CONNECTIVE TISSUE AND SMOOTH MUSCLE MECHANICS

Meiss (p. 5) examines the hypothesis that as airway smooth muscle cells shorten, radially arranged connective tissue constrains radial expansion, thereby causing an increased axial stiffness. Using canine tracheal smooth muscle, he increased radial constraint by placing very thin Silastic bands around the tracheal smooth muscle strip at resting muscle length. The airway smooth muscle was then made to shorten against a very small afterload until restrained by the bands. It was found that subsequent removal of the bands allowed further shortening of the airway smooth muscle with less increase in stiffness at shorter lengths. In a second experimental approach, connective tissue constraints in the tracheal smooth muscle were reduced by partial collagenase digestion. This resulted in further shortening of the airway smooth muscle with less increase in stiffness at shorter lengths. These results demonstrate the importance of extracellular structures in constraining airway smooth muscle shortening. The paper is discussed in an Invited Editorial by Fredberg (p. 3).

	DEVELOPMENTAL PATTERN OF PULMONARY SURFACTANT METABOLISM

Isolated surfactant exists in heavy and light forms. Heavy surfactant, believed to be the functional form, is broken down to light surfactant by convertase, an enzyme present in lamellar bodies. Oulton et al. (p. 71) have investigated changes in convertase activity during development in the rabbit. Lamellar bodies from fetuses, newborns, and adults all showed similarly high convertase activity. However, convertase activity was significantly decreased in heavy surfactant from term fetuses, compared with newborns and adults. This may account for earlier findings that surfactant from fetuses is better at treating respiratory distress in preterm rabbits than is surfactant from adults.

	MUSCLE POWER LOSSES IN SPACEFLIGHT

Antonutto et al. (p. 85) measured maximal explosive power on a force platform and maximal leg cycling power (MCP) on a cycle ergometer as well as muscle mass by nuclear magnetic resonance on four astronauts before and after reentry. Spaceflights were from 31 to 180 days in duration. MCP fell as much as 55% after 180 days in space but fell less, to 75% of the preflight value, after 31 days. Muscle mass decreased relatively less (9 to - 13%). The authors conclude that weightlessness in spaceflight has a major impact on motor unit recruitment.

	VAGAL BRONCHIAL VASODILATOR PATHWAYS

Pissari et al. (p. 105) examined the relative contributions of cholinergic and noncholinergic parasympathetic pathways and of sensory axon reflexes to vagal bronchial vasodilation in chloralose-anesthetized dogs. Electrical stimulation with various current intensities of the peripheral cut end of the cervical vagus nerve revealed that vagal bronchial vasodilation was caused by efferent ganglionic pathways that included both cholinergic and noncholinergic (probably vasoactive intestinal polypeptide) parasympathetic neurotransmitters. Nonganglionic pathways originating from sensory nerve axons were also present but made only a small contribution to the overall bronchial vasodilation evoked by stimulation of the vagus nerve.

	CORTICOSTEROID EFFECTS ON DIAPHRAGM NEUROMUSCULAR JUNCTIONS

The effects of corticosteroid (CS) treatment on muscle fiber and neuromuscular junction (NMJ) morphology were examined in rat diaphragm muscle by Sieck and associates (p. 114). A novel feature of this work was that the effects of CS treatment on the diaphragm were examined in a fiber-specific manner. Whereas CS treatment had little effect on type I and type IIa muscle fibers, muscle fibers expressing the 2X isoform of myosin heavy chain, either alone or together with the myosin heavy chain 2B isoform (type IIx/b), underwent significant atrophy and reduction in NMJ size. However, the reduction in fiber size was disproportionately larger than that observed in the NMJ. This increase in the NMJ-to-fiber size ratio was associated with a CS-dependent reduction in neuromuscular transmission failure in the diaphragm.

	NITRIC OXIDE IN CONDUCTING AIRWAYS

DuBois et al. (p. 159) examined the production and absorption of nitric oxide (NO) in different segments of the human airways. Normal subjects inspired air or mixtures of NO and air, then held their breaths for 1 or 10 s before breathing out. NO was measured in fractions of the expired air corresponding to different regions of the airways. The absorption coefficient, rate of production, and equilibrium concentration of NO for different airway regions were calculated from the dependence of NO concentration on duration of breath holding. Equilibrium concentrations decreased with increasing airway generation. Absorption coefficients and rates of production were higher in bronchioles and respiratory bronchioles than in trachea and bronchi. Equilibrium concentrations and production rates were elevated in one elderly subject.

	TRANSIENT RESPONSE OF A GROWTH FACTOR TO SUSTAINED HYPOXIA

Hypoxia leads to increased brain capillary density in mice. Kuo et al. (p. 260) noted that vascular endothelial growth factor (VEGF) apparently plays an important role in initiating the increased brain vascularity in response to hypoxia. They wondered whether VEGF was also involved in maintenance of the increase during prolonged hypoxia. They found that both the VEGF mRNA and protein expression were transient responses to hypoxia, returning to near-normoxic levels after 7 days of continuous hypoxic exposure. The results suggest that the increased capillary density may substantially correct the error signal responsible for the increased VEGF expression during hypoxia.

	BRAIN COOLING DURING NEAR- DROWNING

Cold-water near-drownings have occurred in which victims survived cold-water immersion for many minutes with full or partial neurological recovery. This phenomenon presumably reflects protective actions of cooling of the body and/or brain. Xu et al. (p. 265) have developed a mathematical model that accounts for direct cooling of the brain via the circulation. The results indicate that conductive heat loss through the skull surface or upper airways is minimal and would not provide significant protection. However, ventilation of cold water together with circulatory cooling would provide substantial cooling of the brain and hypothermic protection.

	RESPIRATORY PATTERNS IN SLEEPING INFANTS

Techniques derived from dynamic systems theory have been applied to an investigation of time series recordings of infant breathing during sleep by Small et al. (p. 359). Using estimation of correlation dimension, surrogate analysis, and reduced linear modeling, the respiratory patterns of 10 healthy infants were investigated at 2 mo of age. The results reveal that the dynamics of infant breathing in quiet sleep can be best described as a nonlinear dynamical system with large-scale, low-dimensional and small-scale, high-dimensional behavior, together with a noise-driven nonlinear system in two-dimensional periodic orbit. The authors conclude that the data are consistent with respiration being chaotic in sleeping infants.