Double-stranded RNA-dependent protein kinase activation modulates endotoxin-induced diaphragm weakness.

Diaphragm caspase-8 activation plays a key role in modulating sepsis-induced respiratory muscle dysfunction. It is also known that double-stranded RNA-dependent protein kinase (PKR) is a regulator of caspase-8 activation in neural tissue. We tested the hypothesis that the PKR pathway modulates sepsis-induced diaphragmatic caspase-8 activation. We first evaluated the time course of diaphragm PKR activation following endotoxin administration in mice. We then determined whether administration of a PKR inhibitor (2-aminopurine) prevents endotoxin-induced diaphragm caspase-8 activation and contractile dysfunction in mice. Finally, we investigated if inhibition of PKR (using either 2-aminopurine or transfection with dominant-negative PKR) blocks caspase-8 activation in cytokine treated C2C12 cells. Endotoxin markedly activated diaphragm PKR (with increases in both active phospho-PKR protein levels, P < 0.03, and directly measured PKR activity, P < 0.01) and increased active caspase-8 levels (P < 0.01). Inhibition of PKR with 2-aminopurine prevented endotoxin-induced diaphragm caspase-8 activation (P < 0.01) and diaphragm weakness (P < 0.001). Inhibition of PKR with either 2-aminopurine or transfection with dominant-negative PKR blocked caspase-8 activation in isolated cytokine-treated C2C12 cells. These data implicate PKR activation as a major factor mediating cytokine-induced skeletal muscle caspase-8 activation and weakness.

Caspase and calpain activation both contribute to sepsis-induced diaphragmatic weakness.

The cecal ligation perforation (CLP) model of sepsis is known to induce severe diaphragm dysfunction, but the cellular mechanisms by which this occurs remain unknown. We hypothesized that CLP induces diaphragm caspase-3 and calpain activation, and that these two enzymes act at the level of the contractile proteins to reduce muscle force generation. Rats (n = 4/group) were subjected to 1) sham surgery plus saline (intraperitoneal); 2) CLP; 3) CLP plus administration of calpain inhibitor peptide III (12 mg/kg ip); or 4) CLP plus administration of a caspase inhibitor, zVAD-fmk (3 mg/kg). At 24 h, diaphragms were removed, and the following were determined: 1) calpain and caspase-3 activities by fluorogenic assay; 2) caspase-3 and calpain I protein levels; 3) the intact diaphragm force-frequency relationship; and 4) the force generated by contractile proteins of single, permeabilized diaphragm fibers in response to exogenous calcium. CLP significantly increased diaphragm calpain activity (P < 0.02), caspase-3 activity (P < 0.02), active calpain I protein levels (P < 0.02), and active caspase-3 protein (P < 0.02). CLP also reduced the force generated by intact diaphragm muscle (P < 0.001) and the force generated by single-fiber contractile proteins (P < 0.001). Administration of either calpain inhibitor III or zVAD-fmk markedly improved force generation of both intact diaphragm muscle (P < 0.01) and single-fiber contractile proteins (P < 0.001). CLP induces significant reductions in diaphragm contractile protein force-generating capacity. This force reduction is mediated by the combined effects of activated caspase and calpain. Inhibition of these pathways may prevent diaphragm weakness in infected patients.

MitoQ administration prevents endotoxin-induced cardiac dysfunction.

Sepsis elicits severe alterations in cardiac function, impairing cardiac mitochondrial and pressure-generating capacity. Currently, there are no therapies to prevent sepsis-induced cardiac dysfunction. We tested the hypothesis that administration of a mitochondrially targeted antioxidant, 10-(6'-ubiquinonyl)-decyltriphenylphosphonium (MitoQ), would prevent endotoxin-induced reductions in cardiac mitochondrial and contractile function. Studies were performed on adult rodents (n = 52) given either saline, endotoxin (8 mg x kg(-1) x day(-1)), saline + MitoQ (500 microM), or both endotoxin and MitoQ. At 48 h animals were killed and hearts were removed for determination of either cardiac mitochondrial function (using polarography) or cardiac pressure generation (using the Langendorf technique). We found that endotoxin induced reductions in mitochondrial state 3 respiration rates, the respiratory control ratio, and ATP generation. Moreover, MitoQ administration prevented each of these endotoxin-induced abnormalities, P < 0.001. We also found that endotoxin produced reductions in cardiac pressure-generating capacity, reducing the systolic pressure-diastolic relationship. MitoQ also prevented endotoxin-induced reductions in cardiac pressure generation, P < 0.01. One potential link between mitochondrial and contractile dysfunction is caspase activation; we found that endotoxin increased cardiac levels of active caspases 9 and 3 (P < 0.001), while MitoQ prevented this increase (P < 0.01). These data demonstrate that MitoQ is a potent inhibitor of endotoxin-induced mitochondrial and cardiac abnormalities. We speculate that this agent may prove a novel therapy for sepsis-induced cardiac dysfunction.

Effect of proteasome inhibitors on endotoxin-induced diaphragm dysfunction.

Infections produce severe respiratory muscle dysfunction. It is known that the proteasome proteolytic system is activated in skeletal muscle in sepsis, and it has been postulated that this degradative pathway is responsible for inducing skeletal muscle weakness and wasting. The objective of this study was to determine if administration of proteasomal inhibitors (MG132, epoxomicin, bortezomib) can prevent sepsis-induced diaphragm weakness. Rats were given either 1) saline (0.5 ml ip), 2) endotoxin (12 mg/kg ip), 3) endotoxin plus MG132 (2.5 mg/kg), 4) endotoxin plus epoxomicin (1 micromol/kg), or 5) endotoxin plus bortezomib (0.05 mg/kg). Animals were killed either 48 or 96 h after injections, and assessments were made of diaphragm proteolysis, force-frequency relationships, mass, protein content, and caspase activation. Endotoxin increased proteolysis (P <0.001). MG132, epoxomicin, and bortezomib each prevented the endotoxin-induced increase in proteolysis (P <0.01). Endotoxin induced severe reductions in diaphragm force generation by 48 h (P <0.01); none of the proteasomal inhibitors prevented loss of force. Endotoxin induced significant reductions in diaphragm mass and protein content by 96 h (P <0.01); neither MG132 nor epoxomicin prevented loss of mass or protein, but bortezomib attenuated the reduction in protein content (P <0.05). Endotoxin increased diaphragm caspase-3 activity (P <0.01); caspase-3 activity remained high when either MG132, epoxomicin, or bortezomib were given. These data suggest proteasomal inhibitors are not an adequate treatment to prevent endotoxin-induced diaphragmatic dysfunction.

Free radicals alter maximal diaphragmatic mitochondrial oxygen consumption in endotoxin-induced sepsis.

Recent studies indicate that sepsis is associated with enhanced generation of several free radical species (nitric oxide, superoxide, hydrogen peroxide) in skeletal muscle. While studies suggest that free radical generation causes uncoupling of oxidative phosphorylation in sepsis, no previous report has examined the role of free radicals in modulating skeletal muscle oxygen consumption during State 3 respiration or inhibiting the electron transport chain in sepsis. The purpose of the present study was to examine the effects of endotoxin-induced sepsis on State 3 diaphragm mitochondrial oxygen utilization and to determine if inhibitors/scavengers of various free radical species would protect against these effects. We also examined mitochondrial protein electrophoretic patterns to determine if observed endotoxin-related physiological derangements were accompanied by overt alterations in protein composition. Studies were performed on: (a) control animals, (b) endotoxin-treated animals, (c) animals given endotoxin plus PEG-SOD, a superoxide scavenger, (d) animals given endotoxin plus L-NAME, a nitric oxide synthase inhibitor, (e) animals given only PEG-SOD or L-NAME, (f) animals given endotoxin plus D-NAME, and (g) animals given endotoxin plus denatured PEG-SOD. We found: (a) no alteration in maximal State 3 mitochondrial oxygen consumption rate at 24 h after endotoxin administration, but (b) a significant reduction in oxygen consumption rate at 48 h after endotoxin, (c) no effect of endotoxin to induce uncoupling of oxidative phosphorylation, (d) either PEG-SOD or L-NAME (but neither denatured PEG-SOD nor D-NAME) prevented endotoxin-mediated reductions in State 3 respiration rates, (e) some mitochondrial proteins underwent tyrosine nitrosylation at 24 h after endotoxin administration, and (f) SDS-page electrophoresis of mitochondria from endotoxin-treated animals revealed a selective depletion of several proteins at 48 h after endotoxin administration (but not at 24 h); (g) administration of L-NAME or PEG-SOD prevented this protein depletion. These data provide the first evidence that endotoxin-induced reductions in State 3 mitochondrial oxygen consumption are free radical-mediated.

Modulation of release of reactive oxygen species by the contracting diaphragm.

Recent reports have demonstrated that superoxide is released by the contracting diaphragm. Moreover, extracellular scavengers of superoxide (i.e., exogenously administered superoxide dismutase) reduce diaphragm fatigue rate, arguing that superoxide released from contracting muscles may have functionally significant effects. The mechanism by which free radical formation and release occurs has not, however, been determined, and all past studies of this phenomenon have been conducted at a single muscle length (the length of maximum force generation, Lo) and at a single level of carbon dioxide. The purpose of the present study was twofold: (1) to examine the effect of blockade of two free radical-generating pathways (i.e., to block cyclooxygenase with indomethacin and xanthine oxidase with oxypurinol) on superoxide release by the contracting diaphragm, and (2) to examine the effect of altering muscle length, carbon dioxide levels, and stimulation frequency on superoxide release during contraction. Studies were performed using an isolated, arterially perfused, rat diaphragm preparation in which superoxide release was assessed in real time by measuring arteriovenous cytochrome c reduction gradients across this muscle. We found that superoxide release during contraction was: (1) not altered by indomethacin administration, (2) partially reduced by oxypurinol administration, (3) reduced by decreasing muscle length, (4) reduced by increasing carbon dioxide concentrations, and (5) reduced by decreasing stimulation frequency. The first two findings indicate that xanthine oxidase pathways contribute to free radical formation under these circumstances but cyclooxygenase does not. The last three findings suggest that these common physiologic alterations have significant effects on free radical release by contracting muscle.

Efficacy of combined inspiratory intercostal and expiratory muscle pacing to maintain artificial ventilation.

Many patients with ventilator-dependent quadriplegia have coincident phrenic nerve injury and therefore cannot be offered phrenic nerve pacing. The purpose of this study was to assess the utility of combined inspiratory intercostal and expiratory muscle pacing to provide complete ventilatory support. Studies were performed in 15 anesthetized dogs. An electrode was positioned on the epidural surface of the upper thoracic spinal cord to activate the inspiratory intercostal muscles; a separate electrode was positioned on the epidural surface of the lower thoracic spinal cord to activate the expiratory muscles. In an attempt to replicate the effects of inspiratory intercostal pacing alone in humans, stimulus parameters during upper thoracic spinal cord stimulation were adjusted to provide suboptimal levels of ventilation (end-tidal PCO2 of 55 to 60 mm Hg). Expiratory muscle activation was triggered electrically by the inspiratory signal with a 4.2-s delay resulting in alternate inspiratory and expiratory muscle pacing at a combined rate of 14 breaths/min. Combined pacing was maintained for an arbitrary period of 3 h. Initial intercostal muscle pacing alone resulted in an end-tidal PCO2 of 57.1 +/- 1.1 mm Hg. After the addition of expiratory muscle pacing, end-tidal PCO2 fell to 36.3 +/- 1.2 mm Hg. Tidal volume during both inspiratory and expiratory muscle pacing and end-tidal PCO2 remained stable throughout the study period. Our results suggest that combined alternate inspiratory and expiratory muscle pacing may be a viable alternative method of artificial ventilation in ventilator-dependent quadriplegic patients.

N-acetylcysteine administration alters the response to inspiratory loading in oxygen-supplemented rats.

Based on recent studies, it has been suggested that free radicals are elaborated in the respiratory muscles during strenuous contractions and contribute to the development of muscle fatigue. If this theory is correct, then it should be possible to attenuate the development of diaphragm fatigue and/or delay the onset of respiratory failure during loaded breathing by administering a free radical scavenger. The purpose of the present experiment was, therefore, to examine the effect of N-acetylcysteine (NAC), a free radical scavenger and glutathione precursor, on the evolution of respiratory failure in decerebrate unanesthetized rats breathing against a large inspiratory resistive load. We compared the inspiratory volume and pressure generation over time in animals pretreated with either saline or NAC (150 mg/kg) and then loaded until respiratory arrest. After arrest, the diaphragm was excised, and samples were assayed for reduced (GSH) and oxidized glutathione. As a control, we also assessed respiratory function and glutathione concentrations in groups of nonloaded saline- and NAC-treated animals. We found that NAC-treated animals were able to tolerate loading better than the saline-treated group, maintaining higher inspiratory pressures and sustaining higher inspired volumes. Administration of NAC also increased the time that animals could tolerate loading before the development of respiratory arrest. In addition, although saline-treated loaded animals had significant reductions in diaphragmatic GSH levels compared with unloaded controls, the magnitude of this reduction was blunted by NAC administration (i.e., GSH averaged 965 +/- 113, 568 +/- 83, 907 +/- 39, and 784 +/- 61 nmol/g for unloaded-saline, loaded-saline, unloaded-NAC, and loaded-NAC groups, P < 0.05, with the value for the loaded-saline group lower than the values for the two unloaded groups; GSH for the loaded-NAC group was not different, however, from unloaded controls). These data demonstrate that administration of NAC, a free radical scavenger, slows the rate of development of respiratory failure during inspiratory resistive loading.

Effect of varying inspired oxygen concentration on diaphragm glutathione metabolism during loaded breathing.

Recent studies have suggested that loaded breathing elicits alterations in diaphragmatic glutathione levels that may be mediated by free radicals and may also be linked to the development of diaphragm fatigue. While free-radical generation in a number of pathophysiologic conditions is known to be a function of ambient oxygen concentrations, the effect of varying inspired oxygen concentration on the diaphragmatic response to loaded breathing (i.e., on diaphragm fatigue and glutathione levels) has not been studied. In this study, we compared the effect of loaded breathing, continued until respiratory arrest in decerebrate rats breathing room air (RA), with the effect of the same load on animals breathing 100% oxygen (O2). After arrest, the animals' diaphragms were excised, force generation was assessed in vitro, and diaphragmatic levels of reduced glutathione (GSH) and oxidized glutathione (GSSG) were determined. Similar measurements were made on unloaded control animals. We found both similarities and differences in the response to loading in O2- and RA-breathing animals. O2-breathing loaded animals had a greater load endurance, lower blood pressure at the end of loading, higher carbon dioxide levels, and greater high-frequency fatigue at the conclusion of loaded trials than did RA-breathing animals. The degree of low-frequency fatigue was similar, however, in the O2- and RA-breathing loaded groups (i.e, twitch force averaged 7.9 +/- 0.6, 8.4 +/- 0.5, 3.8 +/- 0.9, and 4.5 +/- 0.8 N/cm2, respectively, in the RA/unloaded, O2/unloaded, RA/loaded, and O2/loaded groups, p < 0.001).(ABSTRACT TRUNCATED AT 250 WORDS)

N-acetylcysteine administration and loaded breathing.

Recent work has shown that loaded breathing produces alterations in diaphragmatic glutathione metabolism. Moreover, it has been suggested that alterations in glutathione levels may be related to the development of respiratory muscle fatigue and respiratory failure during loading. The purpose of this study was to determine whether it was possible to augment diaphragmatic stores of reduced glutathione (GSH) and thereby delay the development of respiratory failure during loaded breathing by administering N-acetylcysteine (NAC), a glutathione precursor. We compared the effects of massive inspiratory loading on saline- and NAC-treated groups of decerebrate unanesthetized rats with loading continuing until respiratory arrest occurred. As controls, we also studied unloaded saline- and NAC-treated animals. After arrest, diaphragms were excised, measurement was made of diaphragmatic GSH and oxidized glutathione (GSSG) concentrations, and assessment was made of in vitro diaphragmatic contractility (i.e., the force-frequency relationship and in vitro fatigability). We found that loading of saline-treated animals produced reductions in the diaphragmatic force-frequency curve, reductions in GSH, and increases in GSSG levels. NAC administration blunted loading-induced decreases in diaphragmatic GSH levels and reduced the in vitro fatigability of excised diaphragm muscle strips. NAC did not significantly alter the time to respiratory arrest, however, and also failed to alter the effect of loaded breathing on the diaphragmatic force-frequency relationship. These findings suggest that free radical-mediated GSH depletion is not the limiting factor determining the development of respiratory failure in this model of loaded breathing.

Electrical activation of the expiratory muscles to restore cough.

Many patients with spinal cord injury have paralysis of their expiratory muscles and, consequently, lack an effective cough. The purpose of the present study was to evaluate the utility of lower thoracic spinal cord stimulation (SCS) to activate the expiratory muscles. Studies were performed on 15 anesthetized dogs. A quadripolar stimulating electrode (Medtronic Model 3586) was inserted epidurally and on the ventral surface of the lower thoracic spinal cord. Changes in airway pressure, airflow, and internal intercostal and abdominal muscle length were monitored to assess the effects of electrical stimulation. Spinal stimulation applied at the T9-T10 spinal level provided maximal changes in airway pressure generation in preliminary experiments. All subsequent studies were therefore performed with the electrode positioned at this level. The expiratory muscles were stimulated supramaximally over a wide range of lung volumes which were expressed as the corresponding change in airway pressure. The pressure-generating capacity of the expiratory muscles was evaluated by the change in airway pressure produced by SCS during airway occlusion. Peak expiratory airflow was also monitored following release of occlusion. At FRC, deflation (-10 cm H2O) and inflation (+ 30 cm H2O), SCS resulted in positive airway pressures of 44 cm H2O +/- 4 SE, 28 cm H2O +/- 3 SE, and 82 cm H2O +/- 7 SE. The relationship between airway pressure expiratory airflow generation and lung volume was linear (slope = 1.34 +/- 0.04) over the entire vital capacity range. Our results indicate that: (1) a major portion of the expiratory muscles can be activated reproducibly and in concert by electrical stimulation, and (2) this technique may be a clinically useful method of restoring cough in spinal cord injured patients.

Evaluation of intercostal pacing to provide artificial ventilation in quadriplegics.

The purpose of this study was to assess the utility of intercostal muscle pacing by spinal cord stimulation (SCS) to provide artificial ventilation in ventilator-dependent quadriplegic patients. Five ventilator-dependent quadriplegics with phrenic nerve injury (and therefore not candidates for phrenic nerve pacing) were studied. During an initial surgical procedure, a quadripolar epidural disc electrode was positioned on the ventral portion of the upper thoracic spinal cord via a hemilaminectomy and subsequently connected to a radio-frequency receiver implanted subcutaneously over the anterior rib cage. In four of the five patients, initial SCS stimulation resulted in inspired volumes between 150 and 240 ml. Stimulation resulted in no effect in one patient, due to probable cystic degeneration of the thoracic spinal cord. Reconditioning of the intercostal muscles caused substantial increases in inspired volume in three of four patients of 670 to 850 ml. In one patient, reconditioning resulted in a much smaller increase (to 470 ml). The maximum duration that ventilation could be sustained by low-frequency (13 Hz) intercostal pacing ranged between 20 min and 2 3/4 h. Our findings indicate that intercostal pacing via SCS does not result in sufficient inspired volume production to support ventilation for prolonged periods. However, this modality may be a useful adjunct to enhance tidal volume in patients with suboptimal inspired volume by phrenic nerve pacing.

Effects of pentobarbital anesthesia on intercostal muscle activation and shortening.

Although pentobarbital (PB) is a commonly used anesthetic in animal studies examining respiratory motor control, there are virtually no studies that have examined the differential effects of deepening anesthesia on the activation of the various intercostal muscles. In dogs, anesthetized initially with 25 mg/kg of PB, the effects of additional doses of PB (20 mg) provided every 15 min on intercostal electromyogram (EMG) were monitored. In each animal, peak external intercostal (EI) and levator costae (LC) activation progressively decreased with additional doses of PB and were eventually abolished, at which point peak parasternal (PA) EMG had increased to 127 +/- 13% (SE) of control values; peak diaphragm EMG was unaffected. The reductions in EI activation were associated with progressive reductions in EI muscle shortening that, in turn, were associated with progressive reductions in lateral rib cage expansion. PA shortening was not significantly affected. Similar results were obtained in animals breathing supplemental oxygen. These results indicate that 1) activation of EI and LC compared with PA have divergent responses, with EI and LC decreasing and PA increasing; 2) the fall in EI activation results in decrements in EI shortening and lateral rib cage motion; and 3) anesthetic depth is an important variable that must be controlled in studies assessing intercostal muscle activation.

Respiratory muscle rest using nasal BiPAP ventilation in patients with stable severe COPD.

To more systematically evaluate the effect of respiratory muscle rest on indices of ventilatory function, nine outpatients with stable, severe COPD were treated with nasal pressure-support ventilation delivered via a nasal ventilatory support system (BiPAP, Respironics, Inc) for 2 h a day for 5 consecutive days. An additional eight control patients were treated with sham-BiPAP. Maximum inspiratory pressure (MIP), maximum expiratory pressure (MEP), maximum voluntary ventilation (MVV), arterial blood gas values, Borg dyspnea score, dyspnea-associated functional impairment scales, and distance walked in 6 min were measured in subjects prior to and following the week-long trial. Nasal BiPAP produced a 66.3 +/- 6 percent reduction in peak integrated diaphragmatic electromyographic (EMG) activity. There were no statistically significant changes in MIP, MEP, MVV, arterial pH, PaCO2, or PaO2 or in objective measures of functional impairment from dyspnea in either group after ventilator or sham treatment. However, nasal BiPAP reduced the Borg category score during resting, spontaneous breathing from 2.0 +/- 0.4 to 0.7 +/- 0.3 (p < 0.01) after 5 days of treatment. In contrast, sham BiPAP-treated patients had no change in their dyspnea score, which was 1.8 +/- 0.4 and 1.3 +/- 0.4 before and after sham treatment, respectively. Nasal BiPAP also increased distance walked in 6 min from 780 +/- 155 to 888 +/- 151 ft (p < 0.01) (23,400 +/- 4,650 to 26,640 +/- 4,530 cm) (p < 0.01), whereas sham-BiPAP had no effect (768 +/- 96 and 762 +/- 106 ft [23,040 +/- 2,880 and 22,860 +/- 3,180 cm]) before and after sham treatment, respectively). In conclusion, these results indicate that nasal pressure-support ventilation, delivered via nasal BiPAP, improves exercise capacity and reduces dyspnea over the short term in selected outpatients with stable severe COPD. Whether such short-term improvement can be sustained merits further study.

Mechanical action of the internal intercostal muscles in dogs.

The pattern of electrical activation and muscle length changes of the internal intercostal (II) muscles (9th or 10th interspace) of the lower rib cage were evaluated in supine anesthetized dogs. Studies were performed during resting breathing and expiratory threshold loading. Results were compared with simultaneous measurements of the better-studied triangularis sterni muscle (4th interspace). In general, both muscles lengthened with passive inflation and shortened with passive deflation. During resting breathing, both the II and TS muscles were electrically active and shortened below resting length, 7.7 +/- 1.6% (SE) and 5.3 +/- 1.7%, respectively. With the addition of positive end-expiratory pressure, the degree of electrical activation and muscle shortening increased progressively for both muscles, although to a somewhat greater extent for II muscles. Isolated denervation of the II muscles eliminated their shortening during resting breathing and often resulted in muscle lengthening, indicating that II muscle shortening was secondary to its own activation. Expiration was associated with lateral inward movement of the lower rib cage below its relaxation position. This motion was not significantly affected by abdominal muscle section but was markedly reduced by bilateral II denervation (7th-11th spaces). Our results indicate that the II muscles of the lower rib cage 1) are electrically active and shorten below resting length during resting breathing, 2) respond to positive end-expiratory pressure by increasing their level of activation and degree of shortening, and 3) are primarily responsible for inward lateral motion of the lower rib cage below its relaxation position during expiration.

Reflex control of diaphragm activation by thoracic afferents.

Recent studies suggest that chest wall reflexes may have a role in modulating diaphragm activation. The purpose of this study was to more closely examine this issue by assessing the diaphragmatic motor response to airway occlusion. Studies were performed in vagotomized mongrel dogs anesthetized with pentobarbital sodium. Diaphragmatic electromyogram (EMG) and phrenic neurogram (ENG) responses to airway occlusion were evaluated at different precontractile respiratory muscle lengths, achieved by passive inflation and deflation with a volume syringe during the preceding expiration. Lung volume was expressed as the corresponding change in airway pressure. At functional residual capacity, deflation (-5 cmH2O), and large inflation (+25 cmH2O), phrenic ENG during occlusion was 90 +/- 2 (SE), 84 +/- 5, and 86 +/- 3% of the preceding control breaths, respectively (n = 9). Qualitatively similar, but somewhat more pronounced, responses were observed on diaphragmatic EMG. With small lung inflations, the degree of reduction of phrenic ENG with airway occlusion was less. Consequently, the relationship between airway pressure and degree of inhibition was best described as a reverse parabola with the maximum at approximately +10-15 cmH2O. Responses were not significantly affected by bilateral cervical phrenicotomy. Complete section of the spinal cord at the high thoracic level (T1-T2) abolished the observed reduction in phrenic ENG in response to airway occlusion. Our results demonstrate 1) the existence of nonvagal nonphrenic reflex control of diaphragm activation most likely secondary to activation of intercostal afferents and 2) that the magnitude of this reflex is highly dependent on factors related to lung volume.

Failure of vasodilator administration to increase blood flow to the fatiguing diaphragm.

Recent studies have suggested that coronary and limb muscle vessels do not maximally vasodilate under conditions in which cardiac and limb muscle contractile function is dependent on the level of blood flow but, rather, maintain a "vasodilator reserve." If a vasodilator reserve is also present in the fatiguing diaphragm, it may be possible to augment flow to this muscle with vasodilator administration, improving muscle function. The purpose of the present study was therefore to examine the effect of administration of a potent vasodilator, nitroprusside, on the blood flow and contractile function of the fatiguing diaphragm. Studies were performed using an in situ canine diaphragmatic strip preparation that permitted direct measurement of force and blood flow; cardiac output was monitored with a thermodilution catheter. The effects of nitroprusside were examined with the diaphragm rhythmically contracting in response to both subfatiguing and fatiguing stimulation paradigms. For both contraction paradigms, nitroprusside infusions elicited appreciable increases in cardiac output. Nitroprusside infusions also produced significant increases in diaphragmatic blood flow during subfatiguing diaphragmatic contractions but had no effect on flow during fatiguing contractions. Nitroprusside also had no effect on the rate of diaphragmatic fatigue. These data suggest that, under the conditions examined, the diaphragm exhausts its vasodilator reserve during the development of fatigue and vasodilator administration has no appreciable effect on diaphragm blood flow and function. Moreover, although vasodilator drugs with actions similar to nitroprusside are used clinically to augment flow to vital organs, our data would indicate that these drugs have no functionally significant effect on blood flow to the fatiguing diaphragm.

Effects of intraphrenic injection of potassium on diaphragm activation.

The purpose of the present study was to determine whether potassium, injected into the arterial supply of the diaphragm, would reflexly alter efferent diaphragmatic motor outflow and systemic arterial pressure. Studies were performed using in situ canine diaphragm muscle strips in which the inferior phrenic artery and vein were cannulated and all other sources of strip blood flow were ligated. Injection of potassium (0.1 meq) into the inferior phrenic artery elicited a small transient (1-2 breaths) decrease in the peak strip tension developed during spontaneous muscle contractions, in peak integrated strip electromyographic (EMG) activity, and in the peak integrated EMG activity of the contralateral hemidiaphragm. This was followed by a more pronounced and more sustained increase in each of these parameters as well as an increase in systemic arterial pressure. This latter excitatory response was qualitatively similar to that induced by the injection of capsaicin (5 and 25 micrograms) into the phrenic artery. Section of the left phrenic nerve abolished the effects of intra-arterial potassium and capsaicin on systemic arterial pressure and right hemidiaphragm EMG activity. These data support the existence of a potent excitatory phrenic-to-phrenic reflex that can be activated by potassium injection into the diaphragm. Activation of this pathway increases diaphragm motor activation and augments systemic arterial pressure.

Parasternal and external intercostal responses to various respiratory maneuvers.

Recent studies suggest that the external intercostal (EI) muscles of the upper rib cage, like the parasternals (PA), play an important ventilatory role, even during eupneic breathing. The purpose of the present study was to further assess the ventilatory role of the EI muscles by determining their response to various static and dynamic respiratory maneuvers and comparing them with the better-studied PA muscles. Applied interventions included 1) passive inflation and deflation, 2) abdominal compression, 3) progressive hypercapnia, and 4) response to bilateral cervical phrenicotomy. Studies were performed in 11 mongrel dogs. Electromyographic (EMG) activities were monitored via bipolar stainless steel electrodes. Muscle length (percentage of resting length) was monitored with piezoelectric crystals. With passive rib cage inflation produced either with a volume syringe or abdominal compression, each muscle shortened; with passive deflation, each muscle lengthened. During eupneic breathing, each muscle was electrically active and shortened to a similar degree. In response to progressive hypercapnia, peak EMG of each intercostal muscle increased linearly and to a similar extent. Inspiratory shortening also increased progressively with increasing PCO2, but in a curvilinear fashion with no significant differences in response among intercostal muscles. In response to phrenicotomy, the EMG and degree of inspiratory shortening of each intercostal muscle increased significantly. Again, the response among intercostal muscles was not significantly different.(ABSTRACT TRUNCATED AT 250 WORDS)