Biomedical subjects

Newman L Stephens

Publications and source records attributed to Newman L Stephens.

6 recordsLinked to original sources

PubMed2005

Ontogenesis of myosin light chain phosphorylation in guinea pig tracheal smooth muscle.

Increased airway responsiveness occurs in normal young individuals compared to adults. A maturation of airway smooth muscle (ASM) contractility is likely a mechanism of this juvenile airway hyperresponsiveness. Indeed, we showed in guinea pig tracheal smooth muscle (TSM) that maximum shortening velocity decreases dramatically after the first 3 weeks of life. Because the phosphorylation of the 20-kDa myosin light chain (MLC(20)) was shown to be a key event in ASM contractility, in the present work we sought to investigate it during ontogenesis. In three age groups (1-week-old, 3-week-old, and adult guinea pigs), we assessed the amount of MLC(20) phosphorylation achieved either in TSM crude protein homogenates exposed to Mg(2+) . ATP . CaCl(2) or in tracheal strips during electrical field stimulation (EFS). Phosphorylated and unphosphorylated MLC(20) were separated on nondenaturing 10% polyacrylamide gels, and the ratio of phosphorylation was obtained by densitometric analysis of chemiluminescent Western immunoblots. Maximum MLC(20) phosphorylation (% of total MLC(20)) in TSM tissue homogenate was, respectively, 32.6 +/- 5.7, 32.2 +/- 5.7, and 46.8 +/- 5.8 in 1-week, 3-week, and adult guinea pigs. Interestingly, in nonstimulated intact tracheal strips, we found a substantial degree of MLC(20) phosphorylation: respectively, 42.2 +/- 5.8, 36.5 +/- 7.8, and 46.4 +/- 4.7 in 1-week, 3-week, and adult guinea pigs. Maximal EFS-induced MLC(20) phosphorylation (% increase over baseline) in the 3-week age group was attained after 3 sec of EFS, and was 161.2 +/- 17.6, while in 1-week and adult guinea pigs, it was attained at 1.5 sec of EFS and was, respectively, 133.3 +/- 9.3 and 110.2 +/- 3.9 (P < 0.05). We conclude that MLC(20) phosphorylation in guinea pig intact tracheal strips correlates with ontogenetic changes in shortening velocity and changes in myosin light chain kinase content. These results further suggest that the maturation of ASM contractile properties plays a role in the greater airway responsiveness reported in children and young animals.

Animals↗

PubMed2005

Mechanical plasticity and contractile properties of airway smooth muscle.

Smooth muscle has the unique ability to adapt easily and quickly to length changes without compromising its ability to generate force. This ability is referred to as mechanical plasticity and is now considered to be an important aspect of smooth muscle that affects both its contractile and relaxation behaviour. It is therefore important to incorporate knowledge of plasticity into further studies of smooth muscle behaviour. It is also important that future studies be focused on deciphering the mechanism of smooth muscle length adaptation and plasticity. This review outlines some of the proposed mechanisms determining plasticity. However, it should be said that there are other proposed mechanisms not touched upon here, which may be equally as important. This review also focuses on the relevance of smooth muscle plasticity in asthma, but it is important to remember that there are other places where smooth muscle plasticity may play an equally important role.

Adaptation, Physiological↗

PubMed2004

On the terminology for describing the length-force relationship and its changes in airway smooth muscle.

The observation that the length-force relationship in airway smooth muscle can be shifted along the length axis by accommodating the muscle at different lengths has stimulated great interest. In light of the recent understanding of the dynamic nature of length-force relationship, many of our concepts regarding smooth muscle mechanical properties, including the notion that the muscle possesses a unique optimal length that correlates to maximal force generation, are likely to be incorrect. To facilitate accurate and efficient communication among scientists interested in the function of airway smooth muscle, a revised and collectively accepted nomenclature describing the adaptive and dynamic nature of the length-force relationship will be invaluable. Setting aside the issue of underlying mechanism, the purpose of this article is to define terminology that will aid investigators in describing observed phenomena. In particular, we recommend that the term "optimal length" (or any other term implying a unique length that correlates with maximal force generation) for airway smooth muscle be avoided. Instead, the in situ length or an arbitrary but clearly defined reference length should be used. We propose the usage of "length adaptation" to describe the phenomenon whereby the length-force curve of a muscle shifts along the length axis due to accommodation of the muscle at different lengths. We also discuss frequently used terms that do not have commonly accepted definitions that should be used cautiously.

Animals↗

PubMed2003

The biophysics of asthmatic airway smooth muscle.

It is clear that significant advances have been made in the understanding of the physiology, biochemistry and molecular biology of airway smooth muscle (ASM) contraction and how the knowledge obtained from these approaches may be used to elucidate the pathogenesis of asthma. Not to belittle other theories of smooth muscle contraction extant in the field, perhaps the most outstanding development has been the formulation of plasticity theory. This may radically alter our understanding of smooth muscle contraction. Its message is that while shortening velocity and capacity are linear functions of length, active force is length independent. These changes are explained by the ability of thick filament protein to depolymerize at short lengths and to increase numbers of contractile units in series at lengths greater than optimal length or L(ref). Other advances are represented by the report that the major part of ASM shortening is complete within the initial first 20% of contraction time, that the nature and history of loading determine the extent of shortening and that these findings can be explained by the finding that the crossbridges are cycling four times faster than in the remaining time. Another unexpected finding is that late in the course of isotonic relaxation the muscle undergoes spontaneous activation which delays relaxation and smoothes it out; speculatively this could minimize turbulence of airflow. On the applied front evidence now shows the shortening ability of bronchial smooth muscle of human subjects of asthma is significantly increased. Measurements also indicate that increased smooth muscle myosin light chain kinase content, via increased actomyosin ATPase activity could be responsible for the changes in contractility.

Adaptation, Physiological↗

PubMed2003

Structure-function correlation in airway smooth muscle adapted to different lengths.

Airway smooth muscle is able to adapt and maintain a nearly constant maximal force generation over a large length range. This implies that a fixed filament lattice such as that found in striated muscle may not exist in this tissue and that plastic remodeling of its contractile and cytoskeletal filaments may be involved in the process of length adaptation that optimizes contractile filament overlap. Here, we show that isometric force produced by airway smooth muscle is independent of muscle length over a twofold length change; cell cross-sectional area was inversely proportional to cell length, implying that the cell volume was conserved at different lengths; shortening velocity and myosin filament density varied similarly to length change: increased by 69.4% +/- 5.7 (SE) and 76.0% +/- 9.8, respectively, for a 100% increase in cell length. Muscle power output, ATPase rate, and myosin filament density also have the same dependence on muscle cell length: increased by 35.4% +/- 6.7, 34.6% +/- 3.4, and 35.6% +/- 10.6, respectively, for a 50% increase in cell length. The data can be explained by a model in which additional contractile units containing myosin filaments are formed and placed in series with existing contractile units when the muscle is adapted at a longer length.

Adaptation, Physiological↗

PubMed2002

Changes in biophysical and biochemical properties of single bronchial smooth muscle cells from asthmatic subjects.

Whether contractility of bronchial smooth muscle cells (BSMC) from asthmatic subjects is significantly altered has never been validated. We tested the hypothesis that such BSMC show increased contractility. Cells were isolated from endobronchial biopsies. BSMC shortening was measured under an inverted microscope. Statistically significant increases in maximum shortening capacity (Delta L max) and velocity (Vo) were found in asthmatic BSMC compared with normal cells. Mean Delta L max in asthmatic BSMC was 39.05 +/- 1.99% (SE) of resting cell length compared with 28.6 +/- 1.1% in normal cells; mean Vo was 7.2 +/- 0.8% of resting cell length/s in asthmatic cells and 5.23 +/- 0.46% in normal cells. To investigate the mechanism of the increased contractility, we measured mRNA abundance of smooth muscle types of myosin light chain kinase (smMLCK) and myosin heavy chain. RT-PCR data revealed that smMLCK mRNA was higher in asthmatic BSMC (0.106 +/- 0.021 arbitrary densitometric units, n = 7) than in control cells (0.04 +/- 0.008, n = 11; P < 0.05). Messages for myosin heavy chain isoforms showed no difference. Increased kinase message content is an index of the mechanism for the increased velocity and capacity of shortening we report.

Adult↗