Carbon dioxide and acid base balance in the isolated rat diaphragm

Structural and functional changes of peripheral muscles in chronic obstructive pulmonary disease patients

PURPOSE OF REVIEW

The purpose of this review is to identify new advances in our understanding of skeletal muscle dysfunction in patients with chronic obstructive pulmonary disease (COPD).

RECENT FINDINGS

Recent studies have confirmed the relevance of muscle dysfunction as an independent prognosis factor in COPD. Animal studies have shed light on the molecular mechanisms governing skeletal muscle hypertrophy/atrophy. Recent evidence in patients with COPD highlighted the contribution of protein breakdown and mitochondrial dysfunction as pathogenic mechanisms leading to muscle dysfunction in these patients.

SUMMARY

COPD is a debilitating disease impacting negatively on health status and the functional capacity of patients. COPD goes beyond the lungs and incurs significant systemic effects among which muscle dysfunction/wasting is one of the most important. Muscle dysfunction is a prominent contributor to exercise limitation, healthcare utilization and an independent predictor of morbidity and mortality. Gaining more insight into the molecular mechanisms leading to muscle dysfunction/wasting is key for the development of new and tailored therapeutic strategies to tackle skeletal muscle dysfunction/wasting in COPD patients.

Effect of acute sodium bicarbonate ingestion on excess CO2 output during incremental exercise

The effect of bicarbonate ingestion on total excess volume of CO2 output (CO2 excess), due to bicarbonate buffering of lactic acid in exercise, was studied in eight healthy male volunteers during incremental exercise on a cycle ergometer performed after ingestion (0.3 g.kg-1 body mass) of CaCO3 (control) and NaHCO3 (alkalosis). The resting arterialized venous blood pH (P < 0.05) and bicarbonate concentration ([HCO3-]b; P < 0.01) were significantly higher in acute metabolic alkalosis [AMA; pH, 7.44 (SD 0.03); [HCO3-]b, 29.4 (SD 1.5) mmol.l-1] than in the control [pH, 7.39 (SD 0.03); [HCO3-]b, 25.5 (SD 1.0) mmol.l-1]. The blood lactate concentrations ([la-]b) during exercise below the anaerobic threshold (AT) were not affected by AMA, while significantly higher [la-]b at exhaustion [12.29 (SD 1.87) vs 9.57 (SD 2.14) mmol.l-1, P < 0.05] and at 3 min after exercise [14.41 (SD 1.75) vs 12.26 (SD 1.40) mmol.l-1, P < 0.05] were found in AMA compared with the control. The CO2 excess increased significantly from the control [3177 (SD 506) ml] to AMA [3897 (SD 381) ml; P < 0.05]. The CO2 excess per body mass was found to be significantly correlated with both the increase of [la-]b from rest to 3 min after exercise (delta[la-]b; r = 0.926, P < 0.001) and with the decrease of [HCO3-]b from rest to 3 min after exercise (delta [HCO3-]b; r = 0.872, P < 0.001), indicating that CO2 excess per body mass increased linearly with both delta [la-]b and delta [HCO3-]b.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of acidosis on tension development in mammalian skeletal muscle

Effect of carbon dioxide acidosis on the tension development in a rat skeletal muscle (extensor digitorum longus) was examined at different temperatures. Experiments were done in vitro and with direct stimulation, mostly at constant temperatures between 30-35 degrees C and 12-20 degrees C. A decrease of saline pH (8.0 to 6.5) with carbon dioxide increased the twitch and the tetanic tensions and enhanced the tension relaxation in experiments done at high temperatures. At low temperatures the same procedure decreased the tetanic tension and enhanced the tension relaxation. An increased tetanic tension at the high temperatures and a decreased tetanic tension at the low temperatures were also obtained at constant saline pH, with procedures known to decrease intracellular pH. The observations made at higher temperatures are discussed in relation to human muscle performance in exercise.

Intracellular pH regulation during prolonged hypoxia in rats

Conscious rats maintained for three weeks at PB 370-380 Torr were studied in a chamber where PIO2 was maintained at 68-70 Torr at ambient barometric pressure (740-750 Torr). Controls were pair-fed rats maintained at ambient barometric pressure and studied at ambient PIO2 for 4 h. Steady-state intracellular pH (pHi) of left and right ventricle, and of tibialis anterior, quadriceps and diaphragm was determined from the distribution of 5,5-dimethyl-2,4-oxazolidinedione (DMO). Apparent non-bicarbonate buffer value (beta app) was calculated as the ratio of the change in HCO3- concentration to the change in pH elicited by the increase in PCO2. beta app of plasma, tibialis anterior, quadriceps and diaphragm was approximately 2, 3, 6 and 12 times higher, respectively, in hypoxic than in normoxic rats. Neither left nor right ventricular beta app was significantly changed by prolonged hypoxia. In the hypoxic animals, bilateral nephrectomy abolished the increase in beta app of plasma, tibialis anterior and quadriceps, and moderated the increase in beta app of diaphragm. No significant effect of nephrectomy was observed in beta app of either left or right ventricle. The results indicate that in the skeletal muscles studied under conditions of an acid load in the form of increased PCO2, intracellular pH is better regulated in hypoxic than in normoxic rats. The effects of nephrectomy suggest that this is due, at least in part, to a more effective renal compensation in hypoxic than in normoxic rats. Prolonged hypoxia, on the other hand, does not affect the cell pH regulation of right or left ventricle.

Acid-base regulation in prolonged hypoxia: effect of increased PCO2

Conscious rats maintained for 3 wk at PB 370-380 Torr were studied in a chamber where PIO2 was kept at 68-70 Torr at ambient barometric pressure (740-750 Torr). Blood samples were obtained through an arterial catheter. Controls were pair-fed rats maintained at ambient barometric pressure and studied at PIO2 68-70 Torr for 4 h (acute hypoxia) or at ambient PIO2 (normoxia). Arterial blood pH of 3-wk hypoxic rats was not different from that of normoxic rats. Hypercapnia was produced by increasing PICO2 for 4 h. The 3-wk hypoxic rats showed the highest apparent non-bicarbonate buffer value of arterial blood (beta app): 77 mmol/(pH X kg), compared to 38 in normoxia and 43 mmol/(pH X kg) in acute hypoxia. Comparison of beta app at different times of hypercapnia in intact and in nephrectomized rats suggests that the high beta app of prolonged hypoxia is largely due to an increased renal compensation, and, to a smaller extent, to increased chemical buffering. While the extracellular fluid of normoxic and acute hypoxic rats showed a net gain of base of non-renal origin during hypercapnia, the 3-wk hypoxic rats showed a net non-renal base loss, which may be masked by the increased renal compensation.

The influence of muscle respiration and glycolysis on surface and intracellular pH in fibres of the rat soleus

Extracellular pH (pHo) and intracellular pH (pHi) of superficial fibres of the rat soleus muscle were measured in vitro using pH-sensitive glass micro-electrodes. The origin of the pH gradient existing between the bulk phase of extracellular solution and the surface of muscle fibres was investigated. The pHo decreased almost linearly over a distance of 285 microns from bulk solution to fibre surface. The magnitude of the bulk-surface pH gradient is greater in the mid region of the muscle than close to the tendon. Decreasing the superfusate velocity increased the magnitude of the pH gradient. Reducing the buffer capacity of the superfusing solution had the same effect. Inhibiting the aerobic metabolism or stimulating it acidified the fibre surface. Inhibiting glycolysis alone, or both aerobic metabolism and glycolysis, alkalinized the fibre surface. Inhibiting the membrane ionic exchange process involved in pHi regulation had no effect on surface pH. Changing the rate of aerobic or anaerobic metabolism quickly modified pHi in most cases. In conclusion the bulk-surface pH gradient seems to result mainly from diffusion of CO2 and lactic acid across an unstirred layer of fluid covering the surface of muscle fibres.

The effects of hypocapnia on intracellular pH and bicarbonate

The intracellular pH (pHi) and bicarbonate concentration ([HCO3-]icw) of cardiac and skeletal muscles were monitored during respiratory alkalosis in order to further elucidate the homeostatic processes which operate in these tissues to ameliorate deviations from normal acid-base status. Rats were mechanically hyperventilated to induce hypocapnia, pHi was determined by the DMO method, and [HCO-3]icw was calculated from the Henderson-Hasselbalch equation using pHi and the partial pressure of carbon dioxide of vena caval blood. A significant intracellular alkalosis occurred in both cardiac and skeletal muscles during hypocapnia, but the changes in pHi were less in the heart than in skeletal muscle. The decreases in cardiac [HCO-3]icw were greater than those attributable to the physicochemical buffering of the heart. These data are consistent with an intramyocardial source of protons other than physicochemical buffering during respiratory alkalosis. The decreases in skeletal muscle [HCO-3]icw were less than those due to physicochemical buffering. These data are consistent with a net extrusion from skeletal muscles cells of the protons derived from physicochemical buffering during respiratory alkalosis.