STATISTICAL ASSESSMENT OF NEONATAL BREATHING PATTERNS

Breathing is often irregular in infants, and the irregularity may be associated with clinically important apneas and hypopneas. Frey et al. (p. 789) have developed a novel approach to examining irregularity of breathing based on an analysis of the timing of abdominal movements during sleep. The analysis yields an exponent (), which provides a statistical measure of the risk of inadequate breathing. The  increases with postconceptional and postnatal age, indicating a reduction in apneic and hypopneic periods with maturation. The authors suggest that their approach provides a simple, clinically applicable means of characterizing breathing irregularities in infants. The paper is discussed in an Invited Editorial by Bruce (p. 787).

	TRYPTOPHAN AND CENTRAL FATIGUE

Farris et al. (p. 807) infused combinations of tryptophan and glucose into exercising horses to evaluate the hypothesis that peripheral metabolic alterations during fatiguing exercise result in increased plasma free tryptophan, thus allowing tryptophan to cross the blood-brain barrier and modify brain function. Glucose infusion increased endurance time at 50% maximal O₂ consumption but not when tryptophan was also infused. The results support the possibility of a direct involvement of tryptophan in promoting central fatigue, independent of peripheral substrate depletion.

	COFFEE, WITH OR WITHOUT?

Caffeine ingestion increases plasma epinephrine, and there are reports that athletes take caffeine-containing pills or drink coffee to obtain performance-enhancing effects of epinephrine. To determine whether coffee or caffeine contained in coffee affects exercise performance, Graham et al. (p. 883) studied endurance of nine young adults at 85% maximal O₂ consumption with several combinations of caffeine-containing and placebo capsules, regular coffee, and decaffeinated coffee. In the trials with caffeine, plasma paraxanthine concentrations were elevated to the same extent. However, the caffeine-containing capsule resulted in the greatest epinephrine response and endurance time. The authors conclude that some component of coffee moderates the effects of caffeine.

	STOP-FLOW ANALYSIS OF PULMONARY VASCULAR EXCHANGE

Morphological evidence suggests that transport of water and solute across the walls of arterioles, capillaries, and venous segments of the pulmonary vasculature might differ. By using stop-flow methods with solutions containing various markers, Effros et al. (p. 986) have evaluated sites of filtration and uptake in these segments. The results indicate that this approach can be used to identify the fraction of collected samples within exchange vessels of the lung during a stop interval and may help to determine the distribution of solute and water exchange along the pulmonary vasculature.

	HUMAN HYDROMETRY

Total body water and its intracellular and extracellular components have traditionally been measured by the use of isotopes and application of the dilution principle. These cumbersome and constrained measurements have been widely replaced by techniques based on bioelectrical impedance. Ellis and Wong (p. 1056) compared these two approaches by using multifrequency bioelectrical impedance, deuterium dilution, and bromide dilution to estimate total body, extracellular, and intracellular water levels in a large number of normal subjects. The results show a strong correlation between the dilution and impedance measurements, but the authors conclude that further refinement of the constants used in the impedance method is needed to improve its accuracy.

	ENERGY NEEDS IN ELDERLY PEOPLE

Starling et al. (p. 1063) examined determinants of free-living total daily energy expenditure (TEE) and physical activity in older men and women by using doubly labeled water and indirect calorimetry. TEE was predicted from anthropometric, physiological, and physical activity indexes by using multiple regression. Resting metabolic rate and peak O₂ consumption (O_{2 peak}) explained about two-thirds of the variance in TEE. The strongest predictors of energy expenditure for physical activity in both genders were O_{2 peak}, fat-free mass, and body mass. In summary, resting metabolic rate and O_{2 peak} are important independent predictors of energy requirements in the elderly. Cardiovascular fitness and fat-free mass are moderate predictors of physical activity.

	PLATELET-ACTIVATING FACTOR AND THE FETAL CIRCULATION

The decrease in pulmonary vascular resistance at birth is an important part of the transition to extrauterine life, and failure of this response is a significant clinical problem. Understanding the mechanisms involved is an important step in the development of rational therapy for persistent pulmonary hypertension of the newborn. Platelet-activating factor (PAF) is a pulmonary vasoactive agent that is thought to be involved in fetal lung development. Ibe et al. (p. 1079) used a PAF-receptor antagonist to evaluate the possible role of PAF in the maintenance of pulmonary vasomotor tone in fetal lambs. They found that treatment with the antagonist reduced pulmonary vascular resistance in near-term fetal lambs. They also found that the circulating PAF levels fell after delivery, along with the pulmonary vascular resistance. They conclude that PAF contributes to the normally high tone of the fetal pulmonary vessels and that the fall in circulating PAF after birth may be one of the mechanisms involved in the fetal-neonatal transition of the pulmonary vasculature.

	O2 TRANSPORT DURING -ADRENERGIC BLOCKADE

Wolfel et al. (p. 1092) examined the consequences of -adrenergic blockade on submaximal (90-W) exercise O₂ transport and utilization at sea level and at Pikes Peak (4,300 m). At issue was whether an increased stroke volume would offset the lower heart rate and preserve O₂ delivery or whether increased O₂ extraction would occur with blockade. Cardiac output was not maintained after blockade, whereas O₂ consumption at 90 W was unchanged (both at sea level and altitude). Thus O₂ extraction had to increase. Arterial PCO₂ and pH were unaffected by blockade so that arterial lactate levels were also presumably similar. Despite this, perceived effort at 90 W was greater after blockade for reasons that remain unclear. It would be of interest to conduct similar studies at maximal effort, asking how much maximal cardiac output is decreased and whether increased O₂ extraction is still able to preserve maximal O₂ consumption.

	INCREASED PERFUSION DOES NOT MEDIATE EXERCISE-INDUCED ANGIOGENESIS

Recent experiments demonstrate that a single bout of exhaustive exercise results in increased expression of genes encoding for angiogenic growth factors. Roca et al. (p. 1142) compared the effects of acute passive hyperperfusion with those of contraction-induced hyperperfusion on expression of genes encoding for vascular endothelial growth factor (VEGF), basic fibroblast growth factor (bFGF), and transforming growth factor-₁ (TGF-₁) in canine skeletal muscle. Passive hyperperfusion did not alter expression of the growth factor genes for VEGF or bFGF and produced only slightly increased expression of mRNA for TGF-₁. In contrast, VEGF mRNA abundance was increased threefold by electrical stimulation producing similar increases in blood flow, whereas no significant changes were produced in gene expression of the other two growth factors. These results suggest that the increased muscle blood flow of exercise does not account for the increased abundance of angiogenic growth factor mRNA.

	GENDER EFFECTS ON SUBSTRATE UTILIZATION DURING EXERCISE

Lower respiratory exchange ratios and plasma lactate in women during submaximal exercise have pointed to gender differences in substrate utilization. Friedlander and colleagues (p. 1175) tested this possibility and the effect of training directly by infusion of substrates labeled with stable isotopes during exercise. Training caused a reduced glucose flux and oxidation during exercise at fixed work rates, which was explained by changes in relative work intensity. Women differed from men by showing a greater shift toward lipid oxidation in response to training and by relying less on muscle glycogen and lactate during exercise. These data suggest that caution should be exercised in lumping data from both genders in studies involving substrate utilization.