Effects of small changes of blood volume on oxygen delivery and tissue oxygenation

Circulatory effects of small (approximately 10%) changes in blood volume were examined in resting and exercising dogs: controls; group A (-200 ml blood); group B (+200 ml blood); group C (+200 ml 6% dextran). In exercise, cardiac output (Q) increased more in Group A than controls (510.4 ml . kg-1 . min-1 compared to 429.6 ml . kg-1 . min-1; P less than 0.05); oxygen delivery (cardiac output x arterial O2 content) and mixed venous oxygen tension (PVO2) were unchanged from exercising controls. Hypervolemia (group B) did not change Q or O2 delivery compared to controls, but caused a greater reduction in exercise PVO2 (29.3 mmHg compared to 33.1 mmHg in controls; P less than 0.01). Resting PVO2 as raised in group C (50.0 mmHg compared to 46.3 mmHg; P less than 0.05) and exercise PVO2 was reduced less (35.5 mmHg compared to 33.1 mmHg in controls; P less than 0.05). O2 delivery in exercise was higher than in controls (123.4 ml . kg-1 . min-1 compared to 94.3 ml . kg-1 . min-1; P less than 0.001). During exercise, O2 consumption was raised from base line to 34.9 ml . kg-1 . min-1 in controls and raised further to 41.4 ml . kg-1 . min-1 in group A, 44.4 ml . kg-1 . min-1 in group B, and 41.2 ml . kg-1 . min-1 in group C (P less than 0.01). Changes of blood volume that lie within physiological limits thus significantly modify the circulatory response to changed O2 requirements, and also change the metabolic cost of exercise.     

Muscle energy metabolism and electrolyte shifts during low-level prolonged static contraction in man

Seven men performed one-legged isometric knee extension at 5% MVC for 1 h. Total body oxygen uptake amounted to 451 (420-471) ml min-1 and oxygen uptake over the contracting leg to 200 (172-216) ml min-1, with no changes occurring during the 1 h contraction. Venous O2 tension decreased from 29.4 mmHg at rest to 23.1 mmHg with contraction and CO2 tension tended to increase from a resting value of 50.5 mmHg to 57.2 mmHg (n.s.). No similar changes occurred in arterial O2 and CO2 tensions. There was a small but continuous glucose uptake at both rest and throughout the contraction, whereas a lactate release occurred only in the early phase (2 min) of contraction. Muscle glycogen content was 312 mmol kg-1 dry wt at rest, no significant changes had occurred following 30 min or 1 h of contraction. Arterial and venous Hct and Hb values indicated that a flux of water occurred from the vascular bed to the contracting muscle, in which H2O increased from 3.06 l kg-1 dry wt at rest to 3.30 l kg-1 dry wt after 1 h at 5% MVC. Simultaneously potassium (K), was released from the muscle throughout contraction with a mean venous-arterial difference of 0.25 mmol l-1. With a plasma flow of 335 ml min-1 kg-1 wet wt the K loss amounted to 5 mmol kg-1 wet wt or roughly 5% of the total muscle K content.(ABSTRACT TRUNCATED AT 250 WORDS)     

Energy Metabolism of Rat Skeletal Muscle Modulated by the Rate of Perfusion Flow

In order to advance our understanding of the phenomenon of flow-induced increases in the metabolism of the relaxed muscle, the metabolic rate of the isolated rat gracilis muscle was investigated at 28 degrees C in vitro. The muscle was perfused with cell-free Krebs-Henseleit bicarbonate buffer containing 5% bovine serum albumin and 5 mM glucose, saturated with a gas mixture of 95% O2 and 5% CO2 and simultaneously superfused with a medium saturated with with a low O2 gas mixture (1% O2, 5% CO2 and 94% N2). Two different perfusion flow rates (0.054 and 0.100 ml min-1) have been used. Their influence on oxygen consumption and lactate production has been measured. After a 100 min perfusion period, the muscle was freeze-clamped and analysed for ATP, phosphocreatine, creatine, lactate, pyruvate, inorganic phosphate and glycogen content. The energy state of the cell and the proportions of glycolytic and mitochondrial fluxes of ATP synthesis were evaluated. During perfusion at the low flow rate of 0.054 ml min-1, the oxygen uptake was 45 +/- 9 nmol min-1 (g wet wt)-1, accompanied by a dominance of anaerobic glycolytic synthesis of ATP over mitochondrial ATP synthesis, even though the total delivery of oxygen to muscle was three times higher than oxygen consumption. Increasing the perfusion flow rate to 0.100 ml min-1 increased the oxygen uptake to 120 +/- 6 nmol min-1 (g wet wt)-1, thus leading to a prevalence of mitochondrial ATP synthesis over glycolytic ATP synthesis. The inner stores of glycogen served as the main substrate of energy metabolism and the role of exogenous substrates in the flow-stimulated increase of oxygen uptake was negligible. The increase in perfusion rate also enhanced the energy state of the muscle fibres, which was expressed either as the creatine charge or as the value of the change of Gibbs free energy of ATP hydrolysis. Data indicate that the change of perfusion flow rate per se, apart from oxygen and exogenous substrate supply, elicits changes in the regulation of energy metabolism within non-contracting skeletal muscle under open microcirculation.     

Cardiovascular effects of infused adenosine in man: potentiation by dipyridamole

In a single blind randomized study, eight normal subjects (23-40 years) received on two separate days adenosine as a constant i.v. infusion with doses increasing from 0.005-0.1 mg kg-1 min-1 following either i.v. dipyridamole 0.4 mg kg-1 per 10 min or a corresponding volume of saline. Heart rate, blood pressure and skin temperature were measured. Following saline all subjects tolerated adenosine 0.07 mg kg-1 min-1, six also tolerated 0.09 mg kg-1 min-1. Both heart rate and skin temperature increased with adenosine in a dose-related manner. Thus, adenosine 0.09 mg kg-1 min-1 was associated with an increase in heart rate (mean +/- SD) from baseline before saline with 16 +/- 10 b.p.m. (P less than 0.01) and an increase in skin temperature with 1.3 +/- 0.8 degrees C (P less than 0.02). Dipyridamole, which inhibits the cellular uptake of adenosine was associated with a change in heart rate similar to that induced by adenosine. Furthermore, when adenosine was infused following dipyridamole the changes in heart rate and skin temperature were potentiated as compared with adenosine following saline. Thus, following dipyridamole an increase in heart rate with 15 b.p.m. above baseline was obtained with 0.005 mg kg-1 min-1 of adenosine as compared with 0.08-0.09 mg kg-1 min-1 of adenosine following saline. Blood pressure did not change in any of the studies. Exogenous adenosine in man has a dose-related positive chronotropic effect and a local dilatory effect in the skin microcirculation. Dipyridamole potentiates the cardiovascular effects of infused adenosine in man.     

Cardiopulmonary fitness in a sample of Malaysian population

Lung capacity and maximum oxygen uptake (VO2max) were measured directly in 167 healthy males, from all the main races in Malaysia. Their ages ranged from 13 to 59 years. They were divided into five age groups (A to E), ranging from the second to the sixth decade. Lung capacities were determined using a dry spirometer and VO2max was taken as the maximum rate of oxygen consumption during exhaustive exercise on a cycle ergometer. Mean forced vital capacity (FVC) was 3.3 +/- 0.5 l and it correlated negatively with age. Mean VO2max was 3.2 +/- 0.2 l.min-1 (56.8 +/- 3.5 ml.kg-1.min-1) in Group A (13-19 years) compared to 1.7 +/- 0.2 l.min-1 (28.9 +/- 2.9 ml.kg-1.min-1) in Group E (50-59 years). Regression analysis revealed an age-related decline in VO2max of 0.77 ml.kg-1.min-1.year-1. Multiple regression of the data gave the following equations for the prediction of an individual's VO2max: VO2max (l.min-1) = 1.99 + 0.035 (weight)-0.04 (age), VO2max (ml.kg-1.min-1) = 67.7-0.77 (age), where age is in years, weight in kg. In terms of VO2max as an index of cardiopulmonary performance. Malaysians have a relatively lower capacity when related to the Swedish norms or even to those of some Chilean workers. Malaysians were, however, within the average norms of the American Heart Association's recommendations. Age-related decline in VO2max was also somewhat higher in the Malaysians.     

Diet-induced thermogenesis in well-trained subjects

The purpose of this study was to examine the possible relationship between the thermogenic response to a mixed meal and the aerobic capacity in healthy subjects. Fourteen male subjects participated, and their maximal oxygen uptake was determined on a bicycle exercise ergometer. Two groups, each comprising seven individuals, were compared: a well-trained group, with an oxygen uptake of 58 +/- 2 ml min-1 kg-1 and a sedentary group, with an oxygen uptake of 39 +/- 2 ml min-1 kg-1. Respiratory gas exchange was measured continuously for 1 h in the basal state and then for 3 h postprandially. The subjects ingested a test meal in liquid form, consisting of 17% kJ protein, 28% kJ lipids and 55% kJ carbohydrates, and corresponding to 60% of the individually computed 24-h basal energy expenditure. Basal oxygen uptake and energy expenditure were similar in the two groups. After the meal, pulmonary oxygen uptake and energy expenditure rose rapidly and reached a plateau after 1 h. The responses were no different in the two groups: the average rise in pulmonary oxygen uptake above basal during the whole study period was 24.0 +/- 2.1% in well-trained and 26.7 +/- 1.5% in sedentary subjects (NS); the corresponding values for energy expenditure were 25.0 +/- 2.1% and 29.0 +/- 1.6% (NS). Also, when expressed in absolute terms the increments above basal were not significantly different. There was no discernible relationship between the individual thermogenic response and maximal oxygen uptake. In conclusion, the present findings do not indicate that diet-induced thermogenesis is proportional to aerobic capacity in healthy young men.     

Ventilatory response and arterial potassium concentration during incremental exercise in patients with chronic airways obstruction

The relationship of ventilation response (VE) to arterial potassium concentration (K+) during ramp incremental exercise was assessed in nine patients with chronic obstructive pulmonary disease (COPD), and in 10 healthy subjects. For COPD patients the maximum oxygen uptake (VOmax) was 19.6 +/- 3.8 ml kg-1 min-1 (+/- SD), and percentage of forced expired volume at 1 s (% FEV1) was 47.8 +/- 10.4%. In healthy subjects, VO2max was 44.4 +/- 7.0 ml kg-1 min-1 and FEV1 was 89.7 +/- 7.4%. Breath-by-breath determinations for VE, oxygen uptake (VO2) and carbon dioxide output (VCO2), as well as determinations for K+, partial pressure of oxygen (PO2), partial pressure of carbon dioxide (PCO2), pH and lactate in arterial blood were performed during a workout on an exercise bicycle at a ramp function work rate of 20 W min-1, preceded by a 40 min warm-up period. The major findings in the present study are: (1) that there is a linear relation between ventilation and arterial K+ concentration during ramp exercise in both healthy subjects and COPD patients; (2) that the slope of the VE-K+ relationship is significantly lower in COPD patients (16.2 +/- 7.3 l min-1 mM-1) than in normal subjects (37.4 +/- 6.9 l min-1 mM-1, P less than 0.01); and, (3) that the slope of the VE-K+ relationship is significantly related to the ability to ventilate during maximal exercise in both healthy subjects and COPD patients (P less than 0.05). It is thought that the significantly reduced slope of the VE-K+ relationship in the COPD patients could be interpreted as a reduced sensitivity to the stimulus and/or as a mechanical impairment of the ventilation.     

Burrowing and huddling in newborn porcupine: the effect on thermoregulation.

The newborn cape porcupine Hystrix africaeaustralis is a precocial rodent. However, the newborn spends its first 9 weeks in the burrow. Heat production (oxygen consumption--VO2) and body temperature (Tb) were measured at various ambient temperatures (Ta) in newborn (4-8-week-old) porcupines, with a body mass between 1340-1993 g during summer, from the colony kept at the Mammal Research Institute, University of Pretoria. To assess the effect of huddling, these parameters were also measured in adult pairs of porcupines (Ta = 15 and 25 degrees C) kept together in the metabolic chamber and the values were compared with those obtained from single porcupines. Overall minimal thermal conductance was calculated for newborn and paired adult porcupines. The newborn porcupines can regulate their body temperature at Tas between 10-28 degrees C. VO2 measured at lower critical point is 0.602 +/- 0.08 ml/O2/g.h. At Ta = 10 degrees C, VO2 oscillated with a characteristic amplitude and frequency, while body temperature was well regulated. Huddling decreases the lower critical point. The overall minimal thermal conductance (0.044 +/- 0.002 ml O2/g.h.1 degree C) is higher than expected from body mass, so that the young are liable to lose heat rapidly. It is well known that the newborn porcupine spends a long period in the burrow and the results of this study suggest that this may be in order to conserve energy and presumably allocate it to growth. Apart from decreasing vulnerability, delaying foraging also avoids heat loss in the newborn porcupine.     

Osmotic regulation of evaporative water loss and body temperature by intracranial receptors in the heat-stressed cat

The effect on body temperature (Tb) regulation of alterations in the osmotic milieu of the intracranial compartment has been examined by intracerebroventricular (ICV) infusion of water (0.034 ml . min-1) in heat-stressed hydrated and dehydrated cats. At an ambient temperature (Ta) of 38 degrees C, before ICV water infusion, mean Tb (measured in the hypothalamus) of normally hydrated cats was 38.8 +/- 0.1 degree C and mean evaporative water loss (EWL) was 1.32 +/- 0.18 W . kg-1. ICV water infusion was without significant effect (P greater than 0.05, t-test) on either of these values in normally hydrated animals. In dehydrated animals at Ta 38 degrees C, Tb and EWL were both significantly altered (P less than 0.001) from the normally hydrated state and were measured at 39.9 +/- 0.2 degree C and 0.84 +/- 0.09 W . kg-1 respectively. Infusion of water into dehydrated animals significantly altered pre-infusion levels of Tb and EWL so that Tb fell to 39.4 +/- 0.2 degree C (P less than 0.001) and EWL rose to 1.46 +/- 0.09 W . kg-1 (P less than 0.001). No effect of ICV water infusion on mean plasma vasopressin levels (pAVP) was observed in normally hydrated animals (preinfusion pAVP = 1.3 +/- 0.2 microunit . ml-1, post-infusion pAVP equal 1.3 +/- 0.3 microunit . ml-1, P greater than 0.05). However, a significant reduction in pAVP occurred subsequent to infusion in dehydrated animals (pre-infusion pAVP equal 16.6 +/- 1.85 microunits . ml-1, post-infusion pAVP equal 10.7 +/- 2.3 microunits . ml-1, P less than 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)     

Blood oxygen transport and organ weights of two shrew species (S. etruscus and C. russula).

Blood parameters concerning oxygen transport and relative organ weights of 11 Suncus etruscus and 13 Crocidura russula under light halothane anesthesia were investigated. Mean body weight of S. etruscus was 2.5 g and for C. russula was 9 g, hemoglobin concentration was 17.4 and 15.6 g/100 ml blood, hematocrit was 50 and 44%, red blood cells were 18 and 11 X 10(6)/microliter, respectively. Mean corpuscular volume was calculated to be 26 and 41 micron3, mean diameter 5.5 and 7 micron, and mean thickness 1.2 and 1.1 micron, respectively. Mean corpuscular hemoglobin concentration was in the normal range of mammalian red blood cells. A blood oxygen half-saturation pressure of 35 and 34 Torr at pH 7.4, 37 degrees C and a Bohr factor deltalog P50/deltapH of -0.61 and -0.66 was measured. Experiments with stripped hemoglobin showed that 2,3-diphosphoglycerate is the main oxygen affinity reducing allosteric factor. Relative weights of heart, kidney, and liver are remarkably high in S. etruscus. The maximal oxygen transport of 400 ml . kg-1 . min-1 of S. etruscus is feasible by an enormous heart rate, a large relative stroke volume, a high hemoglobin concentration combined with a low oxygen affinity, and a large Bohr effect.     

Oxygen consumption during pouch development of the macropod marsupial Setonix brachyurus

1. Measurements of O(2) consumption at 9 or 10 temperatures in the 20-40 degrees C ambient temperature range were made on joeys with ages selected to cover the 180-day period of pouch occupancy.2. The rate of O(2) consumption of joeys younger than 100 days increased directly with ambient temperature.3. After 100 days of age the O(2) consumption rate at low temperatures rose and at about 140 days of age a constant rate was maintained over the full ambient temperature range.4. Heat transfer from joey to mother commenced after 100 days of age.5. At 150-180 days of age the rate of O(2) consumption at 20 degrees C was approximately 12 times greater than at ages less than 100 days. A thermal neutral zone was established in the range 32-36 degrees C by joeys older than 150 days.6. At the usual pouch temperature of 36.5 degrees C, O(2) consumption per unit wet body weight rose from 12 ml./kg.min at birth to 17 ml./kg.min at the end of pouch life. On a unit dry body weight basis it fell from 120 to 56 ml./kg.min. This decline parallels the decrease in growth rate.     

Blood flow rate, temperature, oxygen tension and consumption in the skin of adults measured by a heated microcathode oxygen electrode

Reliable transcutaneous measurements of arterial oxygen tension are based on a maximum skin blood flow rate which is created by heating the skin, typically at an electrode temperature of 44 to 45 degrees C. This increase in skin blood flow rate creates an arterialization of the oxygen tension in the capillaries and the surrounding tissue. The heat conducted to the skin surface is removed by a combination of convection (skin perfusion) and conduction to the deeper layers of the skin. This heat transport to and through the skin surface causes a measurable temperature profile from the electrode surface to the capillary layer. By a blood flow cessation it is possible to change the temperature profile because the convective part of the heat consumption is eliminated and the conductive part can then be measured and subtracted. Using the forearm as measuring area and a heated tc-PO2 electrode several observations were made. The mean temperature gradient over epidermis down to the capillary layer at an electrode temperature of 43, 44, and 45 degrees C was 2.1, 2.4 and 2.7 degrees C, respectively. The change in temperature profile caused by the blood flow cessation enabled primarily an estimation of the skin blood flow rate by temperature measurements, ranging from 0.07 to 0.24 ml.cm-2.min-1. Increasing blood flow rates correlated to increasing tc-PO2 values. By means of a dynamically, thermally shielded tc-PO2 electrode it was possible to determine the skin blood flow rates in the same arbitrary units computed on the basis of the heat dissipation to the skin surface. Furthermore, it was possible to correlate these blood flow estimations to the cutaneous blood flow rates measured by 133Xe washout technique. By increasing the electrode temperature the cutaneous blood flow rates increased from 12 to 50 ml.(100 g)-1.min-1. It was possible to calculate a conversion factor on the basis of the correlation between the heat determinations of the skin blood flow rate and the 133Xe measurements. Using this conversion factor the highest blood flow rate did not exceed 55 ml.(100 g)-1.min-1. The subcutaneous blood flow rate increased accordingly with increasing electrode temperature. It was concluded that the measured heat consumption of the skin is effected by the heat removing capacity of the cutaneous as well as the subcutaneous blood flow. The cutaneous blood flow, however, was considered predominant in the transport of heat from the skin surface. By 50 times of stripping the skin surface, the cornified epidermal membrane was removed. This procedure increased the tc-PO2 values by on an average 3.6 kPa (27.1 mmHg).(ABSTRACT TRUNCATED AT 400 WORDS)     

The influence of high ambient temperature on thermoregulatory response to intrahypothalamic injections of noradrenaline and serotonin in the pigeon

1. Pigeons with chronically implanted injection cannula in the hypothalamus were injected with noradrenaline (NA, 10 micrograms/microliter) and 5-hydroxytryptamine (5-HT, 10 micrograms/microliter) at the ambient temperature (Ta) of 38 degrees or 42 degrees C. 2. Initial tests at cold (6 degrees C) indicated that birds responded hypothermically to NA and in most instances also to 5-HT. 3. Intrahypothalamic injection of NA (10 micrograms/microliter) had no appreciable effect on oxygen consumption (VO2), body temperature (Tb), foot temperature (Tf), or heart and respiratory rates at Ta 38 degrees C. The increase of VO2, Tb and Tf noted after similar injection at 42 degrees C was in all probability due to observed excitement and bursts of struggling rather than effects on thermoregulatory mechanisms. 4. 5-HT (10 micrograms/microliter) injected at Ta 38 degrees C depressed respiratory frequency from panting (600 breaths.min-1) to normal rate (ca. 50.min-1) within 2--4 min. The absence of panting lasted about 10 min, but only a slight increase of VO2, Tb and Tf followed. At Ta 42 degrees C, no notable changes of VO2, Tb and Tf were recorded after 5-HT injection. 5. It is concluded that 5-HT has an inhibitory action on neuronal pathway controlling panting activity in the pigeon, but NA seems to be ineffective.     

Cardiovascular and metabolic response to adrenaline infusion in weight-losing patients with and without cancer

Fasting is generally accompanied by a decrease in energy metabolism and hormones. On the other hand, indirect evidence has suggested that the response to adrenergic agonists may be maintained or even increased in malnutrition. The present study evaluated whether weight-losing patients with and without cancer have increased plasma concentrations of catecholamines and different responses to intravenously infused adrenaline compared to weight-stable individuals. Eight malnourished cancer and 10 non-cancer patients (11% weight loss) were compared to seven well-nourished and weight-stable patients. Adrenaline was infused i.v. at a rate of 0.005 microgram min-1 kg-1 body weight during 40 min followed by a 40 min rest period (without infusion) and then a final 40 min period with i.v. adrenaline infusion (0.02 microgram min-1 kg-1 body weight). Plasma glycerol concentration at fast was higher in weight-losing patients compared to weight-stable individuals. Whole body oxygen uptake, carbon dioxide production, heart rate and plasma concentrations of free fatty acids (FFA) increased while the mean arterial pressure decreased significantly in response to adrenaline infusion at 0.02 microgram kg-1 min-1 in both weight-losing and weight-stable patients. Adrenaline at 0.005 microgram kg-1 min-1 increased plasma FFA levels by 19% (P less than 0.05) in weight-losing patients while no significant alteration was observed in well-nourished patients. Adrenaline infusion at 0.02 microgram kg-1 min-1 decreased the mean arterial blood pressure and stimulated respiratory gas exchange and heart rate significantly more in weight-losing than in weight-stable patients. The slopes for oxygen uptake, carbon dioxide production, heart rate, plasma FFA and plasma glycerol vs. plasma adrenaline and noradrenaline were all significantly steeper (P less than 0.05-0.01) in malnourished patients than in well-nourished controls. The present study suggests an increased sensitivity to adrenaline in weight-losing patients compared to matched controls with normal nutritional state and stable weight.     

Volume absorption in the pars recta. II. Hydraulic conductivity coefficient

We evaluated the hydraulic conductivity (Pf, micron s-1) of superficial proximal straight tubules isolated from rabbit kidney. Tubules were perfused with hypotonic (270 mosmol/kg H2O) and bathed with isotonic (290 mosmol/kg H2O) NaCl buffers at 25 degrees C. Due to the tendency of transepithelial osmosis plus solute entry to produce osmotic equilibrium along the perfused length, we observed that the total net volume absorption ('JV, nl min-1) increased from 0.64 to 2.21 when the perfusion rate (VO, nl min-1) was increased from 11 to 45 in a group of tubules with an average length of 0.86 mm. From a 'JV of 2.21 nl min-1 at VO = 45 nl min-1 we computed a minimum Pf of 2,200 micron s-1. And extrapolation of the data to VO leads to infinity gave a Pf value of 5,200-7,600 micron s-1. The same perfusion rate dependence of 'JV in a group of tubules with an average length of 3.29 mm gave quantitatively similar results. A theoretical analysis of radial osmosis occurring simultaneously with axial osmotic equilibration showed that Pf values in the range of 3,000-4,000 micron s-1 accurately predicted the observed relations between VO, 'JV, and tubule length.     

Strenuous prolonged exercise elevates resting metabolic rate and causes reduced mechanical efficiency

Resting O2 consumption, net mechanical efficiency during cycling exercise and excess postexercise O2 consumption (EPOC) was measured in 15 army cadets after 3 or 4 days of continuous simulated combat exercises (estimated energy demand: 40 MJ day-1), no organized sleep and virtually no food intake (stress experiment). They exercised for 30 minutes at a work load corresponding to about 50% of maximal O2 uptake. An identical test using the same absolute work load was repeated when the cadets were completely recovered from the combat course (control experiment). Resting O2 consumption increased by 15% from 279 +/- 7 ml min-1 (control) to 320 +/- 8 ml min-1 (stress, P less than 0.001). Mechanical efficiency decreased from 24.6 +/- 0.4% (control) to 20.9 +/- 0.2% (stress, P less than 0.001). EPOC1h increased from 0.58 +/- 0.41 l (control) to 2.24 +/- 0.2% (stress, P less than 0.05). Glucose infusion during exercise (0.20 g kg-1 body weight) had no effect on mechanical efficiency or EPOC. About 1/5 of the increase in exercise O2 uptake can be explained by a substrate shift from carbohydrates to fat, as evidenced by a reduction in R-value during exercise from 0.90 +/- 0.012 (control) to 0.80 +/- 0.010 (stress). Hence, after severe physical stress combined with sleep deprivation and food restriction, O2 uptake is increased both at rest and during submaximal exercise.     

Properties of the macula densa mechanism for renin release in the dog

To study the macula densa mechanism for renin release, both the macula densa and the haemodynamic mechanisms were activated in anaesthetized dogs with denervated kidneys, either by renal arterial constriction to a renal arterial pressure (RAP) of 52 +/- 2 mmHg or by ureteral occlusion to a ureteral pressure of 95-105 mmHg, 20-25 mmHg below RAP. Renal arterial constriction increased renin release from 0.3 +/- 0.2 to 16 +/- 4 micrograms AI min-1. At low RAP, renin release was subsequently reduced to 7 +/- 3 micrograms AI min-1 when sodium excretion was raised far above control values by plasma volume expansion and acetazolamide infusion. Ethacrynic acid (3 mg kg-1 body wt.) restored renin release to pre-expansion values, and a large dose (25 mg kg-1 body wt.) prevented renin release from falling even after unclamping the artery. During ureteral occlusion with stopped glomerular filtration, plasma volume expansion, acetazolamide and ethacrynic acid infusion did not alter renin release. On the other hand, beta-adrenergic stimulation by isoproterenol raised renin release equally (by 30-40 micrograms AI min-1) before and after plasma volume expansion, during both renal arterial constriction and ureteral occlusion. Indomethacin (10 mg kg-1 body wt.) abolished renin release induced by ethacrynic acid infusion and ureteral occlusion. We conclude that the macula densa mechanism for renin release is inactivated by high NaCl reabsorption during plasma volume expansion and acetazolamide infusion, reactivated by inhibition of NaCl reabsorption with ethacrynic acid and completely inhibited by indomethacin. The degree of activation does not influence the renin release induced by beta-adrenergic stimulation.     

Oxygen transport and oxygen consumption in shock

A review of data from patients in shock, or with very low cardiac output, shows that oxygen consumption is maintained above 90 ml . min-1 . m-2, even at the lowest values of arterial oxygen transport. CAIN [3] has found a limiting value of oxygen delivery in anesthetized, hypoxic dogs of about 10 ml . kg-1 . min-1, below which oxygen consumption falls dramatically. No comparable value has been established for man in shock.     

The biliary excretion of 3H-inulin and 3H-terbutaline in the unanesthetized rat.

An animal model which permits sampling of arterial blood and bile from unanesthetized rats with uninterrupted enterohepatic circulation is described. Pharmacokinetic data concerning inulin (0.2 mumol kg-1) and terbutaline (1.8 mumol kg-1) after intraarterial injection are presented. The following values for inulin were obtained Vdbeta equals 0.25 ml g-1, plasma clearance equals 2.3 ml min-1 kg-1 and bile clearance equals 0.03-0.04 ml min-1 kg-1, which mainly are in agreement with data from the literature. For terbutaline the corresponding values were 0.58 ml g-1, 5.2 ml min-1 kg-1 and 0.12-0.98 ml min-1 kg-1, respectively.     

Effect of high pressure on metabolic response to cold in rats

The ability of rats to regulate body temperature in a cool helium-oxygen (heliox) atmosphere at high pressure was studied. Thermoregulatory ability in seven awake rats was assessed in a pressure chamber at 1.5, 21 and 41 ATA by monitoring body temperature and oxygen consumption during a gradual reduction of ambient temperature. Body temperature was measured by a radiotelemetry transmitter implanted into the intraperitoneal cavity. The thermal conductance of the rats increased from 0.37 to 0.75 W kg-1 degrees C-1 as air was substituted by heliox and increased further to 1.47 and 1.81 W kg-1 degrees C-1 as the pressure was increased to 21 and 41 ATA, respectively. At high pressure the rats were able to maintain body temperature until the metabolic rate reached a value of 21.9 +/- 2.0 W kg-1, which corresponds to peak metabolic rate. This occurred at an ambient temperature of approximately 21 degrees C. Below this ambient temperature the rats did not manage to maintain body temperature. The results show that the thermal effector mechanisms of rats respond adequately to a slowly decreasing ambient temperature in hyperbaric heliox environments.