Capillaries measured in canine diaphragm by two methods

Titin-based mechanosensing modulates muscle hypertrophy

BACKGROUND

Titin is an elastic sarcomeric filament that has been proposed to play a key role in mechanosensing and trophicity of muscle. However, evidence for this proposal is scarce due to the lack of appropriate experimental models to directly test the role of titin in mechanosensing.

METHODS

We used unilateral diaphragm denervation (UDD) in mice, an in vivo model in which the denervated hemidiaphragm is passively stretched by the contralateral, innervated hemidiaphragm and hypertrophy rapidly occurs.

RESULTS

In wildtype mice, the denervated hemidiaphragm mass increased 48 ± 3% after 6 days of UDD, due to the addition of both sarcomeres in series and in parallel. To test whether titin stiffness modulates the hypertrophy response, RBM20ΔRRM and TtnΔIAjxn mouse models were used, with decreased and increased titin stiffness, respectively. RBM20ΔRRM mice (reduced stiffness) showed a 20 ± 6% attenuated hypertrophy response, whereas the TtnΔIAjxn mice (increased stiffness) showed an 18 ± 8% exaggerated response after UDD. Thus, muscle hypertrophy scales with titin stiffness. Protein expression analysis revealed that titin-binding proteins implicated previously in muscle trophicity were induced during UDD, MARP1 & 2, FHL1, and MuRF1.

CONCLUSIONS

Titin functions as a mechanosensor that regulates muscle trophicity.

Capillary supply, fibre types and fibre morphometry in rat tibialis anterior and diaphragm muscles after intermittent exposure to hypobaric hypoxia

Three groups of sedentary male rats were exposed to intermittent hypobaric hypoxia (IHH) for 22 days (4 h/day, 5 days/week) in a hypobaric chamber at a simulated altitude of 5,000 m. Tibialis anterior (TA) and diaphragm (DG) were removed at the end of the programme (H group), and 20 or 40 days later (P20 and P40 groups). A control group (C) was maintained at sea-level pressure and their TA and DG were compared to those of the experimental rats at the end of the IHH programme, and also 20 and 40 days later. We measured the fibre morphometry and capillaries of each muscle. Our results demonstrate that IHH does not change the fibre type composition (with reference to either their contractile or oxidative properties) for most muscle regions of the muscles analysed analysed. We found few significant differences in muscle capillarity and fibre morphometry for TA after IHH. However, IHH did induce some statistically significant changes in DG: capillary density of the H rats (736 capillaries/mm2) increased compared to C animals (610 capillaries/mm2). Although IHH did not change the fibre capillarization or morphometric parameters of fast fibre types, we observed reductions ranging from 7 to 13% in fibre area, perimeter and diffusion distances between C and H for slow fibres. Moreover, these morphometric changes accounted for increases of 10-20% in capillarization, fibre unit area and fibre unit perimeter. This indicates that SO fibres are more sensitive to IHH than both fast fibre types.

Efficient design of the diaphragm: distribution of blood flow relative to mechanical advantage

The mammalian diaphragm is composed of two separate muscles (costal and crural) connected by a central tendon that serves as a piston head for drawing air into the lungs. The two muscles are described as having different embryological origins, segmental innervations, and mechanical functions [De Troyer A, Sampson M, Sigrist S, and Macklem PT. Science 213: 237-238, 1981; De Troyer A, Sampson M, Sigrist S, and Macklem PT. J Appl Physiol 53: 30-39, 1982]. On the basis of regional blood flow measurements, the two muscles appear to be nonuniformly recruited at rest, but we anticipated that the two muscles would become uniformly recruited at heavy exercise to efficiently support the high energy requirements of ventilation. We used fluorescent microspheres to measure regional blood flow within the two muscles as an index of muscle recruitment from rest to heavy treadmill exercise in well-trained foxhounds. However, the heterogeneity of blood flow at rest persisted as exercise workloads were increased. Blood flow per gram of muscle remained twofold greater in ventral than dorsal regions of both muscles from rest to heavy exercise. This pattern was matched by a twofold greater regional mechanical advantage in ventral than dorsal regions of the two muscles measured anatomically. Hence blood flow was preferentially and efficiently distributed to those regions capable of generating the greatest inspiratory power independent of muscle mass. The two muscles were recruited from rest to heavy exercise as a single functional unit, not as two muscles under separate control.

Diaphragm structure and function in health and disease

The diaphragm is the primary muscle of inspiration, and as such uncompromised function is essential to support the ventilatory and gas exchange demands associated with physical activity. The normal healthy diaphragm may fatigue during intense exercise, and diaphragm function is compromised with aging and obesity. However, more insidiously, respiratory diseases such as emphysema mechanically disadvantage the diaphragm, sometimes leading to muscle failure and death. Based on metabolic considerations, recent evidence suggests that specific regions of the diaphragm may be or may become more susceptible to failure than others. This paper reviews the regional differences in mechanical and metabolic activity within the diaphragm and how such heterogeneities might influence diaphragm function in health and disease. Our objective is to address five principal areas: 1) Regional diaphragm structure and mechanics (GAF). 2) Regional differences in blood flow within the diaphragm (WLS). 3) Structural and functional interrelationships within the diaphragm microcirculation (DCP). 4) Nitric oxide and its vasoactive and contractile influences within the diaphragm (MBR). 5) Metabolic and contractile protein plasticity in the diaphragm (SKP). These topics have been incorporated into three discrete sections: Functional Anatomy and Morphology, Physiology, and Plasticity in Health and Disease. Where pertinent, limitations in our understanding of diaphragm function are addressed along with potential avenues for future research.

Regulation of ventilatory muscle blood flow

The ventilatory muscles perform various functions such as ventilation of the lungs, postural stabilization, and expulsive maneuvers (e.g., coughing). They are classified in functional terms as inspiratory muscles, which include the diaphragm, parasternal intercostal, external intercostal, scalene, and sternocleidomastoid muscles; and expiratory muscles, which include the abdominal muscles, internal intercostal, and triangularis sterni. The ventilatory muscles require high-energy phosphate compounds such as ATP to fuel the biochemical and physical processes of contraction and relaxation. Maintaining adequate intracellular concentrations of these compounds depends on adequate intracellular substrate levels and delivery of these substrates by arterial blood flow. In addition to the delivery of substrates, blood flow influences muscle function through the removal of metabolic by-products, which, if accumulated, could exert negative effects on several excitatory and contractile processes. Skeletal muscle substrate utilization is also dependent on the ability to extract substrates from arterial blood, which, in turn, is accomplished by increasing the total number of perfused capillaries. It follows that matching perfusion to metabolic demands is critical for the maintenance of normal muscle contractile function. In this article, I review the factors that influence ventilatory muscle blood flow. Major emphasis is placed on the diaphragm because a large number of published reports deal with diaphragmatic blood flow. The second reason for focusing on the diaphragm is because it is the largest and most important inspiratory muscle.

Comparative effects of aging on pharyngeal and diaphragm muscles

We hypothesized that aging is associated with alterations in pharyngeal muscle structural and contractile properties. Sternohyoid and geniohyoid muscles from young (3-4 months) and old (20-21 months) Fischer 344 rats were compared with diaphragm muscle. The pharyngeal muscles had significantly lower proportions of slow oxidative (SO) fibers compared to the diaphragm, and the percentage of fast glycolytic (FG) fibers was significantly higher in the sternohyoid than in both the geniohyoid and the diaphragm. With senescence, there was a small but significant increase in the proportion of FG fibers and a corresponding reduction in the proportion of fast oxidative glycolytic (FOG) fibers in all three muscles. The sternohyoid muscle had significantly faster isometric contractile kinetics and lower fatigue indexes than the diaphragm. Aging was associated with significant worsening of sternohyoid endurance, but no significant alterations in sternohyoid twitch kinetics or diaphragm properties. These results indicate that in rats the pharyngeal dilator muscles have larger proportions of fast fibers, fast contractile kinetics and worse endurance than the diaphragm. Furthermore, aging was associated with a shift to a higher proportion of FG fibers with a concomitant reduction in proportion of FOG fibers, as well as a decline in pharyngeal muscle endurance.