ROLE OF THE PONS IN BREATHING 
More than 70 years have elapsed since Thomas Lumsden's classic studies
of the influence of the pons on respiration, yet this topic remains
unsettled and controversial. Do pontine mechanisms play a central role
in the neurogenesis of breathing or is their action a subordinate and
nonessential modulation of the output of an exclusively medullary
respiratory rhythm generator? Jodkowski et al. (p. 377) contribute new
information to this debate with their report that microinjections of
glutamate at a site in the ventrolateral pons cause reversible
prolongation of neural expiration in anesthetized vagotomized rats. The
injection site is anatomically distinct from the pneumotaxic region in
the dorsolateral pontine tegmentum. The paper is discussed in an
Invited Editorial by St. John (p. 375).
LONG-TERM FACILITATION OF UPPER AIRWAY MUSCLE ACTIVITIES
Regulation of breathing proceeds on many different time scales. Carotid
sinus nerve stimulation in vagotomized cats leads to increased
ventilation for up to 90 min after the stimulation has ended, a
phenomenon termed "long-term facilitation" (LTF). Mateika and
Fregosi (p. 419) demonstrate that LTF occurs in the activities of upper
airway muscles and that LTF is much greater in vagotomized cats than in
those with intact vagi. They suggest that in humans, who have
relatively weak vagal reflex responses to lung inflation, LTF may act
correctively to improve upper airway patency after periods of
chemostimulation by episodes of obstructive sleep apnea.
CLENBUTEROL AND EXERCISE FOR TREATMENT OF MUSCULAR DYSTROPHY?
Clenbuterol, a 
2-agonist, has
potent anabolic properties in skeletal muscle and is thus an attractive
candidate drug for use in the prevention of muscle atrophy in
dystrophin-deficient muscular dystrophy. A potential drawback, however,
is that clenbuterol promotes a fiber type transition from slow-twitch
to fast-twitch fibers, which are preferentially affected by the
dystrophic process. Hayes and Williams (p. 435) report that a
low-intensity exercise program in dystrophic mice prevents the
clenbuterol-induced shift in fiber type while preserving the
strength-increasing effects of the drug. This result has likely
clinical applicability in the treatment of patients with some types of
muscular dystrophy.
POTENTIALS EVOKED BY PHRENIC NERVE STIMULATION
Respiratory sensation undoubtedly involves cortical processing of
afferent activity. Straus et al. (p. 480) recorded scalp and cervical
spinal potentials evoked by transcutaneous stimulation of a phrenic
nerve in human subjects. Consistent negative potentials were recorded
over the second cervical segment at ~7 ms and on the scalp at ~13
ms. The cervical potential may represent the ascending afferent volley.
The scalp potential could have several causes, but the authors
hypothesize that it represents an afferent projection to the anterior
part of the cingulate gyrus
part of the limbic system. The
significance of these potentials for respiratory sensation remains to
be established.
TRANSPLEURAL INERT-GAS TRANSFER IN ISOLATED LUNGS
Li et al. (p. 545) studied the loss of inert gases from the blood
across the pleura of an isolated, ventilated, and perfused rabbit lung
placed within a box with flowing gas surrounding the lung. A
single-compartment model showed transpleural gas flux from the blood
depended on four dimensionless ratios: the perfusate-to-air partition
coefficient, the alveolar ventilation-to-perfusion ratio, the ratio of
pleural diffusing capacity to ventilation conductance, and the box
ventilation-to-alveolar ventilation ratio. The concentrations of six
inert gases were measured in pulmonary arterial blood, venous blood,
exhaled gas, and box gas. The experimental data were adequately
described by the single-compartment model.
EFFECT OF RUNNING ON INTESTINAL PERMEABILITY
Gastrointestinal disorders are commonly reported among long-distance
runners. Although alterations in small intestinal permeability have
been observed, the actual mechanisms responsible are unclear. Pals and
co-workers (p. 571) evaluated the effect of running on small intestinal
permeability, evident by an increase in the urinary excretion ratio of
lactulose to rhamnose, and an index of gastric damage by using sucrose
as a probe. Measurements were made during rest or during and following
60 min of treadmill exercise at 40, 60, and 80% peak oxygen uptake in
six healthy subjects. There was no evidence of gastric damage, as
urinary recovery of sucrose was not altered. Increased small intestinal
permeability was found at the highest exercise intensity, but the
prevalence of gastrointestinal symptoms was small and did not
correspond to increased permeability. The significance and mechanisms
involved in the increased small intestinal permeability after
high-intensity running merit further investigation.
EFFECT OF MUSCLE ISCHEMIA ON BAROREFLEX RESPONSES
Arterial blood pressure is closely controlled by reflexes arising from
the carotid sinus baroreceptors. This mechanism can be quantified in
human subjects by applying pulsatile positive and negative pressures to
the neck and plotting mean arterial pressure as a function of carotid
sinus transmural pressure. The negative slope of such a plot is a
measure of baroreflex sensitivity. Afferents from ischemic muscle also
elevate blood pressurethe muscle chemoreflex. Papelier et al. (p.
577) examined the interaction of these two reflex systems by assessing
the influence of postexercise muscle ischemia on the baroreflex
response curve in seven healthy subjects. Bicycle exercise shifted the
curve upward and rightward, and postexercise leg ischemia, induced by
inflation of thigh cuffs, further elevated mean arterial pressure. The
summation of effects was alinear in that baroreflex sensitivity was
enhanced at low carotid sinus pressures but reduced at high carotid
sinus pressures.
INDUCED PERIODIC BREATHING IN LAMBS
Periodic breathing in association with hypoxia is common in human
adults and infants, especially during sleep, but is relatively uncommon
in neonates of other species. Canet et al. (p. 607) used an
extracorporeal membrane lung to manipulate arterial
PCO2 and
PO2 in awake 2- and 10-day-old lambs.
Substantial reductions in both PCO2
and PO2 consistently induced periodic
breathing in the 10-day-old lambs but only rarely in the 2-day old
animals, which exhibited low chemoresponsiveness. This result is
interpreted to indicate that the induction of periodic breathing by
hypoxia and hypocapnia requires relatively mature ventilatory responses
to chemical stimuli.
EXERCISE TRAINING PROMOTES BLOOD CLOTTING AND FIBRINOLYSIS
Blood coagulation and fibrionolysis constitute two physiological
opponents in hemostasis and thrombus formation. Van den Burg et al. (p.
613) report that previously sedentary subjects engaging in 12 wk of
submaximal exercise training increased factor VIII coagulant activity,
reflected in a decrease in partial thromboplastin time during maximal
exercise. Fibrinolytic potential was also enhanced. The results were
complicated by an opposite effect on fibrinolysis in nonexercising
control subjects studied over the same period (February-June).
Thus training promotes both coagulation and fibrinolytic potential
during exercise and may counteract unfavorable seasonal effects on
fibrinolysis.
REGIONAL HETEROGENEITY OF PULMONARY BLOOD FLOW
Pulmonary perfusion heterogeneity is an important determinant of the
gas- exchange function of the lungs. The role of gravity in determining
the regional distribution of perfusion in the lungs is well known, but
recently there has been considerable interest in observations of
nongravity-dependent heterogeneity (see Invited Editorial by J. M. B. Hughes, J. Appl. Physiol. 81:
1049-1050, 1996). Walther et al. (p. 678) revisited this problem
in the sheep by measuring the three-dimensional flow distribution using
the fluorescent-microsphere method. The researchers found that, in this species, there is a systematic decrease in regional flow with
distance from the hilum that accounts for ~34% of the flow variability in the lungs. They did not detect a vertical gradient attributable to effects of gravity.
ROLE OF THE PONS IN BREATHING
