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Biomedical subjects

T P White

Publications and source records attributed to T P White.

At least 19 recordsLinked to original sources
PubMed2006

Wide-angle coupling into rod-type photonic crystals with ultralow reflectance.

We describe the surprising phenomenon of near-perfect coupling from free space into uniform two-dimensional rod-type photonic crystals over a wide range of incident angles. This behavior is shown to be a generic feature of many rod-type photonic crystal structures that is related to strong forward scattering resonances of the individual cylinders. We explain these results using both semianalytic analysis and two-dimensional numerical calculations and identify the conditions under which efficient, wide-angle coupling can occur. The results may lead to more efficient designs for in-band photonic crystal devices such as superprisms and self-collimation based photonic circuits.

Journal Article↗
PubMed2004

Bloch mode scattering matrix methods for modeling extended photonic crystal structures. I. Theory.

We present a rigorous Bloch mode scattering matrix method for modeling two-dimensional photonic crystal structures and discuss the formal properties of the formulation. Reciprocity and energy conservation considerations lead to modal orthogonality relations and normalization, both of which are required for mode calculations in inhomogeneous media. Relations are derived for studying the propagation of Bloch modes through photonic crystal structures, and for the reflection and transmission of these modes at interfaces with other photonic crystal structures.

Journal Article↗
PubMed2004

Bloch mode scattering matrix methods for modeling extended photonic crystal structures. II. Applications.

The Bloch mode scattering matrix method is applied to several photonic crystal waveguide structures and devices, including waveguide dislocations, a Fabry-Pérot resonator, a folded directional coupler, and a Y-junction design. The method is an efficient tool for calculating the properties of extended photonic crystal (PC) devices, in particular when the device consists of a small number of distinct photonic crystal structures, or for long propagation lengths through uniform PC waveguides. The physical insight provided by the method is used to derive simple, semianalytic models that allow fast and efficient calculations of complex photonic crystal structures. We discuss the situations in which such simplifications can be made and provide examples.

Journal Article↗
PubMed2004

Modes of coupled photonic crystal waveguides.

We consider the modes of coupled photonic crystal waveguides. We find that the fundamental modes of these structures can be either even or odd, in contrast with the behavior in coupled conventional waveguides, in which the fundamental mode is always even. We explain this finding using an asymptotic model that is valid for long wavelengths.

Journal Article↗
PubMed2003

Ultracompact resonant filters in photonic crystals.

A novel design for an ultracompact, high-Q notch-rejection filter is presented, and an analytic expression for the transmission properties is derived. This folded directional coupler shares the properties of a Fabry-Perot resonator and a directional coupler. We compare and contrast the device to high-Q Fabry-Perot cavities in photonic crystal waveguides.

Journal Article↗
PubMed2003

Semianalytic treatment for propagation in finite photonic crystal waveguides.

We present a semianalytic theory for the properties of two-dimensional photonic crystal waveguides of finite length. For single-mode guides, the transmission spectrum and field intensity can be accurately described by a simple two-parameter model. Analogies are drawn with Fabry-Perot interferometers, and generalized Fresnel coefficients for the interfaces are calculated.

Journal Article↗
PubMed2000

Age effects on the adaptive response of the female rat heart following aortic constriction.

Aging is associated with adaptations in the hearts of mammals that diminish the reserve capacity to meet hemodynamic loading challenges. To evaluate potential mechanisms of this phenomenon, the following hypotheses were tested: compared with hearts of adult rats, hearts of aged rats undergoing aortic constriction will exhibit (a) a lower concentration of myofibrillar proteins, (b) a reduced sensitivity to extracellular calcium, and (c) a reduced coronary perfusion. Female Fischer 344 rats aged 9 months (adult) and 27 months (aged) were assigned to control (C) or aortic-constriction (AC) groups and studied at 7 and 28 days post-AC, yielding six groups of rats. Analysis of variance was used to examine the effects of age and AC. The left ventricular (LV) mass/body mass ratio expressed a percentage of age-matched control value averaged AC-7adult, 111%; AC-28adult, 120%; AC-7aged, 106%; AC-28aged, 123% (AC, p < .01). As a percentage of adult rats values, the pressure-generating capacity of the LV averaged Caged, 99%; AC-7aged, 92%; AC-28aged, 92% (age, p < .05). There were no differences attributable to age or AC in either myofibrillar protein concentration or calcium sensitivity. There was, however, a significantly lower concentration of nonmyofibrillar protein (approximately 10%) in the hearts of all three groups of aged rats compared with the adult rats that was unaltered by AC. The percentages of LV myosin heavy chain in the alpha-isoform were Cadult, 77%; AC-7adult, 66%; AC-28adult, 66%; Caged, 45%; AC-7aged, 41%; AC-28aged, 32% (age, p < .01; AC,p < .01). Coronary flow per gram of tissue averaged 9% lower in all three of the aged groups compared with the adult rats and was not significantly affected by AC (age, p < .05). The data suggest that a reduction in nonmyofibrillar protein and a reduced coronary flow, rather than changes in calcium sensitivity or myofibrillar protein, are associated with an impairment in the adaptive response of the aged heart.

Age Factors↗
PubMed1999

Regional variation in cardiac myosin isoforms of female F344 rats during aging.

Myosin heavy chain (MHC) is a major contractile protein of heart muscle consisting of two isoforms in the rat, alpha-MHC that predominates in the hearts of young rats, and beta-MHC that progressively replaces it as the rats age. It was hypothesized that the magnitude of the age-associated decrease in the proportion of cardiac alpha-MHC would be similar in regions of the heart that differed in their initial MHC isoform pattern. MHCs from hearts of female Fischer 344 rats 3, 9, 15, 18, 24, and 27 months of age were separated by gradient gel electrophoresis. Hypertrophy was assessed by indexing regional heart mass to tibial length From 9 through 27 months of age, hypertrophy was 19% and 77% in the left ventricle and left atrial appendage, respectively. There was no significant hypertrophy in either the right ventricular free wall or the right atrial appendage. The proportion of alpha-myosin heavy chain ranged from 86 +/- 1.3% (mean +/- SE) in the right ventricular free wall to 62 +/- 5.8% in left ventricular papillary muscle of 9-month-old rats. In 27-month-old rats, it ranged from 59 +/- 2.7% in the right ventricular free wall to 20 +/- 3.1% in the left ventricular papillary muscle. There was a marked age-associated decrease in the proportion of alpha-myosin heavy chain overall (p <.001) that did not differ significantly among the regions studied (p = .109). These results suggest that the effects of advancing age on the cardiac MHC pattern are independent of age-associated hypertrophy.

Aging↗
PubMed1996

Alterations in skeletal muscle related to short-term impaired physical mobility.

The purpose of this investigation was to describe and compare two methods of recovery of atrophied skeletal muscle following short-term impaired physical mobility. An animal model was used to study morphologic adaptations of atrophied soleus and plantaris muscles to the effects of 7 days of hind-limb suspension (HS) followed by either sedentary recovery or run training during a 28-day recovery period. Significant atrophy, demonstrated by decreased mean fiber area (MFA, in square micrometers), occurred during the 7-day period of HS. During recovery, MFA returned to control values 14 days earlier in the sedentary compared with the trained groups. Runs training following short-term atrophy induced by HS did not result in the high levels of frank muscle damage and type IIC fibers previously reported following long-term (28-day) atrophy.

Analysis of Variance↗
PubMed1996

Functional evaluation at the medial gastrocnemius donor site in rats.

The transfer of a skeletal muscle from a donor to a recipient site creates an initial deficit in the structure and function of the muscle group from which it originates. Removal of the donor muscle induces hypertrophy of the remaining synergistic muscles, which compensate for part of the deficit at the donor site. The medial gastrocnemius (MGN) muscle is a frequently utilized donor muscle. Compared with the mass and force production of the control four-muscle plantar flexor group in rats, removal of the MGN muscle creates an initial deficit of approximately 36 percent. At 60, 90, and 120 days after removal of the MGN muscle, the degree of compensation of the remaining three-muscle plantar flexor group (lateral gastrocnemius, soleus, and plantaris muscles) was evaluated. The mass of the three-muscle group increased 13 percent over the time course studied, but was still 28 percent less than the mass of the control four-muscle group. Similarly, the maximum force of the three-muscle group increased 27 percent, but was 21 percent lower than the control four-muscle group. The authors propose a model that illustrates the function restored at a donor site in terms of the percentage of the total muscle group comprised by the donor muscle and the ability of the remaining muscle group to compensate for its removal.

Adaptation, Physiological↗
PubMed1996

Musculoskeletal adaptations to weightlessness and development of effective countermeasures.

A Research Roundtable, organized by the American College of Sports Medicine with sponsorship from the National Aeronautics and Space Administration, met in November 1995 to define research strategies for effective exercise countermeasures to weightlessness. Exercise was considered both independently of, and in conjunction with, other therapeutic modalities (e.g., pharmacological nutritional, hormonal, and growth-related factors) that could prevent or minimize the structural and functional deficits involving skeletal muscle and bone in response to chronic exposure to weightlessness, as well as return to Earth baseline function if a degree of loss is inevitable. Musculoskeletal deficits and countermeasures are described with respect to: 1) muscle and connective tissue atrophy and localized bone loss, 2) reductions in motor performance, 3) potential proneness to injury of hard and soft tissues, and 4) probable interaction between muscle atrophy and cardiovascular alterations that contribute to the postural hypotension observed immediately upon return from space flight. In spite of a variety of countermeasure protocols utilized previously involving largely endurance types of exercise, there is presently no activity-specific countermeasure(s) that adequately prevent or reduce musculoskeletal deficiencies. It seems apparent that countermeasure exercises that have a greater resistance element, as compared to endurance activities, may prove beneficial to the musculoskeletal system. Many questions remain for scientific investigation to identify efficacious countermeasure protocols, which will be imperative with the emerging era of long-term space flight.

Adaptation, Physiological↗
PubMed1996

Myosin heavy chain of immature soleus muscle grafts adapts to hyperthyroidism more than to physical activity.

The interaction of hyperthyroidism and the elements of physical activity on early regeneration of muscle grafts was investigated. Soleus muscle grafts were studied 15 days after graft operations in eu- and hyperthyroid rats. Hypotheses were tested regarding the adaptation of the myosin heavy chain (MHC) profile of grafts and nongrafted control muscles and whether the effect of hyperthyroidism would predominate over the opposing influence of recruitment and mechanical load on MHC of grafts. Denervation and myectomy of synergist muscles were employed to manipulate the elements of physical activity. Denervation decreased the expression of type I MHC, and hyperthyroidism furthered the shift toward a "fast" isoform profile. For example, in denervated grafts, type IIb was undetected in euthyroid rats and accounted for 31% of MHC in hyperthyroid rats. Compared with control muscles, grafts in the denervated and innervated-normal load groups demonstrated greater plasticity because the adaptive response of MHC to thyroid status was more pronounced. Hyperthyroidism predominated over the opposing influence of the elements of physical activity on the regulation of MHC expression in innervated plus overload grafts. For example, type I MHC was 86% of MHC profile of innervated plus overload grafts in euthyroid rats and was only 49% in hyperthyroid rats. In conclusion, a heightened plasticity for grafts was evidenced in denervated and innervated tissues, and the regulation of MHC by thyroid hormone predominated over that due to the elements of physical activity.

Animals↗
PubMed1995

Mechanical load affects growth and maturation of skeletal muscle grafts.

The purpose of our study was to determine whether the early patterns of growth and maturation of regenerating soleus muscle grafts are sensitive to alterations in mechanical load. We hypothesized that decreased and increased mechanical loading of grafts would reduce and accelerate, respectively, the rate and magnitude of growth and impair and enhance, respectively, the pattern of maturation. On day 0, soleus muscles were grafted and rats were assigned to one of three groups: cage sedentary (normal load), hindlimb suspension (decreased load), or ablation of synergist muscle (increased load). From days 7 to 35, graft mass in cage-sedentary rats increased at a rate of 1.85 mg mass/day. Rates were less for grafts of suspended rats and greater in grafts of ablated rats (-1.06 and 3.89 mg mass/day, respectively; P < 0.01). Neonatal myosin heavy chain (MHC) in grafts reached 10 +/- 1.6% of total MHC at day 7 for cage-sedentary rats, whereas in the suspended animals it reached 11 +/- 2.4% of total MHC at day 14. At days 21 and 35, grafts from the suspended animals had a lower proportion of slow MHC (45 +/- 2.4%) than did grafts from the control and ablated groups (95 +/- 1.5%; P < 0.05). Decreased mechanical load impaired the rate and degree of growth and maturation during regeneration, whereas increased mechanical load enhanced growth characteristics but not maturation.

Animals↗
PubMed1995

Myosin heavy chain phenotype in regenerating skeletal muscle is affected by thyroid hormone.

The purpose was to test hypotheses regarding the affect of thyroid hormone status on development of myosin heavy chain (MHC) protein phenotype in regenerating skeletal muscle. Soleus (SOL) and extensor digitorum longus (EDL) muscle grafts were studied 30 and 60 d following graft operations in eu-, hypo-, and hyperthyroid rats. Hyperthyroidism had a more profound affect on MHC phenotype than did hypothyroidism, and this was noted in SOL grafts more so than EDL grafts. For example, compared with euthyroid hosts at 30 d, SOL grafts from hyperthyroid hosts demonstrated a decrease from 83% to 3% in Type I MHC, and a decrease from 11% to 4% in Type IIa. Furthermore, Type IIx MHC increased from 5% to 12%, and Type IIb MHC increased from 3% to 82%. The change in Type I and IIb MHC noted at 30 d partially or fully reversed to euthyroid values by 60 d, whereas the hyperthyroid-induced differences in Type IIa and IIx were sustained. The adaptation of control muscles to hypo- and hyperthyroidism was qualitatively similar to muscle grafts for all MHC protein isoforms with the exception of Type IIx, which was expressed more so in grafts. For both grafts and control muscles, the adaptive response of MHC phenotype to sustained hyperthyroidism is transient for several of the MHC isoforms.

Animals↗
PubMed1994

Recovery of medial gastrocnemius muscle grafts in rats: implications for the plantar flexor group.

Medial gastrocnemius (MGN) muscles were grafted in 18 rats and evaluated at 60, 90, and 120 days after the operation. Our purpose was to investigate the degree of recovery of the vascularized MGN grafts and the entire plantar flexor muscle group. Compared with control values, muscle mass and maximum force of MGN grafts were decreased by 33 and 38% at 60 days, 22 and 32% at 90 days, and 13 and 15% at 120 days. At 60 and 90 days, the deficits in maximum force for the entire plantar flexor muscle group, including the graft, were 29 and 17%, respectively. No difference was observed at 120 days. At 60 days, the deficit in the total mass of the plantar flexor group was 14% compared with control values, but by 90 days no deficit was observed. The restoration of normal plantar flexor group structure and function indicates that the degree of recovery attained by MGN grafts, although not complete, was sufficient to ensure that the performance of the total muscle group was not compromised.

Adaptation, Physiological↗
PubMed1993

Skeletal muscle regeneration and plasticity of grafts.

The sequence of molecular and cellular events of muscle ontogeny leads to the proliferation, fusion, and differentiation of myoblasts to muscle cells. This sequence is closely paralleled in the grafting-ischemia model in which adult myoblast-satellite cells function as the muscle precursor cells. The study of skeletal muscle regeneration is a fertile and promising area of research in myogenesis. The early regenerative development and maturation of muscle is similar regardless of the perturbation that induced the degeneration-regeneration sequelae. In light of this, we maintain that the skeletal muscle graft model is useful to rigorously evaluate many regulatory aspects of skeletal muscle development and maturation in an adult animal host. One advantage of the graft model is that manipulation of the adult host, such as with exercise or hormone treatment, allows insight into their regulatory roles in muscle development and maturation. These approaches are often not possible for developing skeletal muscle in utero or in ovo. After skeletal muscle grafting, many structural and functional characteristics change with time until they reach a stable value. Successful regeneration requires revascularization, cellular infiltration, phagocytosis of necrotic muscle fibers, proliferation and fusion of muscle precursor cells, reinnervation, and recruitment and loading. The time taken to reach stable values varies among different structural and functional variables, and many reach stable values that are less than those of control skeletal muscle. There are differences in the degree of regenerative success because of the size of muscle mass grafted. In small and large grafts, regeneration is enhanced by facilitation of the reinnervation. Regeneration is evident without vascular repair in grafts of up to approximately 6 g, although in all but the 100 to 150-mg grafts in rats, a significant necrotic core is present. Regeneration is typically unsuccessful when muscle masses greater than 6 g are grafted without vascular repair. Large muscles can be grafted with vascular repair, and in this case, the cellular response is quite different, as the majority of fibers survive rather than degenerate and regenerate. Changing the components of physical activity during skeletal muscle regeneration can alter several attributes of the graft phenotype. The consensus of several experiments supports the interpretation that proper recruitment and force development by grafts are essential variables in the regulation of the development and maturation of muscle grafts. Morphological and physiological attributes of grafts adapt to changes in the habitual level of physical activity in a qualitatively similar fashion to control muscle.(ABSTRACT TRUNCATED AT 400 WORDS)

Adaptation, Physiological↗
PubMed1991

Endurance-training induced changes in skeletal muscle phosphoglycerate kinase of old Wistar rats.

Sufficiently intense, long-term, endurance training has been shown in several studies to induce a variety of adaptations in skeletal muscle, including a substantial restoration of the activities of several muscle enzymes which are known to be modified during biological aging. This activity-restoration may reflect either an increase in the amounts of enzyme proteins or an enhancement of the specific activities of these molecules. The present study examined the effect of long-term endurance training on the status of phosphoglycerate kinase in skeletal muscle of old rats, as compared with the enzyme isolated either from non-trained old or young animals. The kinetics of heat inactivation, which differ markedly between young and old forms of phosphoglycerate kinase, were used as a sensitive probe for the status of the enzyme. The results reveal a remarkable similarity between the heat inactivation patterns of phosphoglycerate kinase from the muscle of old, exercise-trained rats and enzyme purified from young animals, while enzyme samples isolated from sedentary old animals are significantly more heat-stable. Adaptation to endurance-training is thus evident at the molecular level, and maintains phosphoglycerate kinase in its young form. The aging of this enzyme has been previously shown to involve only conformational changes, which develop following a reversible partial oxidation of reactive cysteine residues. Whether the adaptation of the enzyme to endurance-training results from enhancement in its turnover rate (i.e., dwell time in the cell becoming too short for modifications to develop) or is due to increased protection against oxidation (being the first step in the enzyme's aging) remains to be studied.

Aging↗