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Biomedical subjects

L Stevens

Publications and source records attributed to L Stevens.

At least 91 records · Page 5Linked to original sources
PubMed1996

Egg proteins: what are their functions?

The principal functions so far attributed to egg proteins are: (i) provision of nutrient for the developing embryo; (ii) protection of the egg from microbial attack; and (iii) transport of nutrients into the developing embryo. All the proteins contribute to the nutrient role in the form of the amino acid residues they contain. Some also contribute to the phosphate reserves. Protection is provided by a variety of means, including the physical barrier of albumen with its high viscosity, and the chemical barriers in the form of protease inhibitors, nutrient sequesters, a lytic enzyme, and antibodies. Various proteins are involved in nutrient uptake, either by acting as carriers or in the form of membrane receptors.

Amino Acids↗
PubMed1995

Structure-function relationship of soleus muscle fibres from the rhesus monkey.

Functional and structural properties of rhesus monkey skinned fibres were studied in order to examine the relationship between calcium/strontium (Ca/Sr) activation characteristics and protein composition. The fibres were classified according to their Ca/Sr affinity into slow (61%) and fast groups (39%). According to the myosin isoform composition, two additional hybrid types were defined. Thus, four profiles were characterized: two corresponding to slow (S) and fast (F) isoforms and two corresponding to a mixed proportion of slow and fast isoforms. They were called hybrid slow (HS) or hybrid fast (HF) based on the predominant myosin isoform. Tension/pCa parameters and maximal shortening velocities were determined. S fibres showed a higher pCa threshold and affinity as well as shallower slopes of their tension/pCa curve than did F fibres. HS and HF fibres exhibited tension/pCa curves which were positioned close to those of S and F fibres, respectively. No significant difference was observed between S and HS fibres or between F and HF fibres. Maximal shortening velocity values were higher for fibres expressing predominantly fast myosin isoforms. We suggest than when both S and F isoforms of myofibrillar proteins are expressed in a muscle fibre, the functional properties are mainly governed by the predominant isoform.

Animals↗
PubMed1994

The effect of population subdivision on the rate of spread of parasite-mediated cytoplasmic incompatibility.

Over 20 species of arthropods are known to harbor an endosymbiotic bacterium, Wolbachia associated with partial reproductive incompatibility. The micro-organism spreads through a random mating population with positive frequency dependence. Many insect populations, however, are subdivided to some degree. In this paper we model the effect of population subdivision on the rate of spread of Wolbachia and show that it slows the rate of spread of the micro-organism but by itself cannot cause a stable polymorphism. The effects of the micro-organism on host fitness, partial incompatibility, and environmental curing are also investigated, both for panmictic and subdivided populations. In all cases, population subdivision slows the rate of spread relative to a panmictic population, but does not produce a stable polymorphism of infection.

Animals↗
PubMed1994

Usefulness of sequential urinary follicle-stimulating hormone and luteinizing hormone measurements in the diagnosis of adolescent hypogonadotropism in males.

FSH and/or LH deficiency in the second decade of life remains difficult to diagnose with testing at a single point in time because of the partial lack of hormone as well as the dynamic and inherently variable aspects of the pubertal process. A longitudinal study of gonadotropin excretion, therefore, was carried out in 78 normal boys and 157 male patients, aged 10-28 yr, with relative or absolute deficiencies of FSH and/or LH. Seven hundred and fifty-five timed urine samples were extracted with acetone, concentrated, and subjected to RIA. The results from patient groups with multiple tropic hormone deficiencies or isolated gonadotropin deficiency were clearly different from those of normal boys and individuals with constitutional delay in puberty. However, multiple samples obtained over a 2-yr period and, in selected cases, until the late teenage years may be required to diagnose gonadotropin deficiency in some patients, even using stringent predictive modeling criteria.

Adolescent↗
PubMed1993

Effects of chronic electrostimulation on rat soleus skinned fibers during hindlimb suspension.

In order to counteract the changes of the contractile properties of the rat soleus occurring during 10 days of hypokinesia-hypodynamia, due to hindlimb suspension (HS), two different patterns of electrostimulation were applied to the tibial nerve. The contractile properties of single chemically skinned muscle fibers were investigated using the tension-pCa relationship characteristics, the similar or different calcium and strontium affinities, and by measuring the P/tmax kinetic parameters. Our results showed that a pattern similar to firing rates of motoneurons innervating slow twitch muscles inhibited the slow to fast fiber changes observed during HS, whereas a pattern similar to firing rates of motoneurons from fast twitch muscles seemed to favor these changes. Since neither pattern maintained the isometric contractile force developed by the soleus fibers, we concluded that the decrease in mechanical strains imposed on the muscle during unloading was the main factor for the development of atrophy, while the kinetic changes might be predominantly modulated by the nervous command.

Animals↗
PubMed1993

Mechanical analysis of the effects of bacteria and aprotinin on skin wound healing in adult guinea pigs.

Inhibition of wound healing by bacteria may result in part from the conversion of plasminogen to plasmin. This conversion results in dissolution of the fibrin seal in a wound or between skin graft and bed. Aprotinin blocks conversion to plasmin, preserving the fibrin clot. The current study was undertaken to determine the effects of high concentrations of bacteria on wound healing and how these effects are mitigated by aprotinin. Dorsal full-thickness skin incisions were made in 40 anesthetized guinea pigs and closed with nylon sutures. Animals were divided into four groups: (1) control wounds, (2) infected wounds, (3) wounds treated with aprotinin, and (4) infected wounds plus aprotinin (single dose). Animals were killed 3 and 4 weeks after the operation. Skin strips containing segments of the healing wounds were pulled apart by a tensiometer until rupture. Stress-strain curves were generated, and wound strength and toughness were determined. All wounds, including those inoculated with bacteria, appeared healed. The 3-week infected group healed with the least strength and toughness (p < 0.001). A single dose of aprotinin administered with the bacterial inoculum reversed this inhibition. In the 4-week groups, the strongest and toughest wounds resulted from bacterial inoculation alone. Aprotinin alone augmented wound healing when compared with controls. These data suggest that wound healing in both clean and infected wounds is augmented when the plasminogen-plasmin pathway is inhibited.

Journal Article↗
PubMed1993

Functional adaptation of different rat skeletal muscles to weightlessness.

The adaptation to weightlessness of two postural muscles, the slow soleus (SOL) and the fast gastrocnemius lateralis (GL), and a fast muscle used in movements, the extensor digitorum longus (EDL), was studied on five adult Wistar rats. The animals exposed to 14-day spaceflight aboard COSMOS 2044, designated as flight (F), were compared with synchronous (S) animals. The experiments were performed on single skinned fibers whose functional properties were studied. After weightlessness, the SOL exhibited two populations of fibers according to their Sr2+ affinities: 40% remained slow (Fs) and 60% acquired fast-type properties (Ff). Both S and F GL and EDL showed a single distributed population of fast fibers. SOL fibers atrophied insofar as they showed a significant reduction in fiber diameter and absolute maximal tension Po (mg) but not in Po expressed in kg/cm2. GL fibers showed no change in fiber diameter but a decrease in Po in mg and kg/cm2. EDL fibers were not atrophied by weightlessness. The tension/Ca concentration relationships of the Ff SOL and F GL fibers were shifted to the right, indicating a decrease in their Ca2+ affinity. An increase in the contraction kinetics was described for the SOL fibers after weightlessness, whereas no significant modification was found for the GL and EDL. Collectively, the data suggested that the adaptive changes subsequent to weightlessness were more dependent on the muscle function than on the fiber type, since both postural SOL and GL were modified.

Adaptation, Physiological↗
PubMed1993

Seizure outcome in patients with surgically treated cerebral arteriovenous malformations.

A series of 280 cases of cerebral arteriovenous malformations (AVM's) treated surgically between June, 1970, and June, 1989, is reviewed with particular focus on the preoperative seizure history and follow-up seizure status. Follow-up evaluation (mean duration 7.5 years) was achieved in 98% of cases and was accomplished through re-examinations, telephone interviews, and written questionnaires. Overall, 89% of the surviving patients with a follow-up period of greater than 2 years were free of seizures at last examination. Of the 280 patients in this series, 163 had experienced no seizures preoperatively. A recent follow-up study (with a minimum duration of 2 years or to death) was available in 157 of these 163 cases; 21 patients had died. Of the 136 surviving patients, only eight (6%) were having new ongoing seizures. In the 128 (94%) who had remained seizure-free, 73% were receiving no anticonvulsant agents while 27% were taking anticonvulsant prophylaxis. The 2-year minimum follow-up study in 110 of the 117 patients with preoperative seizures revealed that eight (7%) had died. Of the 102 surviving patients, 85 (83%) were seizure-free (with 48% no longer receiving anticonvulsant therapy), while 17 (17%) still suffered intermittent seizures. However, of these 17 patients, 13 reported their seizures to be improved compared to preoperatively; the seizures were the same in two patients and were worse in two patients. An actuarial analysis was conducted comparing the life expectancy of patients following surgery for AVM's with the expected survival of a general white population of the same age and sex in the West Northcentral region of the United States. No statistically significant difference was found. There were seven perioperative deaths (three from cerebral hemorrhage, two from pulmonary emboli, and two from obstruction of venous drainage) and 22 deaths during the follow-up period. Of these 22 deaths, the cause was unknown in four patients, apparently unrelated to the AVM in 13, and directly or indirectly related to the patient's neurological condition prior to surgery or due to surgery performed for resection of the AVM in five. There was a statistically significant relationship between the size and location of the AVM and the clinical presentation. Patients with small AVM's (< 3 cm) were more likely to present with hemorrhage whereas those with large AVM's were more likely to present with seizures.(ABSTRACT TRUNCATED AT 400 WORDS)

Actuarial Analysis↗