Effects of theophylline and enprofylline on diaphragmatic contractility

Xanthines and Phosphodiesterase Inhibitors

Theophylline is an orally acting xanthine that has been used since 1937 for the treatment of respiratory diseases including asthma and chronic obstructive pulmonary disease (COPD). However, in most treatment guidelines, xanthines have now been consigned to third-line therapy because of their narrow therapeutic window and propensity for drug-drug interactions. However, lower than conventional doses of theophylline considered to be bronchodilator are now known to have anti-inflammatory actions of relevance to the treatment of respiratory disease. The molecular mechanism(s) of action of theophylline are not well understood, but several potential targets have been suggested including non-selective inhibition of phosphodiesterases (PDE), inhibition of phosphoinositide 3-kinase, adenosine receptor antagonism and increased activity of certain histone deacetylases. Although theophylline has a narrow therapeutic window, other xanthines are in clinical use that are claimed to have a better tolerability such as doxofylline and bamifylline. Nonetheless, xanthines still play an important role in the treatment of asthma and COPD as they can show clinical benefit in patients who are refractory to glucocorticosteroid therapy, and withdrawal of xanthines from patients causes worsening of disease, even in patients taking concomitant glucocorticosteroids.More recently the orally active selective PDE4 inhibitor, roflumilast, has been introduced into clinical practice for the treatment of severe COPD on top of gold standard treatment. This drug has been shown to improve lung function in patients with severe COPD and to reduce exacerbations, but is dose limited by a range side effect, particularly gastrointestinal side effects.

Clinical Effects of Theophylline on Inspiratory Muscle Drive in Tetraplegia

Theophylline has been shown to restore diaphragmatic function in animals following cervical spinal cord hemisection, which induces hemidiaphragm paralysis. Although theophylline had been used clinically in the treatment of various pulmonary diseases, its effects on respiratory muscle function in cervical spinal cord injured tetraplegics has not been studied. In the present case study, we evaluated a patient injured in 1979 with a chronic asymmetric C5-7 tetraplegia (left C5-6, right C6-7) before and after receiving theophylline chronically by mouth for three weeks and again before and after receiving acute intravenous (IV) aminophylline after the effects of the chronic drug administration wore off. Neural activation to inspiratory muscles was assessed by right and left parasternal intercostal and diaphragm EMGs during quiet breathing and max imal inspiratory efforts. Global respiratory drive was assessed by P100, and inspiratory muscle force was assessed by maximal inspiratory pressures and vital capacity. Both long-term orally administered and acute IV theophylline increased neural activation to the diaphragm, especially on the more affected left side. Theophylline treatment was also associated with an increase in global central respiratory drive and inspiratory muscle force, without changing expiratory airflows. Left diaphragm EMG activity was markedly increased following the administration of theophylline. Of interest, upper parasternal intercostal EMG activity was also recruited on the left in spite of being below the level of cervical injury. We speculate that the administration of theophylline in selected patients with an asymmetric cervical spinal cord injury may activate la tent bulbospinal respiratory pathways and improve inspiratory muscle function, re ducing the likelihood of associated respiratory failure.

Methylxanthines in asthma

Methylxanthines represent a unique class of drugs for the treatment of asthma. The methylxanthine theophylline has demonstrated efficacy in attenuating the three cardinal features of asthma - reversible airflow obstruction, airway hyperresponsiveness, and airway inflammation. At doses achieving relatively high serum levels in which toxic side effects are sometimes observed, direct bronchodilatory effects of theophylline are recognized. At lower serum concentrations, theophylline is a weak bronchodilator but retains its capacity as an immunomodulator, anti-inflammatory, and bronchoprotective drug. Intense investigation into the molecular mechanisms of action of theophylline has identified several different points of action. Phosphodiesterase inhibition and adenosine receptor antagonism have both been implicated in promoting airway smooth muscle relaxation and bronchodilation. Similar mechanisms of action may explain the inhibitory effects of theophylline on immune cells. At lower concentrations that fail to inhibit phosphodiesterase, effects on histone deacetylase activity are believed to contribute to the immunomodulatory actions of theophylline. Since anti-inflammatory and immunomodulatory effects of methylxanthines are realized at lower serum concentrations than are required for bronchodilation, theophylline's predominant role in asthma treatment is as a controller medication for chronic, persistent disease.

Effect of caffeine on respiratory muscle strength and lung function in prematurely born, ventilated infants

The aims of this study were to determine whether caffeine administration increased respiratory muscle function and if this was associated with lung function improvement in prematurely born infants being weaned from mechanical ventilation. Respiratory muscle function was assessed by measurement of the maximum pressures generated during occlusions at end inspiration (Pemax) and end expiration (Pimax) and lung function by measurement of lung volume (functional residual capacity (FRC)) and respiratory system compliance (CRS) and resistance (RRS) in 18 infants with a median gestational age of 28 (range 24-36) weeks. Measurements were made immediately prior to caffeine administration (baseline) and 6 h later. Six hours after caffeine administration compared to baseline, the median Pemax (p = 0.017), Pimax (p = 0.004), FRC (p < 0.001), CRS (p = 0.002) and RRS (p = 0.004) had significantly improved. Our results suggest that caffeine administration facilitates weaning of prematurely born infants from mechanical ventilation by improving respiratory muscle strength.

Recovery of respiratory activity after C2 hemisection (C2HS): involvement of adenosinergic mechanisms

Consequences of spinal cord injury (SCI) depend on the level and extent of injury. Cervical SCI often results in a compromised respiratory system. Primary treatment of SCI patients with respiratory insufficiency continues to be with mechanical ventilatory support. In an animal model of SCI, an upper cervical spinal cord hemisection paralyzes the hemidiaphragm ipsilateral to the side of injury. However, a latent respiratory motor pathway can be activated to restore respiratory function after injury. In this review, restoration of respiratory activity following systemic administration of theophylline, a respiratory stimulant will be discussed. Pharmacologically, theophylline is a non-specific adenosine receptor antagonist, a phosphodiesterase inhibitor and a bronchodilator. It has been used in the treatment of asthma and other respiratory-related diseases such as chronic obstructive pulmonary disease (COPD) and in treatment of apnea in premature infants. However, the clinical use of theophylline to improve respiration in SCI patients with respiratory deficits is a more recent approach. This review will focus on the use of theophylline to restore respiratory activity in an animal model of SCI. In this model, a C2 hemisection (C2HS) interrupts the major descending respiratory pathways and paralyzes the ipsilateral hemidiaphragm. The review also highlights involvement of central and peripheral adenosine receptors in functional restitution. Biochemical binding assays that highlight changes in adenosine receptors after chronic theophylline administration are discussed as they pertain to understanding adenosine receptor-mediation in functional recovery. Finally, the clinical application of theophylline in SCI patients with respiratory deficits in particular is discussed.

Effects of chronic systemic theophylline injections on recovery of hemidiaphragmatic function after cervical spinal cord injury in adult rats

Based on a previous demonstration that acutely administered theophylline induces respiratory-related recovery in an animal model of spinal cord injury, the influence of chronically administered theophylline on maintaining recovery was assessed. The absence of respiratory-related activity in the left phrenic nerve and hemidiaphragm of rats subjected to an ipsilateral C2 spinal cord hemisection was confirmed electrophysiologically 24 h after injury. Theophylline was then injected i.p. for 3-30 consecutive days. Recovery of respiratory-related activity was observed in the majority (29 out of 32) of the experimental animals. We conclude that theophylline not only induces, but also maintains recovery for prolonged periods after cervical spinal cord injury.

Aminophylline enhances resting Ca2+ concentrations and twitch tension by adenosine receptor blockade in Rana pipiens

1. We hypothesized that the xanthine aminophylline acts to block adenosine receptors on the surface of skeletal muscle fibres, thereby inhibiting a depressant action of endogenous adenosine. We further hypothesized that this action results in increased concentrations of intracellular resting Ca2+ and enhanced twitch tension upon muscle stimulation. 2. Peak twitch tension (Pt) of the semitendinosus muscle in normal frog Ringer solution (NFR) ranged from 6.8 to 9.4 g. Intracellular Ca2+ concentrations in control resting fibres ranged from 67 to 70 nM. Aminophylline at 100 microM produced increases of 26 and 22% in Pt and Ca2+ concentrations, respectively. 3. The adenosine receptor antagonists 8-phenyltheophylline (8-PT) and 1,3-dipropyl-7-methylxanthine (1,3-d-7-M) both increased Pt by 32% over values in NFR. In addition, 1,3-d-7-M increased resting Ca2+ concentrations by 29% over control levels. 4. Adenosine deaminase increased twitch tension and resting intracellular Ca2+ concentrations by 22 and 26% over controls, respectively. 5. N6-(2-phenylisopropyl)adenosine (R-PIA, 1 microM), a potent adenosine analogue, partially blocked both the increase in Pt and intracellular Ca2+ concentrations induced by the xanthines, possibly by competing for the adenosine receptor. 6. The data herein provide support for the existence of adenosine receptors on the membranes of skeletal muscle fibres and suggest a possible role for adenosine receptors in the regulation of twitch tension.

Effect of a beta 2-agonist (broxaterol) on respiratory muscle strength and endurance in patients with COPD with irreversible airway obstruction

The effect of broxaterol, a new beta 2-agonist, on respiratory muscle endurance and strength was studied in a double-blind, placebo-controlled, randomized crossover clinical trial in 16 patients with chronic obstructive pulmonary disease (COPD) with irreversible airway obstruction (FEV1 = 57.1 percent of predicted). One patient withdrew from the study because of acute respiratory exacerbation. Inspiratory muscle strength was assessed by maximal inspiratory pressure (MIP) and endurance time was determined as the length of time a subject could breathe against inspiratory resistance (target mouth pressure = 70 percent of MIP, Ti/Ttot = 0.4). Broxaterol (B) or placebo (P) was given orally for seven days at the dose of 0.5 mg three times a day with a washout period of 72 h between study treatments. Measurements were performed before administration of B or P and 2 h (six patients) or 8 h (nine patients) after the end of each treatment. No significant changes in FEV1 or FRC were observed after B or P suggesting that diaphragmatic length was maintained constant with each treatment. The MIP did not significantly change, while endurance time increased after B in the patients tested at 2 h (from 234.8 +/- 48.1 s to 284.0 +/- 48.0 s, p less than 0.05) and at 8 h (from 187.2 +/- 31.1 s to 258.2 +/- 40.4 s, p less than 0.005). No changes were observed after P. Minute ventilation, airway occlusion pressure (P0.1), integrated electromyographic activities of the diaphragm (Edi), and intercostal parasternals (Eic) (normalized to the value obtained during MIP) showed no change during the endurance run with different treatments. We conclude that in a group of COPD patients with irreversible airway obstruction, B significantly improves respiratory muscle endurance, and that this does not arise as a result of an effect on neuromuscular drive or pulmonary mechanics, but may be mediated by peripheral factors.

Lung function, hypoxic and hypercapnic ventilatory responses, and respiratory muscle strength in normal subjects taking oral theophylline

Methylxanthines are known to be respiratory stimulants and are thought by some to augment hypercapnic and hypoxic ventilatory drive and improve respiratory muscle strength. Hypoxic and hypercapnic ventilatory responses were measured in 10 normal subjects before, during, and after administration of theophylline for three and a half days. Pulmonary function, carbon dioxide production, and mouth pressures during maximal static inspiratory and expiratory efforts were also measured. The mean (SD) serum theophylline concentration was 13.8 (3.2) mg/l. Lung volumes and flow rates did not change significantly with theophylline. The mean (SD) values for maximum static inspiratory pressure were 152 (27), 161 (25), and 160 (24) cm H2O, respectively before, during, and after theophylline. Neither these values nor peak expiratory pressure measurements were significantly changed. The slopes of the hypercapnic ventilatory responses were 2.9 (0.9), 3.3 (1.2), and 3.3 (1.4) l/min/mm Hg carbon dioxide tension (PCO2) respectively before, during, and after theophylline administration. The respective values for the slopes of the hypoxic response were -1.4 (0.9), -1.3 (0.8), and -1.1 (0.9) l/min/1% oxyhaemoglobin saturation. None of these values changed significantly with theophylline. Theophylline, however, increased carbon dioxide production (200 to 236 ml/min) and alveolar ventilation (4.7 to 5.7 l/min) significantly, with a concomitant fall of end tidal PCO2 (35.5 to 32.9 mm Hg). It is concluded that in man oral theophylline at therapeutic blood concentrations increases carbon dioxide production and ventilation without changing pulmonary function, respiratory muscle strength, or the hypoxic or hypercapnic ventilatory response significantly.

Adenosine depresses spontaneous transmitter release from frog motor nerve terminals by acting at an A1-like receptor

Adenosine (1 microM to 1 mM) depressed spontaneous transmitter release from frog motor nerve terminals without producing any observable postsynaptic effects. Since this action of adenosine was blocked by 20 microM theophylline and 1 microM 8-phenyltheophylline, adenosine probably acts at a specific receptor on motor nerve terminals to reduce spontaneous transmitter output. The effects of the adenosine analogs, L-N6-phenylisopropyladenosine (L-PIA, 100 pM to 1 microM), D-PIA (100 nM to 100 microM), and 5'-N-ethylcarboxamidoadenosine (NECA, 10nM to 100 microM), were tested on spontaneous transmitter release at the frog neuromuscular junction. L-PIA depressed mepp frequency at a threshold concentration of about 1 nM, was thirteen times more potent than NECA, and was 294 times more effective than D-PIA. The rank-order potency of these analogs indicates that adenosine acts at an A1-like receptor to depress spontaneous transmitter release. Inhibitory actions of maximally effective concentrations of adenosine and L-PIA were also blocked by the A1-specific antagonist, 1-3-dipropyl-8-cyclopentylxanthine (DPCPX) at a concentration of 100 nM. Micromolar concentrations of NECA, an agonist with approximately equal affinity for the A1 and A2 receptors, produced biphasic effects on mepp frequency. Thus, a second adenosine receptor, perhaps of the A2 subtype, may be present on motor nerve terminals and may mediate an increase in spontaneous transmitter release.

In vivo effects of theophylline on diaphragm, bicep, and quadricep strength and fatigability

Aminophylline has been demonstrated to increase in vitro contractility in skeletal muscle, including diaphragm. In vivo studies report significant increases in diaphragm contractility in patients with chronic obstructive pulmonary disease but only small increases in control subjects. The present study determined the effects of aminophylline on strength and fatigability in the diaphragm, the biceps brachii, and the quadriceps of normal individuals. Seven healthy subjects were tested with placebo and drug conditions on separate days in a randomized, double-blind fashion. Mean theophylline levels of 15 +/- 2 mg/L SD were maintained by constant intravenous infusion. Strength of the diaphragm was measured as maximum inspiratory pressure. Strength of the biceps and quadriceps were measured isometrically during arm flexion (90 degrees) and leg extension (115 degrees) against an electronic load cell. Fatigue was measured as the decrease in tension during a 30-second contraction and during a 6-minute period of alternating 5-second maximal contraction and 5-second rest. Therapeutic levels of theophylline had no effect on strength or fatigability during a maximal contraction in any muscle group studied.

Pharmacotherapy of respiratory muscles

This article assesses the role of pharmacotherapy in the management of respiratory muscle dysfunction. It focuses on two classes of drugs, the methylxanthines and the sympathomimetic agents. A prospective section focuses also on the particularities of the diaphragm among the skeletal striated muscles. In addition, a new approach to respiratory pharmacotherapy, which may be beneficial in patients with respiratory muscle dysfunction, is suggested.