of excitability and a reduction in the maximum of the reaction to stressors of greater strength. The reaction to repeated stimulation at small strengths of the initial stressor proved to be larger or equal to the response of the system to the initial stimulation. At greater strengths of the stressor the reaction to repeated stimulation is significantly less than to the initial stimulation.

Supercontracted state of vertebrate smooth muscle cell fragments reveals myofilament lengths

Isolated cell preparations from chicken gizzard smooth muscle typically contain a mixture of cell fragments and whole cells. Both species are spontaneously permeable and may be preloaded with externally applied phalloidin and antibodies and then induced to contract with Mg ATP. Labeling with antibodies revealed that the cell fragments specifically lacked certain cytoskeletal proteins (vinculin, filamin) and were depleted to various degrees in others (desmin, alpha-actinin). The cell fragments showed a unique mode of supercontraction that involved the protrusion of actin filaments through the cell surface during the terminal phase of shortening. In the presence of dextran, to minimize protein loss, the supercontracted products were star-like in form, comprising long actin bundles radiating in all directions from a central core containing myosin, desmin, and alpha-actinin. It is concluded that supercontraction is facilitated by an effective uncoupling of the contractile apparatus from the cytoskeleton, due to partial degradation of the latter, which allows unhindered sliding of actin over myosin. Homogenization of the cell fragments before or after supercontraction produced linear bipolar dimer structures composed of two oppositely polarized bundles of actin flanking a central bundle of myosin filaments. Actin filaments were shown to extend the whole length of the bundles and their length averaged integral to 4.5 microns. Myosin filaments in the supercontracted dimers averaged 1.6 microns in length. The results, showing for the first time the high actin to myosin filament length ratio in smooth muscle are readily consistent with the slow speed of shortening of this tissue. Other implications of the results are also discussed.

Smooth operators. The molecular mechanics of smooth muscle contraction

Smooth muscle cells squeeze the blood back to your heart, raise the hackles on your neck and change the F-stop of your eyes. The past year has provided penetrating new insights into their mechanism of contraction.

Periodic organization of the contractile apparatus in smooth muscle revealed by the motion of dense bodies in single cells

To study the organization of the contractile apparatus in smooth muscle and its behavior during shortening, the movement of dense bodies in contracting saponin skinned, isolated cells was analyzed from digital images collected at fixed time intervals. These cells were optically lucent so that punctate structures, identified immunocytochemically as dense bodies, were visible in them with the phase contrast microscope. Methods were adapted and developed to track the bodies and to study their relative motion. Analysis of their tracks or trajectories indicated that the bodies did not move passively as cells shortened and that nearby bodies often had similar patterns of motion. Analysis of the relative motion of the bodies indicated that some bodies were structurally linked to one another or constrained so that the distance between them remained relatively constant during contraction. Such bodies tended to fall into laterally oriented, semirigid groups found at approximately 6-microns intervals along the cell axis. Other dense bodies moved rapidly toward one another axially during contraction. Such bodies were often members of separate semirigid groups. This suggests that the semirigid groups of dense bodies in smooth muscle cells may provide a framework for the attachment of the contractile structures to the cytoskeleton and the cell surface and indicates that smooth muscle may be more well-ordered than previously thought. The methods described here for the analysis of the motion of intracellular structures should be directly applicable to the study of motion in other cell types.

Analysis of the birefringence of the smooth muscle anococcygeus of the rat, at rest and in contraction. I

The birefringence of the rat anococcygeus muscle was measured at rest and in contraction. A large increase (+30%) of the optical retardation was observed in muscles fully stimulated by Noradrenaline, in isometric conditions. This was associated with a reduction of the muscle thickness (-12%), so that the birefringence increased by 48%. These changes were reversed upon relaxation. The relationship between the birefringence increase and the mechanical response was investigated as a function of time and of Noradrenaline concentration. Possible origins for the birefringence increase are considered: an increased density of birefringent material, mainly filaments, seems the most likely explanation of the results.

Morphologic characterization of cultured smooth muscle cells isolated from the tracheas of adult dogs

The goals of our study were to isolate smooth muscle cells from the trachealis muscle of adult dogs and to characterize the cells morphologically when they were maintained in primary culture. Enzymatic digestion of the muscle yielded 4.8 +/- 1.8 X 10(6) viable smooth muscle cells per gram of tissue. When placed in culture, these cells rapidly proliferated until confluence was reached. The proliferating cells in culture differed from the cells in the intact tissue in that they stained less intensely for smooth muscle myosin, developed immunofluorescent staining for the intermediate filament protein vimentin, and lost many of the ultrastructural properties of the intact muscle. Only within nodules of cells in the confluent cultures were these ultrastructural properties preserved. Cultures of canine tracheal fibroblasts differed from these smooth muscle cell cultures in that the fibroblasts did not stain for smooth muscle myosin and did not form nodules at confluence. We concluded that adult canine airway smooth muscle cells may be maintained in primary culture, that the confluent cultures contain nodules of cells with many morphologic characteristics of the intact muscle, and that these preparations may be distinguished from cultured canine tracheal fibroblasts on specific morphologic grounds.

Corkscrew-like shortening in single smooth muscle cells

The slower and more economical contraction of smooth muscle as compared to that of skeletal muscle may relate to the arrangement of its contractile apparatus. Because the arrangement of the contractile apparatus determines the manner in which a single smooth muscle cell shortens, shortening of a contracting cell was examined by tracking of marker bead movements on the cell surface by means of digital video microscopy. Smooth muscle cells were observed to freely shorten in a unique corkscrew-like fashion with a pitch of 1.4 cell lengths (that is, the length change required for one complete rotation of cell) at a rate of 27 degrees per second. Corkscrew-like shortening was interpreted in terms of a structural model in which the contractile apparatus or cytoskeleton (or both) are helically oriented within the cell. Such an arrangement of these cytoarchitectural elements may help to explain in part the contractile capabilities of smooth muscle.

Ion transport and electrophysiology of the early proximal colon of rabbit

The ion transport properties of the mammalian descending colon have been the subject of numerous investigations during the last decade. In contrast, relatively few studies have investigated proximal segments of this organ. In the present study, we assessed transepithelial transport of Na+, K+ and Cl- in the isolated initial segment (P1) of rabbit colon in vitro using radioisotopic tracer fluxes and electrophysiological techniques. Like the rabbit descending colon, the proximal colon actively absorbs sodium and chloride, however, its transport systems are markedly different. In vivo, this segment absorbs potassium, however in vitro active potassium secretion was observed. Unlike the descending colon, Na+ absorption is relatively insensitive to amiloride and only a slight inhibition was obtained even at 1 mM concentrations of this drug. Na+ and Cl- absorption appeared to be coupled (directly or indirectly) since the absorption of each ion was inhibited by the removal of the other. Serosal ouabain also inhibited Na+ and Cl- absorption and net K+ secretion. Unlike the descending colon, the proximal P1 segment did not have a net absorptive K+ transport system that was detectable in the presence of ouabain. Electrically, the early proximal colon has a low transepithelial resistance compared to descending colon (RT = 133 +/- 7 omega cm2) but a larger short-circuit current (Isc = 178 +/- 12 microA/cm2). The transepithelial potential averaged -21 +/- 1 mV, in excellent agreement with values measured in vivo. The apical and basolateral membrane potentials averaged -21 +/- 1 mV and -42 +/- 1 mV and intracellular potassium activity was 70 +/- 2 mM.(ABSTRACT TRUNCATED AT 250 WORDS)

Identification and characterization of major ionic currents in isolated smooth muscle cells using the voltage-clamp technique

Voltage-clamp experiments were carried out on freshly dissociated single vertebrate smooth muscle cells from the stomach muscularis of Bufo marinus. Conventional two-microelectrode methodology was used, thus avoiding rapid dialysis of the cytosol. Four major phases of current were identified upon voltage jumps from negative holding levels to more positive levels. The first phase of current was an initial, inward current. This current was blocked by external Mn2+ and was of the correct magnitude to account for the rising phase of the Ca2+-dependent, TTX-independent action potentials found in these cells. Following this initial, inward Ca2+ current, a large outward current was observed which reached its peak over a period of hundreds of milliseconds and then decayed over a period of seconds to a steady-state level. The peak outward current and the steady-state outward current constitute the second and third major currents. The peak outward current was the largest current observed, with a magnitude as large as tens of nanoamps whereas the inward current was at most about one nanoamp. The peak outward current was reduced more than tenfold in the presence of external TEA. It was also decreased or abolished when the preceding inward current was diminished or eliminated by using external Mn2+ or less negative holding potentials. In this way the peak outward current was identified as a Ca2+-activated K+ current whose slow decay was hypothesized to result from removal of internal Ca ions by cellular mechanisms following the initial rise in [Ca2+]i resulting from the inward current. A fourth major current was an early transient outward current observed most clearly upon voltage jumps to more positive potentials when the inward current was eliminated by using less negative holding potentials or external Mn2+. A classical steady-state inactivation relationship as a function of membrane potential was constructed for the inward current. A substantial portion of this inactivation curve lies at potentials negative to the apparent threshold for activation of inward current, suggesting a true voltage-dependent inactivation. Although additional Ca2+-dependent inactivation could not be ruled out, neither could evidence for it be found.

Electrical PD, short-circuit current and fluxes of Na and Cl across avian intestine

In vitro measurements were made of transmural potential difference (PD), short-circuit current (Isc), resistance and unidirectional fluxes of 22Na and 36Cl across the duodenum, jejunum, ileum and colon of normal sodium-replete domestic fowl (Gallus domesticus). The PD ranged from about 1 mV across the duodenum to 8 mV across the colon while the Isc was, respectively, 2.8 and 64 microA X cm-2. The jejunum and ileum exhibited values between these extremes. Unidirectional fluxes (under short-circuit conditions) of Na and Cl were lowest across the duodenum where there was no evidence of active transport of these ions. Unidirectional fluxes of Na and Cl were less across the jejunum than across the ileum or colon. A net active transport of Na (but not Cl) was observed in the ileum (= 106% of the Isc) and colon (= 50% of Isc). The possible physiological significance of these observations in the domestic fowl are discussed and are compared to that of a mammal, the rabbit.

Coordination of mural elements and myofilaments during arteriolar constriction

Arterioles undergo major morphological changes during vasoconstriction. We used transmission electron microscopy to study wall morphology in both dilated and constricted microvessels to understand the cellular basis of these changes. The relation between the orientation and density of myofilaments and the distribution of dense bodies was analyzed with respect to the level of microvessel tone. The data show a strong correlation between the degree of arteriolar constriction and both the orientation and density of myofilaments. In dilated arterioles, myofilament orientation was predominantly circumferential across the entire smooth muscle cell, averaging 84 +/- 2 degrees (SEM) relative to a radial reference line. In vessels constricted to 50% of their maximal diameter, myofilament orientation was dependent upon the location within the cell, being largely circumferential at the adventitial border (77 +/- 4 degrees) and shifting to a radial arrangement at the intimal border (36 +/- 5 degrees). The reorganization of myofilaments during constriction was associated with a decrease in myofilament density at the intimal-medial border of the smooth muscle cells. The decrease in myofilament density resulted from a selective withdrawal of myofilaments from periluminal areas where "ridges" had formed. Our observations suggest that an ordered distribution of membrane-associated dense bodies along the periluminal aspect of the smooth muscle cells is responsible for both the myofilament reorganization and ridge formation during vasoconstriction. Results of the present study are incorporated into a hypothetical model of arteriolar ultrastructure compatible with the mural reorganization observed during vasoconstriction.

Large-conductance Ca2+-activated K+ channels in freshly dissociated smooth muscle cells

Freshly dissociated cells from the stomach muscularis of the toad Bufo marinus have been employed to carry out a systematic set of electrophysiological studies on the membrane properties of smooth muscle. The existence of Ca2+-activated K+ channels became apparent during the first studies under current clamp. In subsequent studies under voltage clamp, a Ca2+-activated. TEA-sensitive outward current was evident, and it was more than an order of magnitude larger than any other current observed in the cells. The channel responsible, at least in part, for this large outward current has been identified on the basis of single-channel records, and some of its main characteristics have been studied. It is similar in many respects to the large-conductance, Ca2+-activated K+ channel seen in other preparations. This channel has now been found in a considerable diversity of smooth muscle types.

Action potentials and net membrane currents of isolated smooth muscle cells (urinary bladder of the guinea-pig)

Cells were isolated by incubating chunks of tissue from the urinary bladder of the guinea-pig in a high potassium, low chloride medium containing 0.2 mM calcium plus the enzymes collagenase and pronase. After isolation, the cells were superfused with a physiological salt solution (PSS) containing 150 mM NaCl, 3.6 mM CaCl2 and 5.4 mM KCl (35 degrees C). Patch electrodes filled with an isotonic KCl-solution were used for whole cell recordings. With a single electrode voltage clamp we measured a capacitance of 50 +/- 5 pF per cell, an input resistance of 200 +/- 25 kOhm X cm2 and a series resistance of 44 +/- 4 Ohm X cm2. The cells had resting potentials of -52 +/- 2 mV. They did not beat spontaneously but responded to stimuli with single action potentials (APs) which rose from the threshold (-38 mV) with a maximal rate of 6.5 +/- 1.8 V/s to an overshoot of 22 +/- 3 mV. The AP lasted for 36 +/- 4 ms (measured between threshold and -40 mV). Continuous cathodal current produced repetitive activity, a pacemaker depolarization followed the AP and preceded the next upstroke. Net membrane currents evoked by clamp steps to positive potentials were composed of an inward and an outward component. The inward component generating the upstroke of the AP was carried by Ca ions (iCa, Klöckner and Isenberg 1985). The repolarization resulted from a potassium outward current iK. Ca-channel blockers (5 mM NiCl2) reduced iK suggesting that (part of) iK was Ca-activated. iK rose within about 100 ms to a peak of 40-200 muA/cm2 from which it inactivated slowly and incompletely. The inactivating iK followed a bell-shaped voltage-dependence, the noninactivating iK an outwardly rectifying one. Both parts had similar steady state inactivation curves with a half maximal inactivation potential at -36 mV and a slope of 9 mV. Repolarization to -50 mV induced outward tail currents which reversed polarity at -85 mV (the calculated potassium equilibrium potential). The amplitude and the time course of the envelope of the tail currents varied in proportion to iK during the prestep. Thus, the tail current is suggested to reflect the turning off of a potassium conductance which had been activated during the prepulse. iK was largely reduced but not blocked by 20 or 150 mM tetraethylammonium (TEA). TEA did not significantly change the resting potential, but it prolonged the AP and facilitated upstroke and overshoot.(ABSTRACT TRUNCATED AT 400 WORDS)

Regional differences in electrolyte, short-chain fatty acid and water absorption in the hindgut of two species of arboreal marsupials

Short-chain fatty acid, electrolyte and water absorption from the hindgut of two arboreal marsupial species, the greater glider (Petauroides volans) and the brush-tail possum (Trichosurus vulpecula) were studied in vivo using a single perfusion technique. Qualitative and quantitative differences in the net movement of sodium, potassium and chloride were found between the different hindgut segments and between the two species. ALl transport processes exhibited active characteristics. Net Na+ transport in all segments was concentration-dependent in the range of 45-135 mmol . 1(-1) Na+. The proximal colon of the greater glider showed a net Na+, Cl- and water secretion and K+ absorption, all electrolyte movements being against the electrochemical gradient. Water followed passively the osmotic gradient generated mainly by the net movement of Na+. Short-chain fatty acids were absorbed according to their chain length in a constant ratio of 1.0:1.2:1.3 for acetate, propionate and butyrate, respectively. Our data indicate that absorptive and secretory processes in the hindgut of these marsupials are basically similar to those of eutherians, even in epithelia differing significantly in the direction of net solute transport.

Preliminary pharmacological characterization of the isolated circular strips of gastric muscularis muscle of Bufo marinus: a new preparation

This paper describes a preliminary pharmacological study of isolated circular strips of gastric muscularis muscle of Bufo marinus under isometric conditions. The stomach of Bufo marinus was surgically removed and the muscularis muscle was isolated and divided into 12 equal parts from the fundus to the pylorus. The preparation contracted to acetylcholine (ACh), DL-muscarine, and K+. Histamine and 5-hydroxytryptamine had no effect, while dopamine caused relaxation. Prostaglandin E2 (PGE2) caused relaxation of the tonic contractions and potentiation of spontaneous contractions. A challenge of the preparation with ACh (1.5 X 10(-5) M) initiated tonic and spontaneous contractions, of which three types were identified. These were: type A (predominantly tonic contractions on strip #1); type B (tonic contractions over which spontaneous contractions were superimposed on strips #2-8; and type C (mostly tonic contractions and some superimposed spontaneous contractions on strips #9-12). Lower concentrations of ACh (1.0 X 10(-6)M) elicited tonic contractions, while higher concentrations of ACh (3.0 X 10(-5)M) were necessary for the initiation of tonic and spontaneous contractions in type B preparations.

Distribution of alpha-actinin in single isolated smooth muscle cells

In order to probe the organization of the contractile machinery in smooth muscle, we have studied the distribution of alpha-actinin, a protein present in high concentration in dense bodies, structures apparently analogous to the Z-disks of striated muscle. Localization of alpha-actinin in single isolated smooth muscle cells of the stomach muscularis of Bufo marinus was determined by analysis of the pattern of anti-alpha-actinin staining in single fluorescence photomicrographs, stereo pair micrographs, and computerized three-dimensional reconstructions from multiple image planes. The distribution of anti-alpha-actinin and antitubulin staining was compared in contracted and relaxed cells. The studies revealed that alpha-actinin is present in high concentrations in fusiform elements (mean axial ratio = 4.82) throughout the cytoplasm and in larger, more irregularly shaped plaques along the cell margins. Many of the fusiform-stained elements are organized into stringlike arrays characterized by a regular repeating pattern (mean center-to-center interspace = 2.2 +/- 0.1 micron). These linear arrays appear to terminate at the anti-alpha-actinin stained larger plaques along the cell margin; several of these strings often run in parallel with their elements in lateral register. While this general pattern of organization is maintained in cells during contraction, the distance between successive stained elements in stringlike arrays is decreased. We suggest that the decrease in the distance between elements in these strings results from shortening of materials that constitute these linear arrays. We do not believe that the shortening within these arrays reflects compression by forces generated elsewhere within the cell, as the reorganization of noncontractile microtubules is qualitatively different from the changes in the pattern of anti-alpha-actinin staining.

Active potassium transport by rabbit descending colon epithelium

Previous studies of rabbit descending colon have disagreed concerning potassium transport across this epithelium. Some authors reported active K+ secretion under in vitro short-circuited conditions, while others suggested that K+ transport occurs by passive diffusion through a highly potassium-selective paracellular route. For this reason, we re-examined potassium fluxes across the colon in the presence of specific and general metabolic inhibitors. In addition, electrochemical driving forces for potassium across the apical and basolateral membranes were measured using conventional and ion-sensitive microelectrodes. Under normal conditions a significant net K+ secretion was observed (JK net = -0.39 +/- 0.081 mueq/cm2hr) with 42K fluxes, usually reaching steady-state within approximately 50 min following isotope addition. In colons treated with serosal addition of 10(-4)M ouabain, JKsm was lowered by nearly 70% and JKms was elevated by approximately 50%. Thus a small but significant net absorption was present (JKnet = 0.12 +/- 0.027 mueq/cm2hr). Under control conditions, the net cellular electrochemical driving force for K+ was 17mV, favoring K+ exit from the cell. Cell potential measurements indicated that potassium remained above equilibrium after ouabain, assuming that passive membrane permeabilities are not altered by this drug. Net K+ fluxes were abolished by low temperature. The results indicate that potassium transport by the colon may occur via transcellular mechanisms and is not solely restricted to a paracellular pathway. These findings are consistent with our previous electrical results which indicated a nonselective paracellular pathway. Thus potassium transport across the colon can be modeled as a paracellular shunt pathway in parallel with pump-leak systems on the apical and basolateral membranes.

Noise analysis reveals K+ channel conductance fluctuations in the apical membrane of rabbit colon

In this paper we describe current fluctuations in the mammalian epithelium, rabbit descending colon. Pieces of isolated colon epithelium bathed in Na+ or K+ Ringer's solutions were studied under short-circuit conditions with the current noise spectra recorded over the range of 1-200 Hz. When the epithelium was bathed on both sides with Na+ Ringer's solution (the mucosal solution contained 50 microM amiloride), no Lorentzian components were found in the power spectrum. After imposition of a potassium gradient across the epithelium by replacement of the mucosal solution by K+ Ringer's (containing 50 microM amiloride), a Lorentzian component appeared with an average corner frequency, fc = 15.6 +/- 0.91 Hz and a mean plateau value So = (7.04 +/- 2.94) x 10(-20) A2 sec/cm2. The Lorentzian component was enhanced by voltage clamping the colon in a direction favorable for K+ entry across the apical membrane. Elimination of the K+ gradient by bathing the colon on both sides with K+ Ringer's solutions abolished the noise signal. The Lorentzian component was also depressed by mucosal addition of Cs+ or tetraethylammonium (TEA) and by serosal addition of Ba2+. The one-sided action of these K+ channel blockers suggests a cellular location for the fluctuating channels. Addition of nystatin to the mucosal solution abolished the Lorentzian component. Serosal nystatin did not affect the Lorentzian noise. This finding indicates an apical membrane location for the fluctuating channels. The data were similar in some respects to K+ channel fluctuations recorded from the apical membranes of amphibian epithelia such as the frog skin and toad gallbladder. The results are relevant to recent reports concerning transcellular potassium secretion in the colon and indicate that the colon possesses spontaneously fluctuating potassium channels in its apical membranes in parallel to the Na+ transport pathway.

Effects of secretagogues on the K+ permeability of mucosal and serosal borders of rabbit colonic mucosa

(1) K+ efflux rates from the mucosal and serosal surfaces of sheets of rabbit colonic mucosa have been determined by measuring net K+ loss into K+-free Ringer solution bathing each side of the tissue. (2) Initially, there is a high rate of K+ loss from the tissue, this falls to a lower steady-state rate after 20 min. Loss of K+ from the tissue into the serosal bath is 6-8 fold faster than loss to the mucosal bath. (3) A number of intestinal secretagogues, e.g. theophylline, cyclic AMP, carbachol, ionophore A23187, as well as the laxative bisacodyl, raise the K+ efflux rate across the mucosal border by 200-300%. In the case of K+ efflux induced by carbachol the effect is shown to be dependent on raised levels of intracellular Ca2+. Ca2+-calmodulin complex does not appear to be be involved in activation of K+ efflux across the mucosal border. (4) Amiloride does not block mucosal K+ efflux, but tetraethyl-ammonium does inhibit K+ efflux across the mucosal border, induced by either bisacodyl or raised intracellular Ca2+. (5) The results suggest that laxatives may increase the rate of K+ secretion into the colonic lumen by raising the K+ permeability of the mucosal border.