Journal of Applied Physiology
Vol. 83, No. 4, pp. 1405-1405, October 1997
EXERCISE AND MUSCLE

Respiratory muscle reserve in rats during heavy exercise

Luc E. Gosselin, David Megirian, Joshua Rodman, Donna Mueller, and Gaspar A. Farkas

Department of Physical Therapy, Exercise, and Nutrition Sciences, State University of New York at Buffalo, Buffalo, New York 14214

ABSTRACT

INTRODUCTION

METHODS

RESULTS

DISCUSSION

ACKNOWLEDGEMENTS

FOOTNOTES

REFERENCES

ABSTRACT

Gosselin, Luc E., David Megirian, Joshua Rodman, Donna Mueller, and Gaspar A. Farkas. Respiratory muscle reserve in rats during heavy exercise. J. Appl. Physiol. 83(4): 1405-1409, 1997.The extent to which the respiratory pump muscles limit maximal aerobic capacity in quadrupeds is not entirely clear. To examine the effect of reduced respiratory muscle reserve on aerobic capacity, whole body peak oxygen consumption (O_{2 peak}) was measured in healthy Sprague-Dawley rats before and after Sham, unilateral, or bilateral hemidiaphragm denervation (Dnv) surgery. O_{2 peak} was determined by using a graded treadmill running test. Hemidiaphragm paralysis was verified after testing by recording the absence of electromyographic activity during inspiration. Before surgery, O_{2 peak} averaged 86, 87, and 92 ml · kg¹ · min¹ for the Sham, unilateral, and bilateral Dnv groups, respectively. Two weeks after surgery, there was no significant change in O_{2 peak} for either the Sham or unilateral Dnv group. However, O_{2 peak} decreased ~19% in the bilateral Dnv group 2 wk after surgery. These findings strongly suggest that the pulmonary system in rats is designed such that during heavy exercise, the remaining respiratory pump muscles are able to compensate for the loss of one hemidiaphragm, but not of both.

hemiparalysis; diaphragm; denervation; peak oxygen consumption

INTRODUCTION

THE RESPIRATORY SYSTEM must generate adequate ventilation to maintain normal arterial blood-gas and pH homeostasis. A robust system has evolved that involves complex neural integration of respiratory muscle output to achieve effective chest wall displacement and subsequent pulmonary ventilation and gas exchange. Although the diaphragm is considered the primary inspiratory muscle of ventilation during quiet breathing, other respiratory muscles are also recruited (4). The relative contribution of these inspiratory muscles varies depending on posture, states of consciousness, and metabolic demand (10). The integrated responses by the respiratory muscles not only maintain arterial blood-gas and pH status but also minimize the work (or oxygen cost) of breathing (3).

During heavy exercise, minute ventilation increases substantially in response to increased metabolic demand. This exercise hyperpnea is associated with an increased work of breathing, as evidenced by a significant increase in respiratory muscle oxygen consumption (1), increased blood flow to the diaphragm (13), increased oxygen extraction by the diaphragm (16), and increased diaphragm muscle glycogen utilization (11). The increased work of breathing during exercise is reflected in the significant increases in rat diaphragm muscle oxidative enzyme capacity as a consequence of whole body endurance training (7). This adaptation enhances aerobic production of ATP and delays the onset of skeletal muscle fatigue (5). However, despite these significant alterations in indexes of respiratory muscle work and positive adaptations associated with endurance exercise, it is unclear whether the respiratory pump muscles actually impose a limitation on peak whole body oxygen consumption (O_{2 peak}).

Therefore, the purpose of this study was to determine whether decreasing the amount of available respiratory muscle mass by either unilateral or bilateral diaphragm paralysis decreases O_{2 peak} in healthy untrained rats. We hypothesized that if the respiratory muscles as a whole reached their maximum capacity during heavy exercise, loss of one-half or all of the diaphragm in healthy rats would have a deleterious impact on O_{2 peak}.

METHODS

Measurement of O_{2 peak}.

RESULTS

Mean body weights of the Sham and Dnv groups are listed in Table 1. All rats gained weight over the 2-wk postsurgical period, ranging from 8.5% in the bilateral Dnv group to 11.4% in the unilateral Dnv group.

Table  1.   Body weight before and 2 wk after Sham, unilateral, and bilateral Dnv surgery Group n Body Wt, g Presurgery Postsurgery %Change from presurgery Sham 6 321 ± 14  354 ± 11  10.3 Unilateral Dnv 7 317 ± 12  353 ± 8  11.4 Bilateral Dnv 7 331 ± 3  359 ± 6  8.5 Values are means ± SE; n, no. of rats. Sham, phrenic nerves intact; Dnv, hemidiaphragm denervation.

Presence or absence of Dnv was verified in all rats. In the Sham animals during spontaneous inspiration, visual signs of shortening of both sides of the diaphragm were associated with EMG activity (Fig. 1A). In the unilateral Dnv group (Fig. 1B), EMG activity in the right hemidiaphragm was absent and corresponded with visual signs of muscle paralysis (lengthening of the Dnv right hemidiaphragm during contraction of the contralateral side). In the bilateral Dnv group, EMG activity was absent in both hemidiaphragms but was present during inspiration in the parasternal intercostal (Fig. 2).

Pre- and postsurgical running time to exhaustion (Table 2) and maximal running speed did not differ in either the Sham or unilateral Dnv group. Additionally, compared with presurgery, postsurgical O_{2 peak} did not change in either the Sham or unilateral Dnv group (Table 2). The presurgery O_{2 peak} in the bilateral Dnv group averaged 92 ml · kg¹ · min¹ and did not differ from that in either the Sham or unilateral Dnv groups. However, unlike in the latter two groups, O_{2 peak} decreased by ~19% (P < 0.05) after bilateral Dnv. The decrease in O_{2 peak} was associated with a reduction in the running time to exhaustion (Table 2) and a similar percent decrease in maximum running speed. O_{2 peak} values in each animal are shown in Fig. 3.

Table  2.   O2 peak and running time to exhaustion before and 2 wk after Sham, unilateral, and bilateral Dnv surgery Group Presurgery Postsurgery %Change From Presurgery O2 peak, ml · kg1 · min 1 Sham 86.2 ± 1.3  89.2 ± 5.2  +3.5 Unilateral Dnv 87.5 ± 3.2  88.4 ± 5.6  +1.0 Bilateral Dnv 91.8 ± 4.3  74.5 ± 3.5*  18.8 Time to exhaustion, min Sham 14.2 ± 0.48  14.2 ± 0.48  0 Unilateral Dnv 14.0 ± 0.22  14.0 ± 0.53  0 Bilateral Dnv 13.4 ± 0.53  12.0 ± 0.82*  10.6 Values are means ± SE; n = 6 Sham and 7 unilateral and bilateral Dnv rats, respectively. O2 peak, peak O2 consumption. * Significantly different from control, P < 0.05.

DISCUSSION

We hypothesized that the loss of just one hemidiaphragm would significantly decrease exercise O_{2 peak} as a result of decreased respiratory muscle reserve. However, despite unilateral hemidiaphragm paralysis, the rats achieved a O_{2 peak} similar to that obtained before surgery. With complete diaphragm paralysis, O_{2 peak} was decreased by ~19%. Therefore, the results of this study strongly suggest that the respiratory musculature in rats has an adequate reserve to account for the loss of one-half of the diaphragm during maximal exercise, but not for both halves.

Previous studies have examined the effect of diaphragm paralysis on resting arterial blood gases and ventilatory response in rats. In unanesthetized rats with hemidiaphragm paralysis achieved by spinal cord hemisection, respiratory rate increased significantly from 78 breaths/min before surgery to 104 breaths/min 24 h after surgery (6). Pressures of arterial oxygen (Pa_O2) and carbon dioxide (Pa_CO2) were essentially unaffected (6). However, in unanesthetized rats with bilateral hemidiaphragm paralysis, Pa_O2 significantly decreased by ~11 Torr, whereas Pa_CO2 increased ~3 Torr (17). These changes in arterial blood gases were attributed to alterations in breathing pattern. At rest, bilaterally phrenicotomized rats responded by increasing breathing frequency at the expense of a lower tidal volume, thus increasing dead space ventilation (18).

For the rat to achieve a similar O_{2 peak} with unilateral Dnv, or to achieve even the 80% of O_{2 peak} observed after bilateral Dnv, the animal must increase ventilation to match increased metabolic demand. We were unable to measure minute ventilation in our system and therefore do not know to what extent peak exercise minute ventilation is affected as a consequence of unilateral Dnv. The results of this study suggest peak exercise minute ventilation in unilaterally Dnv rats is affected minimally, at worst. Maskrey et al. (18) reported that in awake, bilaterally Dnv rats exposed to hypoxia, minute ventilation increased to ~87% of that observed in Sham animals. The increase in ventilation, however, appeared largely to be because of an increase in frequency rather than in tidal volume (VT) because the maximal VT in the bilaterally Dnv rats was only ~73% of that in Sham animals.

To compensate for the loss of the diaphragm, the rat alters its breathing strategy and recruits other respiratory muscles to maintain adequate ventilation. At rest, bilaterally Dnv rats increase activity of the internal and external intercostal muscles during inspiration (19). In addition, activity of the external and internal oblique muscles also increases during expiration, presumably to cause caudal displacement of the lower rib cage and thus assist lung deflation. This alteration in recruitment pattern may also serve as a mechanism to increase breathing frequency. Changes in recruitment strategy during breathing at rest are also noted in the canine Dnv model. Although paralysis of a hemidiaphragm does not appear to alter the resting length of the intact hemidiaphragm, EMG activity of the intact hemidiaphragm increases significantly during inspiration and is associated with greater shortening (12). With bilateral paralysis, costal diaphragm end-expiratory length increases slightly from control, and costal shortening is significantly less compared with control and unilateral paralysis. Other respiratory muscles are also recruited to compensate for the loss of the diaphragm (12). For example, shortening of the transverse abdominis is significantly higher than control by 22.7 and 42.5% after unilateral and bilateral hemidiaphragm paralysis, respectively. Compared with control, shortening of the parasternal intercostal muscles also is significantly higher after both unilateral and bilateral hemidiaphragm paralysis. Because of the altered recruitment strategy, dogs at rest are able to maintain normal resting end-tidal CO₂ levels after both unilateral and bilateral hemidiaphragm paralysis.

The recruitment strategy the rats in the present study adopt to compensate for the loss of one or both hemidiaphragms during exercise is unknown. However, on the basis of data obtained from resting rats, we can safely hypothesize that recruitment of both inspiratory and expiratory accessory respiratory muscles is greater. Further studies are necessary to determine the precise recruitment pattern employed by rats to achieve the necessary exercise hyperpnea.

In addition to alterations in recruitment strategy during breathing, other adaptations may occur to optimize ventilation. The effectiveness of the diaphragm depends, in part, on the extent of fiber shortening, which results in flattening of the diaphragm and subsequent lung expansion. Loss of an active hemidiaphragm will result in lengthening of the paralyzed muscle, thereby altering the extent of diaphragm flattening. In a separate group of rats, the concentration of collagen in the Dnv hemidiaphragm was increased by 40% 2 wk after Dnv (L. Gosselin, unpublished observation). Such an increase presumably results in increased stiffness of the Dnv hemidiaphragm and may assist diaphragm flattening during contraction of the intact contralateral side. However, further studies are required to support this idea.

The nature of the reduction in O_{2 peak} is not entirely clear. Assuming all other factors remain constant, alveolar minute ventilation would have to decrease ~35-45% to cause a 20% reduction in O_{2 peak}. There was no difference in the slope of the oxygen cost of running before or after bilateral phrenic nerve transection. Thus the reduction in O_{2 peak} in the bilaterally Dnv rats appears to be due primarily to a decrease in maximal running speed and time to exhaustion. Because arterial blood gases were not measured in this study, it is unknown whether these rats were hypoxic during treadmill running. Another possibility for the reduction in O_{2 peak} is that efficiency of ventilation is markedly decreased without the diaphragm, and therefore the accessory chest wall muscles have a higher metabolic demand that "steals" blood flow from the locomotor skeletal muscles. We think this is unlikely because additional blood generally reserved for the diaphragm should be available for any increased work by the accessory respiratory muscles. This point is highly speculative, and future studies are required before any conclusive statement can be made.

The effectiveness of the diaphragm depends, in part, on its ability to generate force. A modest reduction (15-25%) in maximal specific force of the diaphragm muscle has been observed in several experimental models, including chronic obstructive pulmonary disease (14), hypothyroidism (9), aging (8, 20), and malnutrition (15). It has been speculated that such a reduction may significantly impair pulmonary gas exchange, especially during periods when the ventilatory demand is high. However, the results of the present study suggest that despite a 50% reduction in the available mass of the diaphragm, an adequate reserve exists in the remaining respiratory musculature to compensate for this loss. Moreover, even with complete diaphragm paralysis, rats are still able to achieve 80% of their presurgical O_{2 peak} (with the diaphragm intact). These findings highlight the remarkable reserve of the respiratory musculature, as well as the ability of the pulmonary system to alter its recruitment strategy to compensate for the loss of part or all of the diaphragm.

ACKNOWLEDGEMENTS

The authors are grateful to Dr. Frank Cerny for his input on this project.

FOOTNOTES

Address for reprint requests: L. E. Gosselin, Dept. of Physical Therapy, Exercise, and Nutrition Sciences, 405 Kimball Tower, State University of New York at Buffalo, Buffalo, NY 14214 (E-mail: gosselin{at}acsu.buffalo.edu).

Received 10 April 1997; accepted in final form 23 July 1997.

REFERENCES