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VASCULAR RESPONSES OF FEED ARTERIES TO SKELETAL MUSCLE |
Jasperse and Laughlin (p. 441) determined the nature of
endothelium-dependent and -independent vascular responses to
vasodilators in isolated rat soleus feed arteries. In addition, they
examined the effect of exercise training on the responses of these
arteries to vasodilator substances. The endothelium-dependent
vasodilator acetylcholine relaxed the arteries in a dose-dependent
manner, an effect that was attenuated by arginine analogs. In contrast, the endothelium-dependent dilators bradykinin, substance P, and adenosine had no effect on the arteries. The article is discussed in an
Invited Editorial by Bohlen (p. 439).
| |
BED REST ALTERS MYOSIN HEAVY CHAIN GENE EXPRESSION |
A model of bed rest (37 days) was used by Andersen et al. (p. 455) to
unload human skeletal muscle (medial vastus lateralis) to examine the
relative "matching" of myosin heavy chain (MHC) mRNAs with their
respective isoprotein translational products. Bed rest elicited a
marked increase in the extent of "mismatch" between MHC mRNAs and
MHC protein. Specifically, bed rest increased the number of fibers with
rare or unusual MHC mRNA and isoprotein combinations (e.g., type I
fibers containing only mRNA for type 2X MHC). The results of this study
indicate that 37 days of bed rest produce skeletal muscle fibers in a
transitional state that demonstrates changes in MHC expression at the
mRNA level; these changes are not temporally reflected by similar
changes at the MHC protein level.
| |
ELASTIC MODULI OF LUNG PARENCHYMA DURING BRONCHOCONSTRICTION |
Does bronchoconstriction increase the tethering of airways by the lung
parenchyma? To approach this question, Okazawa et al. (p. 496) studied
the effects of brochoconstriction by carbachol on the bulk (
) and
shear moduli (µ) of lung parenchyma in isolated rabbit lungs. The was measured from small pressure-volume loops at inflation pressures
(PL) of 4, 6, 8, 12, and 16 cmH2O; µ was measured by an
indentation test. Both and µ increased at the higher
PL values. The increased
after carbachol nebulization, and the fractional increase decreased
monotonically as PL increased, whereas the fractional increase in µ with carbachol that was present at 4-12 cmH2O
PL vanished at 16 cmH2O
PL. Thus a shear modulus-induced parenchymal tethering that attenuates bronchoconstriction may be most
effective below 12 cmH2O.
| |
MECHANISM OF VAGALLY MEDIATED CARDIAC SLOWING |
Sears et al. (p. 510) investigated the role played by the muscarinic
receptor-coupled nitric oxide (NO) pathway in the cholinergic control
of heart rate. In anesthetized rabbits, the NO donor molsidomine significantly increased the bradycardic response to vagal stimulation. Similarly, in isolated guinea pig atria, the NO donor sodium
nitroprusside increased the atrial slowing evoked by vagal stimulation.
The increased cardiac slowing was mimicked by a stable analog of cGMP, which is the intracellular messenger for NO. The effect was not evoked
by carbamylcholine chloride, a stable analog of acetylcholine. The
authors conclude that the NO enhancement of the cardiac slowing evoked
by vagal stimulation involves a presynaptic site of action.
| |
STRUCTURE AND FUNCTION OF THE PULMONARY ARTERIAL TREE |
The relationships between structure and function in the pulmonary
arterial tree have been studied extensively but are sufficiently complex as to be difficult to summarize. How do the structural changes
accompanying normal growth or the vascular remodeling seen in certain
diseases affect pulmonary hemodynamics? Dawson et al. (p. 569) have
addressed this problem by developing a mathematical model representing
a rigid or distensible tapered tube, through which blood flow decreases
with distance from the inlet, reflecting diversion of blood at branch
points along the way. Using morphometric data from dog lungs to
establish model parameters, they have examined the sensitivity of model
pressure-flow behavior to structural and mechanical properties.
| |
LUNG BLOOD FLOW DISTRIBUTION IN THE BABOON |
The importance of gravity in determining the distribution of pulmonary
blood flow remains controversial. High-resolution measurements based on
microsphere deposition in animals suggest that gravity has a modest
influence, but the question has been raised as to whether this
conclusion is applicable to humans, with their upright posture and
large size. Glenny et al. (p. 623) have approached this problem by
studying a nonhuman primate, the baboon. The results are similar to
those for other animals, but the question remains whether baboons are
similar enough to humans to define the importance of gravity in the
human pulmonary circulation.
| |
NEONATAL RESPIRATORY INJURY BY LYSOPHOSPHATIDYLCHOLINE |
Grossman et al. (p. 633) induced lung injury in neonatal rabbits by
pulmonary instillation of lysophosphatidylcholine (lyso-PC), a
breakdown product of phosphatidylcholine. Compliance and alveolar volume density were reduced in a dose-dependent manner, and
histological evidence of lung injury was present. When exogenous
surfactant was administered at birth with the lyso-PC, lung compliance
was normal, and evidence of lung injury was substantially reduced. However, if surfactant was administered 30 min after lyso-PC, compliance was decreased, and lung injury was as severe as with lyso-PC
alone. Presence of lyso-PC in the lungs of ventilated neonates may
contribute to lung injury by several pathophysiological mechanisms,
including destabilization of existing surfactant, toxic action on the
lung epithelium, and further inactivation of surfactant by plasma
proteins leaking into air spaces. This injury can be attenuated by
administration of exogenous surfactant but only if treatment is
performed early in the disease process.
| |
CHRONIC RESISTIVE LOADING ALTERS DIAPHRAGMATIC FUNCTION |
Evidence suggests that strenuous or "fatiguing" muscle
contractile activity elicits lipid and protein peroxidation reactions that may be responsible for diminished muscle contractile function. This hypothesis derives from work on in vitro muscle preparations maintained under arguably artificial experimental conditions. Supinski
et al. (p. 651) have demonstrated that increasing diaphragmatic load
via tracheal banding in intact rats was sufficient to increase markers
of lipid and protein peroxidation and to diminish in diaphragm muscle
force development. Thus the findings previously demonstrated in vitro
are confirmed in vivo.
| |
CEREBRAL HEMODYNAMICS IN ACUTE MOUNTAIN SICKNESS |
Acute mountain sickness (AMS) is common on rapid ascent to altitudes of
2,500 m or more. Its development may relate to changes in cerebral
blood flow (CBF) brought on by the opposing stimuli of hypoxemia
(dilator) and hypocapnia (constrictor). In persons with AMS, hypoxemia
is more severe than in those with no AMS. Jansen et al. (p. 681), in a
study conducted at 4,243 m, report that resting CBF was higher in AMS
patients than in those without symptoms and that voluntary
hyperventilation reduced CBF more in patients with AMS than in
nonsufferers. Arterial O2
saturation was also reduced. The data suggest roles for CBF and
hypoxemia in AMS, but what is cause and what is effect remain difficult questions.
| |
THE EYE PROVIDES A VIEW OF O2 TRANSPORT |
The study of O2 metabolism in any
tissue or organ requires some approach to intracellular assessment of
O2 and its utilization or to
measurement of tissue blood flow and arterial and/or venous O2 concentrations.
Ideally, both kinds of information should be obtained. Beach et al. (p.
748) have devised a method using differential spectroscopy of
hemoglobin via a fundus camera to measure retinal arterial and venous
O2 saturation. Good agreement was
found between independently measured arterial saturation and that of
the new method over a range of values. However, direct confirmation of the accuracy of venous saturation values cannot currently be achieved. The technology should be useful in studies of
O2 transport in the retina, both
for particular retinal diseases and for more systemic diseases with
retinal manifestations.
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