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Biomedical subjects

G Bowes

Publications and source records attributed to G Bowes.

121 records · Page 7Linked to original sources
PubMed1981

Upper airway occlusion induced by diaphragm pacing for primary alveolar hypoventilation: implications for the pathogenesis of obstructive sleep apnea.

This report describes a patient with primary alveolar hypoventilation who, after 2 yr of successful treatment with nocturnal oxygen, developed severe hypoxemia and hypercapnia during sleep, morning headaches, and daytime fatigue. Sleep studies demonstrated prolonged periods of hypoventilation and apnea without evidence of upper airway occlusion. Therefore, a phrenic nerve stimulator was implanted to allow pacing of the diaphragm during sleep. However, diaphragm pacing was accompanied by paradoxical movement of the rib cage and upper airway occlusion during sleep, and was unsuccessful in maintaining adequate ventilation. Therefore, the patient underwent a tracheostomy after which diaphragm pacing maintained adequate nocturnal ventilation; however, paradoxical movement of the rib cage persisted. The induction of upper airway occlusion as a result of diaphragm pacing, in contrast to the absence of occlusion during spontaneous breathing, highlights the importance of the normal temporal coordination of inspiratory activation of the upper airway muscles and diaphragm. The findings have important implications for the pathogenesis of obstructive sleep apneas in general.

Adult↗
PubMed1981

Role of metabolic CO2 production in ventilatory response to steady-state exercise.

We examined the role of metabolic CO2 production in the hyperpnea of muscular exercise by comparing the response of alveolar ventilation to moderate levels of exercise with the response to venous infusion of CO2 at rest. Studies were performed in four awake sheep that were trained to run on a treadmill. The sheep had been cannulated for veno-venous extracorporeal perfusion so that CO2 could be infused into the peripheral venous blood through membrane lungs in the perfusion circuit. The sheep breathed room air through an endo-tracheal tube inserted through a tracheostomy, and samples of expired gas were collected for measurement of the rates of CO2 production and O2 consumption. All measurements were made in the steady state. In each of the four sheep, the relationship between alveolar ventilation and the rate of CO2 production could be described by a single linear function (r greater than 0.99; P less than 0.001), regardless of whether CO2 production was increased by exercise, venous CO2 infusion, or combinations of both procedures. This relationship applied for values of CO2 production up to 350% of control. In contrast, no unique relationship was found between the rate of alveolar ventilation and either the rate of O2 consumption, cardiac output, or mixed venous blood gas pressures. The findings indicate that the hyperpnea of mild to moderate steady-state exercise can be attributed to the associated increase in the rate of CO2 production. Therefore, there is no need to invoke obligatory nonmetabolic stimuli to account for the ventilatory response to steady-state exercise.

Animals↗
PubMed1980

C(4) Acid Metabolism and Dark CO(2) Fixation in a Submersed Aquatic Macrophyte (Hydrilla verticillata).

The CO(2) compensation point of the submersed aquatic macrophyte Hydrilla verticillata varied from high (above 50 microliters per liter) to low (10 to 25 microliters per liter) values, depending on the growth conditions. Plants from the lake in winter or after incubation in an 11 C/9-hour photoperiod had high values, whereas summer plants or those incubated in a 27 C/14-hour photoperiod had low values. The plants with low CO(2) compensation points exhibited dark (14)CO(2) fixation rates that were up to 30% of the light fixation rates. This fixation reduced respiratory CO(2) loss, but did not result in a net uptake of CO(2) at night. The low compensation point plants also showed diurnal fluctuations in titratable acid, such as occur in Crassulacean acid metabolism plants. However, dark fixation and diurnal acid fluctuations were negligible in Hydrilla plants with high CO(2) compensation points.Exposure of the low compensation point plants to 20 micromolar (14)CO(2) resulted in 60% of the (14)C being incorporated into malate and aspartate, with only 16% in sugar phosphates. At a high CO(2) level, the C(4) acid label was decreased. A pulse-chase study indicated that the (14)C in malate, but not aspartate, decreased after a long (270-second) chase period; thus, the C(4) acid turnover was much slower than in C(4) plants.Phosphoenolpyruvate carboxylase activity was high (330 micromoles per milligram chlorophyll per hour), as compared to ribulose bisphosphate carboxylase (20 to 25), in the plants with low compensation points. These plants also had a pyruvate, Pi dikinase activity in the leaves of 41 micromoles per milligram chlorophyll per hour, which suggests they are not C(3) plants. NAD- and NADP(+)-malate dehydrogenase activities were 6136 and 24.5 micromoles per milligram chlorophyll per hour, respectively. Of the three decarboxylating enzymes assayed, the activities of NAD- and NADP(+)-malic enzyme were 104.2 and 23.7 micromoles per milligram chlorophyll per hour, while phosphoenolpyruvate carboxykinase was only 0.2.Low compensation point Hydrilla plants fix some CO(2) into C(4) acids, which can be decarboxylated for later refixation, presumably into the Calvin cycle. Refixation would be advantageous in summer lake environments where the CO(2) levels are high at night but low during the day. Hydrilla does not fit any of the present photosynthetic categories, and may have to be placed into a new group, together with other submersed aquatic macrophytes that have environmentally variable CO(2) compensation points.

Journal Article↗
PubMed1980

Effect of sleep fragmentation on ventilatory and arousal responses of sleeping dogs to respiratory stimuli.

We studied the effects of sleep fragmentation on arousal and ventilatory responses to hyperoxic hypercapnia, isocapnic hypoxia, and chemical stimulation of the larynx during sleep in 5 dogs. Sleep fragmentation was induced by repeatedly arousing the dogs with acoustic stimuli throughout 2 to 3 consecutive nights. Responses to respiratory stimuli were then studied during a subsequent daytime sleep. Arterial O2 saturation was measured with an ear oximeter, and sleep stage was determined by electroencephalographic and behavioral criteria. Hypercapnic and hypoxic ventilatory responses were unimpaired by sleep fragmentation. In contrast, alveolar PCO2 levels at arousal increased after sleep fragmentation, from a mean +/- SEM of 52.2 +/- 1.4 mm Hg to 55.6 +/- 1.5 mm Hg (p < 0.05) during slow-wave sleep, and from 57.9 +/- 1.5 mm Hg to 61.3 +/- 2.2 mm Hg (p < 0.05) during rapid-eye movement sleep. Similarly, arterial O2 saturation at arousal decreased after sleep fragmentation from 80.1 +/- 1.0% to 70.2 +/- 2.7% (p < 0.05) during slow-wave sleep, and from 66.3 +/- 3.6% to < 55% (p < 0.05) during rapid-eye-movement sleep. Arousal responses to laryngeal stimulation were also impaired after sleep fragmentation. We conclude that arousal responses to respiratory stimuli are decreased by sleep fragmentation.

Animals↗
PubMed1978

Photosynthetic and Carbohydrate Metabolism in Isolated Leaf Cells of Digitaria pentzii.

Mesophyll cells and bundle sheath strands were isolated rapidly from leaves of the C(4) species Digitaria pentzii Stent. (slenderstem digitgrass) by a chopping and differential filtration technique. Rates of CO(2) fixation in the light by mesophyll and bundle sheath cells without added exogenous substrates were 6.3 and 54.2 micromoles of CO(2) per milligram of chlorophyll per hour, respectively. The addition of pyruvate or phosphoenolpyruvate to the mesophyll cells increased the rates to 15.2 and 824.6 micromoles of CO(2) per milligram of chlorophyll per hour, respectively. The addition of ribose 5-phosphate increased the rate for bundle sheath cells to 106.8 micromoles of CO(2) per milligram of chlorophyll per hour. These rates are comparable to those reported for cells isolated by other methods. The K(m)(HCO(3) (-)) for mesophyll cells was 0.9 mm; for bundle sheath cells it was 1.3 mm at low, and 40 mm at higher HCO(3) (-) concentrations. After 2 hours of photosynthesis by mesophyll cells in (14)CO(2) and phosphoenolpyruvate, 88% of the incorporated (14)C was found in organic acids and 0.8% in carbohydrates; for bundle sheath cells incubated in ribose 5-phosphate and ATP, more than 58% of incorporated (14)C was found in carbohydrates, mainly starch, and 32% in organic acids. These findings, together with the stimulation of CO(2) fixation by phosphoenolpyruvate for mesophyll cells and by ribose 5-phosphate plus ATP for bundle sheath cells, and the location of phosphoenolpyruvate and ribulose bisphosphate carboxylases in mesophyll and bundle sheath cells, respectively, are in accord with the scheme of C(4) photosynthesis which places the Calvin cycle in the bundle sheath and C(4) acid formation in mesophyll cells.Starch and reducing sugars were present in both mesophyll and bundle sheath cells following a period of photosynthesis by whole leaves. However, when isolated cells were exposed to (14)CO(2) in the light, even with appropriate exogenous substrates, only bundle sheath cells accumulated appreciable amounts of labeled carbohydrates. Incubation of mesophyll cells in the light with ATP and either pyruvate and inorganic phosphate, or phosphoenolpyruvate, or 3-phosphoglycerate resulted in large increases in total carbohydrates. The 3-phosphoglycerate treatment produced the greatest increase. These results could not be explained on the basis of increased CO(2) fixation. They suggest that mesophyll cells are able to metabolize exogenously supplied 3-carbon compounds to carbohydrates, despite the apparent inability of these cells to utilize CO(2) for this purpose, and support the view that in the whole leaf 3-phosphoglycerate is transported from bundle sheath to mesophyll cells, where it is reduced to carbohydrate.Sucrose and sucrose-phosphate synthetases and invertase were localized mainly in bundle sheath cells. ADP-Glucose starch synthetase and amylase were present mainly in bundle sheath cells whereas starch phosphorylase was present mainly in mesophyll cells.

Journal Article↗
PubMed1976

Comparison of the photosynthetic characteristics of three submersed aquatic plants.

Light- and CO(2)-saturated photosynthetic rates of the submersed aquatic plants Hydrilla verticillata, Ceratophyllum demersum, and Myriophyllum spicatum were 50 to 60 mumol O(2)/mg Chl.hr at 30 C. At air levels of CO(2), the rates were less than 5% of those achieved by terrestrial C(3) plants. The low photosynthetic rates correlated with low activities of the carboxylation enzymes. In each species, ribulose 1,5-diphosphate carboxylase was the predominant carboxylation enzyme. The apparent K(m)(CO(2)) values for photosynthesis were 150 to 170 mum at pH 4, and 75 to 95 mum at pH 8. The K(m)(CO(2)) of Hydrilla ribulose 1,5-diphosphate carboxylase was 45 mum at pH 8. Optimum temperatures for the photosynthesis of Hydrilla, Myriophyllum, and Ceratophyllum were 36.5, 35.0, and 28.5 C, respectively. The apparent ability of each species to use HCO(3) (-) ions for photosynthesis was similar, but at saturating free CO(2) levels, there was no indication of HCO(3) (-) use. Increasing the pH from 3.1 to 9.2 affected the photosynthetic rate indirectly, by decreasing the free CO(2). With saturating free CO(2) (0.5 mm), the maximum photosynthetic rates were similar at pH 4 and 8. Carbonic anhydrase activity, although much lower than in terrestrial C(3) plants, was still in excess of that required to support HCO(3) (-) utilization.Hydrilla and Ceratophyllum had CO(2) compensation points of 44 and 41 mul/l, respectively, whereas the value for Myriophyllum was 19. Relatively high CO(2) compensation points under 1% O(2) indicated that some "dark" respiration occurred in the light. The inhibition of photosynthesis by O(2) was less than with terrestrial C(3) plants. Glycolate oxidase activity was 12.3 to 27.5 mumol O(2)/mg Chl.hr, as compared to 78.4 for spinach. Light saturation of photosynthesis occurred at 600 to 700 mueinsteins/m(2).sec in each species grown under full sunlight. Hydrilla had the lowest light compensation point, and required the least irradiance to achieve the half-maximal photosynthetic rate.Field measurements in a Hydrilla mat indicated that in the afternoon, free CO(2) dropped to zero, and O(2) rose to over 200% air saturation. Most photosynthetic activity occurred in the morning when the free CO(2) was highest and O(2) and solar radiation lowest. The low light requirement of Hydrilla probably provides a competitive advantage under these field conditions.

Journal Article↗
PubMed1975

pH Dependence of the Km(CO(2)) of Ribulose 1,5-Diphosphate Carboxylase.

The Km(CO(2)) values of ribulose 1,5-diphosphate carboxylase in freshly ruptured spinach (Spinacia oleracea L.) chloroplasts and in the purified form isolated from spinach leaves were found to be pH dependent. Raising the pH of the assay solution produced a substantial decrease in the Km(CO(2)) of both enzyme systems. In freshly ruptured chloroplasts at pH 7.2 the Km(CO(2)) was 25 mum, at pH 8 it decreased to 19 mum, and at pH 8.8 a further decrease to 7 mum was found. With the purified enzyme at pH 7.2 the Km(CO(2)) was 147 mum, while the corresponding Km values for pH 8 and 8.8 were 34 and 15 mum CO(2), respectively. The latter figure approximates the physiological Km(CO(2)) of 10 mum estimated for photosynthesizing leaves and intact chloroplasts. The maximum velocity for both enzyme systems at optimum substrate levels was at pH 8, but the highest calculated rate of CO(2) uptake at atmospheric CO(2) levels occurred at pH 8.8. These results support the proposal that the light-induced efflux of protons out of the chloroplast stroma may be a major factor involved with the reported in vivo light activation of ribulose 1,5-diphosphate carboxylase.

Journal Article↗