Relaxation of human isolated bronchial smooth muscle. Role of prostacyclin and prostaglandin F2 alpha in muscle tone

Mechanisms of intrinsic force in small human airways

We quantified the magnitude and investigated mechanisms regulating intrinsic force (IF) in human airway smooth muscle (hASM). IF was identified by reducing extracellular calcium (Ca2+) concentration to nominally zero in freshly isolated isometrically mounted 2mm human bronchi. Our results show: (1) the magnitude of IF is ∼50% of the maximal total force elicited by acetylcholine (10(-5) M) and is epithelial independent, (2) IF can also be revealed by β-adrenergic activation (isoproterenol), non-specific cationic channel blockade (La3+) or L-type voltage gated Ca2+ channel blockade (nifedipine), (3) atropine, indomethacin, AA-861, or pyrilamine did not affect IF, (4) IF was reduced by the intracellular Ca2+ ([Ca2+]i) chelating agent BAPTA-AM, (5) ω-conotoxin had no effect on IF. In studies in cultured hASM cells nominally zero Ca2+ buffer and BAPTA-AM reduced [Ca2+]i but isoproterenol and nifedipine did not. Taken together these results indicate that rapid reduction of [Ca2+]i reveals a permissive relationship between extracellular Ca2+, [Ca2+]i and IF. However IF can be dissipated by mechanisms effecting Ca2+ sensitivity. We speculate that an increase of IF, a fundamental property of ASM, could be related to human airway clinical hyperresponsiveness and must be accounted for in in vitro studies of hASM.

Intravenous alprostadil, an analog of prostaglandin E1, prevents thiamylal-fentanyl-induced bronchoconstriction in humans

Prostaglandin (PG) E(1) relaxes airway smooth muscle in animals. However, no clinical data have been published on the bronchorelaxant effects of IV alprostadil, an analog of PGE(1). We have described experimental thiamylal-fentanyl-induced bronchoconstriction in humans; we now report the effect of IV alprostadil on thiamylal-fentanyl-induced bronchoconstriction. Thirty-two patients were allocated randomly to a control group (n = 16) and alprostadil group (n = 16). Anesthesia was induced with thiamylal 5 mg/kg and vecuronium 0.3 mg/kg and maintained with a continuous infusion of thiamylal 15 mg. kg(-1). h(-1). The lungs of the patients were ventilated with 50% nitrous oxide in oxygen. Twenty minutes after the induction of anesthesia, patients in the control group were given a continuous infusion of normal saline 20 mL/h, and those in the alprostadil group received a continuous infusion of alprostadil 0.2 micro g. kg(-1). min(-1) (20 mL/h), both for 60 min. Both groups were then given fentanyl 5 micro g/kg. Systolic and diastolic arterial blood pressure, heart rate, mean airway resistance (Rawm), expiratory airway resistance (Rawe), and dynamic lung compliance (Cdyn) were measured at the baseline, just before the fentanyl injection (T30), at three consecutive 6-min intervals after fentanyl injection (T36, T42, and T48), and 30 min after fentanyl injection (T60). Baseline Rawm, Rawe, and Cdyn values were comparable between groups. In the control group, both Rawm and Rawe were significantly increased at T36-60, and Cdyn was significantly decreased at T36-60 compared with the baseline. Patients given alprostadil showed no change in Rawm, Rawe, or Cdyn at T36-60. Thus, IV alprostadil seems to have a bronchodilator effect in humans.

IMPLICATIONS

IV alprostadil, an analog of prostaglandin E(1), prevents thiamylal-fentanyl-induced bronchoconstriction in humans. This finding suggests that IV alprostadil has a bronchodilator effect.

Prostaglandin E1 produces spasmolytic effects on histamine-induced bronchoconstriction in dogs

OBJECTIVE

In this study, we evaluated the spasmolytic effect of intravenous prostaglandin (PG) E1 on histamine-induced bronchoconstriction with a direct visualization method using a superfine fiberoptic bronchoscope.

SETTING

A university research laboratory.

SUBJECTS

Mongrel dogs.

INTERVENTIONS

The bronchial cross-sectional area (BCA) of mongrel dogs was measured by a direct visualization method using a superfine fiberoptic bronchoscope. Bronchoconstriction was elicited with histamine (H) infusion: 10 microg/kg iv bolus + 500 microg/kg/h continuous iv. The first protocol (n = 7) was designed to determine the effects of intravenous bolus of PGE1: 0 (saline), 0.01, 0.1, 1.0 and 10 microg/kg on H-induced bronchoconstriction. BCA was assessed before and 30 min after starting the H infusion and 5 min after each dose of intravenous PGE1. The second protocol was designed to determine whether continuous intravenous infusion of PGE1 reverses H-induced bronchoconstriction. In the PG group (n = 6), PGE1 was continuously infused at 0.1 microg/kg/min (20 mL/hr). In the control group (n = 6), saline was administered at a rate of 20 mL/hr iv. BCA was assessed before and 30 min after starting the H-infusion and at 5, 10, 30 and 60 min after commencing the PGE1 or saline infusion. Arterial blood was obtained simultaneously for measurement of plasma concentrations of epinephrine and norepinephrine by gas chromatography mass spectrometry.

MEASUREMENTS AND MAIN RESULTS

In the first protocol, PGE1 produced a dose-dependent increase in the percentage of BCA and 10 microg/kg of PGE1 almost fully reversed the H-induced bronchoconstriction. Plasma catecholamines did not change significantly. In the second protocol, continuous infusion of PGE1 produced a time-dependent reversal of H-induced bronchoconstriction (percentage of BCA increased to 80.0+/-9.0% 60 min after the start of PGE1 infusion), whereas saline infusion did not reverse the bronchoconstriction. Plasma catecholamines did not change significantly in either group.

CONCLUSIONS

Both intravenous bolus and continuous intravenous infusion of PGE1 reversed the H-induced bronchoconstriction. PGE1 may be used safely for patients with the hyperreactive airway and might be useful as a therapeutic agent for these patients.

Inherent tone of human bronchus: role of eicosanoids and the epithelium

1. Airway preparations of different species possess varying degrees of inherent tone which is the result of different metabolites of arachidonic acid in different species. In human bronchial smooth muscle in vitro we have investigated the effects of 5-lipoxygenase inhibition (zileuton, 10 microM), cyclo-oxygenase inhibition (indomethacin, 1 microM) and mechanical epithelium removal on inherent tone. The shunting of arachidonic acid by inhibition of one or other of these enzymes, as a possible explanation for the effects observed, has also been investigated. 2. Zileuton caused a significant fall in tone either alone (-107 +/- 33 mg) or after cyclo-oxygenase inhibition (-203 +/- 48 mg) and this effect was not significantly altered by epithelial removal (-191 +/- 43 mg alone; -333 +/- 88 mg after indomethacin). Indomethacin increased tone when applied alone (160 +/- 94 mg), but this effect only reached statistical significance after 5-lipoxygenase inhibition, (210+/-81 mg; P<0.05). Epithelial removal did not alter the effect of indomethacin when applied alone (213+/-97 mg), but significantly reduced the effect of indomethacin after 5-lipoxygenase inhibition (34 +/- 23 mg; P<0.05). 3. These data suggest that inherent tone in human bronchus is largely the result of contractile 5-lipoxygenase products. However, the involvement of cyclo-oxygenase products cannot entirely be discounted, since in the presence of 5-lipoxygenase inhibition contractile and relaxant eicosanoids originating from the bronchial epithelium appear to influence significantly inherent tone.

Pre-junctional inhibitory action of prostaglandin E2 on excitatory neuro-effector transmission in the human bronchus

The effects of prostaglandin E2 (PGE2) and indomethacin on excitatory neuro-effector transmission in the human bronchus were investigated by tension recording and microelectrode methods. PGE2 (10(-10)-10(-9)M) suppressed the amplitude of twitch contractions and excitatory junction potentials (e.j.ps) evoked by field stimulation at a steady level of basal tension obtained by the combined application of indomethacin (10(-5) M) and FPL55712 (10(-6) M). In doses over 10(-8)M, PGE2 reduced the muscle tone and dose-dependently suppressed the amplitude of twitch contractions. Indomethacin (10(-5) or 5 x 10(-5) M) reduced the muscle tone and enhanced the amplitude of twitch contractions and e.j.ps evoked by field stimulation in the presence of FPL55712. PGE2 (10(-9) M) had no effect on the post-junctional response of smooth muscle cells to exogenously applied acetylcholine (ACh) (4 x 10(-7) M). However, indomethacin (10(-5) M) significantly enhanced the ACh-induced contraction of the human bronchus. These results indicate that PGE2 in low concentrations has a pre-junctional action to inhibit excitatory neuro-effector transmission in addition to a post-junctional action, presumably by suppressing transmitter release from the vagus nerve terminals in the human bronchial tissues.

The spontaneous electrical and mechanical activity of human bronchial smooth muscle: its modulation by drugs

1. Tissue taken at operation was used to study the electrical and mechanical properties of human bronchial smooth muscle with intracellular microelectrodes and isometric recording of tension changes. 2. Over 90% of the muscle strips exhibited spontaneous tone and 70% produced spontaneous phasic contractions. The resting membrane potential of the smooth muscle cells ranged between -40 to -50 mV with a mean value of -44.9 +/- 5.2 mV (n = 92 +/- s.d.). Spontaneous oscillations of the membrane potential (slow waves) were observed in 90% of the cells examined. 3. The electrical slow waves, phasic contractions and spontaneous tone were greatly reduced by FPL 55712 (10(-6)-10(-5) M). Indomethacin (1-5 x 10(-5) M), atropine (10(-6) M) or a 5-lipoxygenase inhibitor, AA 861 (5 x 10(-6) M) each reduced spontaneous mechanical tone. Indomethacin and atropine each caused minor reduction in the amplitude of electrical slow waves. 4. Leukotriene C4 (10(-8) M), physostigmine (10(-6) M) and K+-rich physiological salt-solution (containing atropine 10(-6) M) each caused tone development in tissue treated with AA 861 (5 x 10(-6) M). In the case of leukotriene C4 and physostigmine, phasic contractions were superimposed on the developed tone. 5. Electrical field stimulation evoked an excitatory junction potential (e.j.p.) followed by a small group of slow waves. Repetitive field stimulation (2-20 stimuli at 20 Hz) markedly enhanced the amplitude of oscillatory slow waves. FPL 55712 (1.9 x 10(-6) M) abolished the oscillatory slow waves following the e.j.p., and physostigmine (10(-6) M) enhanced the amplitude of the e.j.p. and slow waves. 6. These results indicate that, under in vitro conditions, the electrical activity of human bronchial smooth muscle comprises slow wave discharge which may be related to the spontaneous contractions and generation of basal tone.

Mechanisms of intrinsic tone in bullfrog lung: relaxant effects of indomethacin, ouabain and potassium

Bullfrog hemilungs showed minimal relaxation (9 +/- 2% of the maximal relaxant effect of theophylline, Imax) after a 16 h incubation in 10(-5) M indomethacin, indicating that prostaglandin synthesis plays little or no role in the high intrinsic tone characteristic of this preparation. A higher concentration of indomethacin (10(-4) M) produced greater relaxation (23 +/- 3% of Imax), but also markedly potentiated isoprenaline-induced relaxation. The interaction with isoprenaline was similar to that previously found for papaverine, a phosphodiesterase inhibitor. Ouabain (10(-5) and 10(-4) M) produced an initial contraction followed by marked relaxation (50% of Imax), indicating that a ouabain-sensitive mechanism is of major importance in the maintenance of intrinsic tone. Ouabain-treated hemilungs showed reversal (relaxation) of the normal contractile response to 26 mM potassium and marked impairment of the contractile response to calcium in calcium-depleted preparations. These effects suggest that ouabain-induced relaxation reflects a drug action on calcium movements. The marked relaxation (30 to 40% of Imax) produced by 26 mM potassium in ouabain-treated hemilungs is of particular interest in that it indicates a mechanism of potassium-induced relaxation distinct from stimulation of sodium-potassium ATPase.

Some properties of calcium-induced contraction in the isolated human and guinea-pig tracheal smooth muscle

We studied the effects of the cyclooxygenase inhibitors, indomethacin and aspirin, the leukotriene antagonist, FPL 55712 and the Ca antagonist, verapamil on basal tone and Ca-induced contractions in the human and guinea-pig tracheal muscle. Indomethacin and aspirin usually increased spontaneous tone or Ca-induced contractions in human strips, while consistently decreased tension development in the guinea-pig muscle. FPL 55712 strongly reduced contractions in the human, whereas it had a very weak effect on the guinea-pig muscle. Verapamil had a small inhibitory effect in the human trachea, either at 5.9 or 40 mM K, but markedly suppressed Ca-induced contractions of guinea-pig trachea in 40 mM K and had little effect on contractions in 5.9 mM K in this tissue. It is concluded that active tone in the two different tracheal muscles were controlled by different processes; that different arachidonate by-products may exert an effect on basal tone in the two different muscles; and that different plasma membrane Ca channels may be operative under basal conditions in the different muscles.

Nanomolar concentrations of prostaglandin F2 alpha potentiate cholinergic contractions of rabbit isolated tracheal muscle

The effects of prostaglandin F2 alpha (PGF2 alpha) on stimulation- or acetylcholine-evoked contractions were studied in isolated airway muscle preparations from rabbits, guinea-pigs and humans. Low concentrations of PGF2 alpha (10(-9) to 9 X 10(-8) mol/1) produced a dose-related (10-300%) increase in the contractile responses of the rabbit trachealis muscle to electrical stimulation at 2 Hz. This effect was inversely related to the rate of stimulation. In seven out of forty two preparations the resting muscle tone was also increased by 1.1 X 10(-8) mol/1 or higher concentrations of PGF2 alpha. This substance enhanced the contractile responses to acetylcholine (1-2.7 X 10(-8) mol/1) to the same extent as those to electrical stimulation. The potentiation produced by PGF2 alpha was not affected by indomethacin, mepyramine, methysergide or phenoxybenzamine. Electrically evoked contractions of isolated tracheal strips of guinea-pig or segments of human bronchial muscles were not changed significantly in the presence of 0.1-5 X 10(-8) mol/1 of PGF2 alpha. These results suggest that PGF2 alpha may modulate airway muscle tone by enhancing the postsynaptic stimulatory effect of acetylcholine released from the pulmonary cholinergic nerve endings. This modulation seems to be species-dependent.