On the mechanisms whereby temperature affects excitation-contraction coupling in smooth muscle

Aqueous Humor Outflow Requires Active Cellular Metabolism in Mice

Purpose

Conventional wisdom posits that aqueous humor leaves the eye by passive bulk flow without involving energy-dependent processes. However, recent studies have shown that active processes, such as cell contractility, contribute to outflow regulation. Here, we examine whether inhibiting cellular metabolism affects outflow facility in mice.

Methods

We measured outflow facility in paired enucleated eyes from C57BL/6J mice using iPerfusion. We had three Experimental Sets: ES1, perfused at 35°C versus 22°C; ES2, perfused with metabolic inhibitors versus vehicle at 35°C; and ES3, perfused at 35°C versus 22°C in the presence of metabolic inhibitors. Inhibitors targeted glycolysis and oxidative phosphorylation (2-deoxy-D-glucose, 3PO and sodium azide). We also measured adenosine triphosphate (ATP) levels in separate murine anterior segments treated like ES1 and ES2.

Results

Reducing temperature decreased facility by 63% [38%, 78%] (mean [95% confidence interval (CI)], n = 10 pairs; P = 0.002) in ES1 after correcting for changes in viscosity. Metabolic inhibitors reduced facility by 21% [9%, 31%] (n = 9, P = 0.006) in ES2. In the presence of inhibitors, temperature reduction decreased facility by 44% [29%, 56%] (n = 8, P < 0.001) in ES3. Metabolic inhibitors reduced anterior segment adenosine triphosphate (ATP) levels by 90% [83%, 97%] (n = 5, P<<0.001), but reducing temperature did not affect ATP.

Conclusions

Inhibiting cellular metabolism decreases outflow facility within minutes. This implies that outflow is not entirely passive, but depends partly on energy-dependent cellular processes, at least in mice. This study also suggests that there is a yet unidentified mechanism, which is strongly temperature-dependent but metabolism-independent, that is necessary for nearly half of normal outflow function in mice.

Lowering Perfusate Temperature From 37°C to 32°C Diminishes Function in a Porcine Model of Ex Vivo Kidney Perfusion

BACKGROUND

Ex vivo perfusion (EVP) is a novel method of preservation. However, optimal perfusion conditions remain undetermined. Reducing the temperature of the perfusate to subnormothermia may be beneficial during EVP and improve early graft function. The aim of this study was to investigate whether subnormothermia would influence the conditioning effect of EVP when compared with normothermic perfusion, and standard cold static storage (CS).

METHODS

Porcine kidneys underwent static CS for 23 hours followed by 1 hour of EVP using leukocyte-depleted blood at a mean temperature of 32°C or 37°C. After this, kidneys were reperfused with whole autologous blood at 37°C for 3 hours to assess renal function and injury. These were compared with a control group that underwent 24 hours CS.

RESULTS

During EVP, kidneys perfused at 37°C had a higher level of renal blood flow and oxygen consumption compared with EVP at 32°C (P = 0.001, 0.002). During reperfusion, 32°C EVP kidneys had lower creatinine clearance and urine output than control (P = 0.023, 0.011) and a higher fractional excretion of sodium, serum potassium, and serum aspartate transaminase than 37°C EVP kidneys (P = 0.01, 0.023, 0.009).

CONCLUSIONS

Tubular and renal functions were better preserved by a near-physiological temperature of 37°C during 1 hour of EVP, when compared to EVP at 32°C or cold storage.

Measurement of smooth muscle function in the isolated tissue bath-applications to pharmacology research

Isolated tissue bath assays are a classical pharmacological tool for evaluating concentration-response relationships in a myriad of contractile tissues. While this technique has been implemented for over 100 years, the versatility, simplicity and reproducibility of this assay helps it to remain an indispensable tool for pharmacologists and physiologists alike. Tissue bath systems are available in a wide array of shapes and sizes, allowing a scientist to evaluate samples as small as murine mesenteric arteries and as large as porcine ileum - if not larger. Central to the isolated tissue bath assay is the ability to measure concentration-dependent changes to isometric contraction, and how the efficacy and potency of contractile agonists can be manipulated by increasing concentrations of antagonists or inhibitors. Even though the general principles remain relatively similar, recent technological advances allow even more versatility to the tissue bath assay by incorporating computer-based data recording and analysis software. This video will demonstrate the function of the isolated tissue bath to measure the isometric contraction of an isolated smooth muscle (in this case rat thoracic aorta rings), and share the types of knowledge that can be created with this technique. Included are detailed descriptions of aortic tissue dissection and preparation, placement of aortic rings in the tissue bath and proper tissue equilibration prior to experimentation, tests of tissue viability, experimental design and implementation, and data quantitation. Aorta will be connected to isometric force transducers, the data from which will be captured using a commercially available analog-to-digital converter and bridge amplifier specifically designed for use in these experiments. The accompanying software to this system will be used to visualize the experiment and analyze captured data.

Differential effect of hyperthermia on nerves and smooth muscle of the mouse ileum

BACKGROUND AND OBJECTIVES

Cytoreductive surgery (CRS) and hyperthermic intraperitoneal chemotherapy (HIPEC) gained wide acceptance as the treatment of choice for selected patients with peritoneal surface malignancies. Patients tend to suffer from prolonged ileus following CRS + HIPEC, complicating their recovery. We studied the effects of hyperthermia on the intestine to gain insight into mechanisms of ileus post-HIPEC.

METHODS

Segments of mouse ileum were incubated at 36°C. Electrical field stimulation (EFS) was applied, stimulating nerves, and the resultant muscle contraction was measured. The response was measured at varying temperatures (38-43°C) at exposure times of up to 120 min. We also stimulated the tissues with 10(-6) M carbachol, a muscarinic receptor agonist, which acts directly on smooth muscle.

RESULTS

Response to EFS decreased at high temperatures, especially above 41°C. This effect was irreversible for 120 min after decreasing temperature. When stimulating with carbachol, both transient and plateau responses decreased at 43°C (plateau more than transient) but the effect reversed on returning to 36°C.

CONCLUSION

The irreversible decline in responses to nerve stimulation when exposed to high temperatures was not seen with direct muscle stimulation. This indicates that smooth muscle is resilient and that the main effect of hyperthermia is on nerves. These results have significance for HIPEC.

Temporal and spatial variations in spontaneous Ca events and mechanical activity in pregnant rat myometrium

OBJECTIVES

The aim of this study was to investigate the temporal and spatial characteristics of spontaneous Ca signals in pregnant rat myometrium.

STUDY DESIGN

Confocal imaging of longitudinal strips of 21-day pregnant rats loaded with the Ca sensitive indicator Fluo-4, was combined with measurements of mechanical activity in uterine smooth muscle cells, in situ and freshly isolated.

RESULTS

Our results show that the Ca transients in pregnant uterine tissue are composed of Ca spikes, which are associated with the spike-like action potentials. There is large variation in the pattern of spontaneous activity in myometrium, ranging from non-propagating Ca spikes confined to individual smooth muscle cells, through to regional and global propagating Ca spikes. Irrespective of the pattern of activity displayed, the Ca signals were always in the form of Ca spikes, singularly or in bursts. These Ca spikes did not show fixed initiations sites, propagated in longitudinal and transverse directions from the initiation regions, and had a variable pattern of propagation in preparations which were not synchronously active. In preparations which showed synchronous activity, Ca spikes singularly or bursts propagated mainly in the transverse direction from the initiation regions. The amplitude of force generated by single spikes was dependent on the number of bundles recruited by the propagating Ca spike within the strip, and was about 30-40% of the maximal force produced by carbachol or high-K stimulation. If Ca spikes appeared in the form of bursts they generated longer lasting fused contractions, the amplitudes of which were dependent on the number and the frequency of Ca spikes in the burst.

CONCLUSIONS

Longitudinal myometrium from pregnant rats generates spontaneous Ca spikes which vary in their initiation sites, spatial spread and frequency and are associated with the spike-like action potentials. They are sensitive to the L-type Ca channel blocker, nifedipine. Contractile activity was dependent on the spatial spread of individual Ca spikes and when fully synchronized, produced single submaximal phasic contraction. The number and frequency of bursts of Ca spikes controlled the amplitude and duration of contraction.

In Situ Calcium Signaling: No Calcium Sparks Detected in Rat Myometrium

Controlled uterine smooth muscle activity is essential for our reproductive health. While we understand reasonably well the steps that produce contraction following a rise in intracellular [Ca], the mechanism controlling excitability and thus the rise of Ca, is less well understood. Here we examine the role of the internal Ca sore, the sarcoplasmic reticulum (SR), and its relation to surface membrane ion channels. We show that despite having a well-developed SR, the rat uterus does not produce the elemental and local Ca signals, known as Ca sparks. This in turn has consequences for excitability, as the negative feedback loop between these Ca signals and Ca-activated K (BK) channels on the surface membrane is lost. This may be important for producing the powerful long-lasting contractions of the uterus required during labor.

Effects of temperature on pacemaker potentials in the mouse small intestine

The effects of temperature on the generation of pacemaker potentials recorded from myenteric interstitial cells of Cajal (ICC-MY) distributed in the mouse small intestine were investigated using intracellular recording techniques. In response to increasing temperatures in the range of 26-40 degrees C, the frequency and maximum rate of rise (dV/dt (max)) of pacemaker potentials were increased while their duration was decreased. The resting membrane potential and amplitude of the pacemaker potentials were not affected by change in temperature. Elevation of temperature decreased the amplitude, duration, and rise time of unitary potentials generated spontaneously during intervals between the pacemaker potentials. Metabolic inhibition (KCN and iodoacetic acid) decreased the frequency of pacemaker potentials with no alteration to the amplitude and dV/dt (max). Cyclopiazonic acid (3 muM), an inhibitor of the internal Ca(2+) pump, abolished pacemaker potentials in low-temperature conditions (<29 degrees C) but not at high-temperature conditions (>38 degrees C). These results suggest that the primary and plateau components of pacemaker potentials have different temperature sensitivities: the primary component is highly temperature-sensitive and is activated at higher temperatures, while the plateau component is formed by activation of temperature-insensitive mechanisms. The results also suggest that the mitochondria-induced intracellular Ca(2+) handling system seems to be involved in the initiation of the generation of pacemaker potentials but not in their configuration.

Temperature Changes Stimulate Contraction in the Human Radial Artery and Affect Response to Vasoconstrictors

BACKGROUND

Radial artery conduits are increasingly used in coronary artery bypass grafting as an additional arterial graft to the internal thoracic artery. Their reactive nature remains a concern, often necessitating the routine use of topically applied vasodilators, such as glyceryl trinitrate, papaverine, phenoxybenzamine, or calcium channel antagonists, in theatre. During preparation prior to surgery and grafting, radial artery conduits are exposed to cooling and rewarming. We investigated how these temperature changes would affect radial artery contractility and how commonly used topical treatments might be used to prevent this.

METHODS

Human radial artery was obtained excess to surgery and arterial sections used in organ bath tension experiments or for the culture of smooth muscle cells from medial explants.

RESULTS

The radial artery responded to rapid cooling by the addition of 22 degrees C buffer with contraction. Gradual cooling, over a 20 to 30 minute period, reduced basal tension and the response to potassium chloride (KCl) and noradrenaline. Subsequent rewarming from 22 degrees C to 37 degrees C reestablished contraction at precooled levels and led to an elevation of the basal tension. Increases in tension measured in the radial artery were paralleled by increases in intracellular calcium in smooth muscle cells. Contraction induced by rapid temperature changes could be blocked by glyceryl trinitrate but not by phenoxybenzamine. Papaverine and calcium channel blockers had only limited activity.

CONCLUSIONS

Temperature changes commonly encountered in theatre during the preparation of radial artery grafts are likely to cause contraction. If rapid temperature change cannot be avoided during graft preparation, then topically applied glyceryl trinitrate will block these responses.

Airway smooth muscle dysfunction precedes teratogenic congenital diaphragmatic hernia and may contribute to hypoplastic lung morphogenesis

Fetal intervention aims to improve lung growth and survival in congenital diaphragmatic hernia (CDH). Airway smooth muscle (ASM) is important in lung development: ASM progenitors produce a key growth factor for lung morphogenesis (fibroblast growth factor 10); ASM contractility is also coupled to growth. ASM hyperreactivity occurs in postnatal CDH and may exacerbate barotrauma via impaired lung compliance. We hypothesize that ASM hyperreactivity and its sequelae are based on an early developmental lesion of ASM activity in hypoplastic lung. Sprague-Dawley rats were fed 100 mg nitrofen on Day 9.5 of pregnancy to induce lung hypoplasia in offspring (controls had vehicle alone). Normal and hypoplastic lung primordia were cultured from Day 13.5 of gestation at 37 degrees C in 5% CO(2) and loaded at 54 or 78 h with Ca(2+)-sensitive indicators: Fluo-4 for confocal imaging and Indo-1 or Fura-2 for photometric measurements of [Ca(2+)](i). Hypoplastic lung features spontaneous propagating ASM Ca(2+) transients with reduced frequency, increased amplitude, and significantly prolonged plateau duration, relative to control lung. Nonetheless, hypoplastic lung exhibits normal requirement for extracellular calcium entry and intracellular calcium release in initiation and regulation of ASM Ca(2+) waves. Early ASM dysfunction in lung hypoplasia is apparent as specific anomalies of Ca(2+) transients that indicate a problem with plasmalemmal ion channels/action potential generation. Elucidation of such an ASM lesion may allow pharmacologic amelioration not only of ASM hyperreactivity and its sequelae, but also of hypoplastic lung growth itself.

Spontaneous Propagating Calcium Waves Underpin Airway Peristalsis in Embryonic Rat Lung

Prenatal airways from diverse species exhibit spontaneous peristaltic contractions (airway peristalsis). These contractile waves appear coupled to and may function to regulate prenatal lung growth. They are unaffected by atropine or tetrodotoxin but abolished by nifedipine. Nevertheless, the mechanisms by which these contractile waves are generated, regulated, and propagated remain obscure. Using calcium imaging and whole embryonic lung organ culture, we demonstrate for the first time that peristalsis of the embryonic airway is driven by spontaneous, regenerative, temperature-sensitive calcium (Ca2+) waves. These Ca2+ waves propagate between individual airway smooth muscle cells coupled via gap junctions, are likely to be action potential-mediated, and are dependent on not only extracellular calcium entry via L-type voltage-gated channels but also intracellular Ca2+ stores. Thus, if airway peristalsis regulates lung growth, these findings mean that airway smooth muscle Ca2+ waves in turn regulate prenatal lung morphogenesis.

Intracavernosal Irrigation by Cold Saline as a Simple Method of Treating Iatrogenic Prolonged Erection

AIMS AND METHODS

The aim of this study was to evaluate the efficacy of aspiration and irrigation of the corpora cavernosa with cold saline as a simple outpatient method for treating prolonged penile erection after intracavernous injection of vasoactive agents.

RESULTS

Aspiration and irrigation was needed in 70 out of 122 cases with iatrogenic priapism in whom cooling of the penis and perineum failed to achieve detumescence. According to the temperature of the saline used, patients were randomized into four different groups: A, B, C, and D with a saline temperature 10, 15, 20, and 37 degrees C, respectively.

MAIN OUTCOME

We used a significantly larger volume of saline in groups C and D compared to group A. On the other hand there was no significant difference in the volume of saline used between groups A and B. Complete detumescence was achieved in 24/25 (96%) of cases in group A compared to 9/15 (60%) of cases in group D.

CONCLUSION

We recommend corporal aspiration and irrigation with 10 degrees C saline for patients with prolonged penile erection who failed to respond to the noninvasive measures using ice-cold compresses and physical exercise.

Augmentation of α1-Adrenoceptor-Mediated Contraction by Warming Without Increased Phosphorylation of Myosin in Rat Caudal Arterial Smooth Muscle

We previously reported the relationship between alpha1-adrenoceptor-mediated contraction and phosphorylation of 20-kDa myosin light chain (LC20) in de-endothelialized rat caudal arterial smooth muscle at room temperature (Mita M, Walsh MP. Biochem J. 1997;327:669-674). We now describe the effect of increasing the temperature to 37 degrees C on this relationship. The EC50 value (76.6 +/- 18.2 nM) for cirazoline (alpha1-adrenergic agonist)-induced contraction of the strips at room temperature (23 degrees C) was significantly greater than that (14.5 +/- 1.9 nM) at 37 degrees C. The initial rate of the contraction to a sub-maximal concentration of cirazoline (0.3 microM) was similar at the two temperatures. However, cirazoline-induced maximal force at 37 degrees C was approximately 1.8 times that at room temperature. LC20 phosphorylation in response to cirazoline at room temperature and 37 degrees C closely matched the time courses of contraction, but values were not significantly different at the two temperatures: resting phosphorylation levels were 0.09 +/- 0.04 mol P(i)/mol LC20 at 37 degrees C and 0.22 +/- 0.06 mol P(i)/mol LC20 at room temperature; maximal cirazoline-stimulated LC20 phosphorylation levels were 0.58 +/- 0.08 mol P(i)/mol LC20 at room temperature and 0.49 +/- 0.05 mol P(i)/mol LC20 at 37 degrees C. We conclude, therefore, that the enhanced cirazoline-induced contraction at 37 degrees C is not due to increased LC20 phosphorylation.

Role of ERK1/2 in uterine contractility and preterm labor in rats

The present study tested the hypothesis that ERK activation is an essential step in the onset of labor in a rat model of preterm labor. The administration of RU-486, an antiprogesterone agent, to rats induced preterm delivery 22.2 +/- 0.24 h after treatment. Changes in basal signaling events were studied in myometrial tissue from CO(2)-euthanized rats. Rats treated with RU-486 displayed a dramatically increased in vitro uterine contractility compared with gestational stage-matched, sham-treated rats. In vitro contractility was not significantly different from that during spontaneous labor. During RU-486-induced preterm labor, as previously described for spontaneous labor, ERK phosphorylation levels increased, as did phosphorylation of caldesmon at Ser(789), an ERK phosphorylation site. Also, a small but significant increase in 20-kDa myosin light chain phosphorylation was seen at a constant intracellular pCa of 7. When rats were chronically treated with an agent that prevents ERK activation, U-0126, the onset of RU-486-induced preterm labor was delayed in a statistically significant manner. Chronic in vivo treatment with U-0126 also significantly inhibited the RU-486-induced increase in in vitro contractility and ERK and caldesmon phosphorylation but did not alter the RU-486-induced increase in 20-kDa myosin light chain phosphorylation. These data indicate that ERK activation is a component of the multiple events leading to the development of labor in this rat model. We suggest that the ERK pathway could possibly be used to identify targets for the development of a novel class of tocolytic agents.

Receptor-independent activation of Rho-kinase-mediated calcium sensitisation in smooth muscle

1. The aim of this work was to determine whether Rho-kinase-mediated calcium sensitisation contributes to contractions of the mouse anococcygeus smooth muscle and, if so, whether the process was activated by receptor-dependent or receptor-independent mechanisms. 2. The Rho-kinase inhibitor Y27632 produced concentration-dependent decreases in tone raised by either the muscarinic receptor agonist carbachol (CCh), or the sarco-endoplasmic reticulum calcium ATPase inhibitor thapsigargin (Tg) (EC(50) values against CCh and Tg of 8.4+/-3.3 (n=6) and 6.1+/-2.1 (n=7) micro M, respectively). Pretreatment of tissues with Y27632 also inhibited contractions produced by 65 mM external potassium (69+/-7% (n=4) inhibition using 10 micro M Y27632). Y27632 had no effect on contractions produced by the inhibitor of smooth muscle myosin light-chain phosphatase, calyculin-A. 3. In beta-escin-permeabilised preparations, both CCh and Tg produced significant increases in tone over-and-above that produced by a combination of calcium (1 micro M) and GTP (100 micro M). These responses to CCh and Tg were inhibited by Y27632 (10 micro M). 4. Western blot analysis of fractionated tissue samples probed for RhoA immunoreactivity, indicated that both CCh and Tg were able to induce translocation of RhoA from the cytosol to the membrane. 5. These findings suggest that Rho-kinase-mediated calcium sensitisation is activated by both receptor-dependent and receptor-independent mechanisms in the mouse anococcygeus.

Calyculin-A inhibits nitrergic relaxations of the mouse anococcygeus

The aim was to determine whether blockade of store-operated Ca(2+) entry, or inhibition of Ca(2+) sensitisation, is the predominant mechanism by which neuronally released nitric oxide mediates relaxation of the mouse anococcygeus. Nitrergic relaxations to field stimulation (10 Hz, 10 s trains) were unaffected by the sarcoplasmic reticulum Ca(2+) ATPase blocking agent thapsigargin (100 nM), known to prevent nitric-oxide-induced inhibition of store-operated Ca(2+) entry. Conversely, the myosin phosphatase inhibitor calyculin-A (1 microM) caused almost complete abolition of nitrergic relaxations. The results provide evidence that inhibition of Ca(2+) sensitisation is the major cellular mechanism underlying nitrergic relaxation of the mouse anococcygeus.

Pharmacology and thermosensitivity of the dartos muscle isolated from rat scrotum

1 The dartos is a thin sheet of smooth muscle closely associated with the skin of the scrotum. Although known to play an important role in scrotal thermoregulation, there has been no detailed study into the pharmacology, or thermosensitivity, of the dartos from any species. Here, we investigate these two parameters in the isolated dartos muscle from rat. 2 Field stimulation of the rat dartos caused contractions that were abolished by tetrodotoxin, phentolamine and guanethidine, but unaffected by atropine or L-N(G)-nitroarginine. Exogenous noradrenaline also produced contractions blocked by both phentolamine and prazosin. In muscles with raised tone and negated sympathetic function, field stimulation failed to elicit relaxation. The dartos muscle did not contract in response to carbachol, nicotine, histamine, 5-hydroxytryptamine (all up to 100 micro M) or substance P (up to 1 micro M). 3 Contractile responses to field stimulation and noradrenaline were much greater at 30 degrees C compared with 40 degrees C; indeed, contractions to 1 micro M noradrenaline at 30 degrees C were relaxed by around 80% on heating to 40 degrees C. Similar heat-induced relaxations were observed during contractions to both U46619 (100 nM) and high K (70 mM). 4 In contrast, contractile responses to the myosin phosphatase inhibitor calyculin-A (1 micro M), either in the presence or absence of external calcium, were resistant to relaxation by heating. In calcium-free medium at 30 degrees C, U46619 continued to produce contractions that were again relaxed by 80% on heating to 40 degrees C. However, in the presence of calyculin-A, this heat-induced relaxation was greatly reduced. 5 Thus, the rat dartos muscle receives a functional sympathetic innervation and contracts to noradrenaline via alpha-adrenoceptors. There is no functional inhibitory innervation. Experiments with calyculin-A suggest that myosin phosphatase is a major contributor to the marked thermosensitivity of the dartos muscle.