LEUKOCYTES RESPOND TO MECHANICAL DEFORMATION 
Kitagawa et al. (p. 1397) show that extrusion of
polymorporphonuclear leukocytes (PMN) through polycarbonate filters of
3-µm-pore diameter leads to deformation without the cytoskeletal
reorganization typical of migrating cells. Levels of intracellular free
Ca2+ and F-actin increase, as does
expression of CD18 and CD11b. Levels of L-selectin are unchanged, and
H2O2
production is not induced. Passage through 5-µm pores has similar
effects, although of lesser magnitude. Thus mechanical deformation of
PMN induces structural and functional changes that might influence
their passage through pulmonary capillaries. The paper is discussed in
an Invited Editorial by Downey (p. 1395).
OXIDATIVE STRESS IN CARBON MONOXIDE POISONING
Some patterns of severe exposure to carbon monoxide result in lipid
peroxidation and irreversible brain damage that is not directly
attributable to carboxyhemoglobin formation. Rats exposed in this way
by Thom et al. (p. 1424) showed increased plasma levels of oxidized
glutathione (GSSH), oxidized proteins, and other oxidation products.
Other patterns of carbon monoxide exposure caused similar increases in
plasma carboxyhemoglobin, without he appearance of oxidation products.
Plasma GSSH may have been derived from the action of peroxynitrite on
erythrocytes; the source of the other oxidized products was not
determined. The authors conclude that plasma levels of oxidized
products might identify those victims of carbon monoxide poisoning who
are most in danger of developing irreversible brain damage.
LONGITUDINAL AGING EFFECTS DESPITE CONTINUED PHYSICAL TRAINING
Pollock and associates (p. 1508) provide unique longitudinal data on
the effects of aging at age 50, 60, and 70 yr on maximal O2 consumption
(
O2 max) and body
composition in 21 track athletes of varying levels of continued
habitual activity. Physiological capacity as measured by
O2 max and maximal
heart rate declined over the two decades despite vigorous training
regimens. Decrements in capacity averaged 8-15% per decade.
Fat-free body weight also declined with aging, but this change was more
influenced by the subjects' training habits.
WORK OF BREATHING AND BLOOD FLOW DISTRIBUTION IN MAXIMAL
EXERCISE
Harms et al. (p. 1573) tested the hypothesis that during exercise at
maximal O2 consumption the high
demand for respiratory muscle blood flow would elicit locomotor muscle
vasoconstriction and compromise limb blood flow. Inspiratory muscle
work in seven male cyclists was either decreased via a proportional
assist ventilator, increased via graded resistive loads, or not
manipulated. A significant and substantial inverse correlation was
found between work of breathing and limb blood flow. Similarly, a
significant and substantial positive correlation was found between work
of breathing and leg vascular resistance. The authors concluded that
the work of breathing incurred during maximal exercise causes
vasoconstriction in locomotor muscles.
STRESS FAILURE OF PULMONARY CAPILLARIES IN THOROUGHBRED RACEHORSES
Exercise-induced arterial hypoxemia, pulmonary hypertension, and
pulmonary hemorrhage occur commonly in Thoroughbred horses during heavy
exercise, and this has been attributed to "stress failure" of the
lung's blood-gas barrier. Birks et al. (p. 1584) isolated and perfused
the lungs of Thoroughbred horses over a wide range of capillary
transmural pressures (Ptr; i.e., capillary minus airway
pressure). They determined that a threshold of Ptr of
75-100 mmHg was sufficient to cause a significant disruption of
the capillary endothelium, especially in the dorsal caudal lung
regions. Thus the blood-gas barrier of the horse appears to be more
resistant to stress failure than does that of either the rabbit or the
dog. Nevertheless, pressures in this range are frequently achieved by
the Thoroughbred during heavy exercise.
DOBUTAMINE AS A COUNTERMEASURE IN SIMULATED MICROGRAVITY
Because astronauts have decreased exercise performance after
spaceflights, in-flight countermeasures are being considered to ensure
adequate endurance in emergency egress conditions in the future. Tipton
and Sebastian (p. 1607) examined the efficacy of dobutamine
administration as a countermeasure for reduced exercise performance after prolonged inactivity or simulated microgravity. Three
groups of rats were included in the study: normal cage-confined rats,
head-down tail-suspended rats, and head-down tail-suspended animals
treated with dobutamine hydrochloride. Treadmill performance was
examined before suspension, at 14 days, and after 21 days. The results
reveal that head-down suspended rats treated with dobutamine had normal
peak oxygen consumptions, plasma lactic acid concentrations, and
arterial blood pH. However, these beneficial effects of dobutamine
treatment did not restore exercise performance. Thus both groups of
suspended rats had 30% shorter treadmill run times, reduced mechanical
efficiency, and increases in their rate of heating relative to the
controls. The results suggest that dobutamine has limited potential as
a countermeasure for postflight egress emergencies and that other
countermeasures must be considered.
FINITE ELEMENT MODEL OF THE DIAPHRAGM
Previous studies in dogs show that the shape of the costal diaphragm is
nearly constant with either passive or active lung inflation, that
there is little change in diaphragm length in the direction transverse
to the muscle fibers, and that the central tendon is inextensible.
Boriek and Rodarte (p. 1626) have used a finite-element model to
examine the effects of fiber and central tendon stiffness on diaphragm
shape changes in response to surface pressure. Shape changes became
limited when transverse muscle stiffness was increased or when the
central tendon was made inextensible. These findings with the model
provide a plausible explanation for the experimental finding that the
diaphragm shape remains constant during inflation.
HOW DOES CHRONIC CIGARETTE SMOKE EXPOSURE CAUSE PULMONARY
HYPERTENSION?
It has been difficult to determine the contribution of specific changes
in vascular morphometry to the changes in pulmonary hemodynamics
associated with pulmonary diseases involving vascular remodeling.
Yamato et al. (p. 1644) found that the percentage of muscularized
peribronchiolar arterioles was greater and the mean capillary density
and diameter were smaller in guinea pigs exposed to cigarette smoke for
6 mo than they were in the nonsmoking control group. The pulmonary
arterial pressure was also higher in the smokers. However, the authors
did not find significant correlations between the measured morphometric
variables and pulmonary arterial pressure. Thus they conclude that the
measured morphometric variables were not responsible for the elevated
pulmonary arterial pressure.
HYDRAULIC ENERGY FROM MUSCLE CONTRACTIONS
Can the energy of skeletal muscle contractions be effectively harnessed
to assist the heart? Trumble and Magovern (p. 1704) describe the
initial in vitro testing of a prototype muscle-energy converter
designed to transform the power of in situ muscle contractions into
hydraulic form. The converter resembles a simple piston pump and is
designed for implant beneath the humeral insertion of the latissimus
dorsi muscle. Muscle-energy converter transfer capacity was found to be
>98% of the input power, which was converted into hydraulic energy
and preload work. These results demonstrate that a significant amount
of contractile energy can be efficiently transformed to hydraulic power
by this device.
LEUKOCYTES RESPOND TO MECHANICAL DEFORMATION
