Search PubMed⌕ Search

Biomedical subjects

R Nassar

Publications and source records attributed to R Nassar.

36 records · Page 2Linked to original sources
PubMed1991

Force-pCa relation and troponin T isoforms of rabbit myocardium.

We have previously reported the existence of at least four troponin T isoforms in rabbit ventricular muscle and described the changes in their distribution with development. In this report we test whether the proportions of the troponin T isoforms are related to the sensitivity of the myofilaments to calcium. We measured the force-pCa relations in 12 detergent-skinned ventricular strands of cardiac muscle from newborn (2-5-day-old) rabbits. We determined from each strand the amount of each troponin T isoform relative to the total amount of troponin T by using sodium dodecyl sulfate-polyacrylamide gel electrophoresis and densitometric scans of Western blots probed with a cardiac-specific troponin T monoclonal antibody, MAb 13-11. To assess the presence of different relative amounts of cardiac and slow skeletal troponin I among the strands, we determined the amount of cardiac troponin I relative to tropomyosin. We determined the Hill coefficient and the pCa for half-maximal force, pCa50, for each strand. pCa50 was related directly to the relative amount of troponin T2 (pslope = 0.037). Our results do not indicate a relation between the Hill coefficient and troponin T2. We also did not find a relation between pCa50 and the cardiac troponin I/tropomyosin ratio, which suggests that the correlation between pCa50 and troponin T2 was not a result of changes in the relative amounts of cardiac and slow skeletal muscle troponin I. Our findings indicate that a relation exists between the force-pCa characteristics of rabbit myocardium and the troponin T isoforms that it expresses, suggesting a role for troponin T in modulating the sensitivity of cardiac myofilaments to calcium.

Animals↗
PubMed1991

To excite a heart: a bird's view.

Ultrastructural investigations of avian cardiac muscle, including ratite hearts, have provided great insights into the mechanisms as to how excitation leads to contraction in the heart. The geometry of the conduction fibers of ratite hearts confirms earlier observations on birds showing that the geometry of the conduction system and its component cells is adapted to hearts of different sizes and rates of contraction so as to maintain a differential in conduction velocities between the conduction system and the working fibers. The study of the ratite conduction fibers bears out the idea of an inverse relationship between the size of the gap junctions and the input resistance of cardiac cells. The anomalous extended junctional SR typical of all avian hearts, proscribes the notion of direct contact transduction into calcium release for contraction of an excitatory signal propagating at the cell surface. Couplings appear well suited to maintain direct, if transitory, connections to the extracellular space in addition to harboring channels for intracellular calcium release.

Actin Cytoskeleton↗
PubMed1990

The in utero left ventricle of the fetal sheep: the effects of isoprenaline.

1. Isoprenaline was infused into in utero fetal lambs to examine the effect of this beta-adrenoceptor agonist on left ventricular stroke volume and left ventricular output and test the generally held assumption that the fetal ventricle is markedly limited in its ability to increase ventricular output. 2. Seven in utero lambs (121-133 days of gestation) were instrumented with left ventricular dimension transducers, an ascending aortic electromagnetic flow transducer, a brachiocephalic arterial catheter and electrocardiographic and atrial pacing electrodes. 3. On the day of study, 5-8 days following surgery and 129-137 days of gestation, each lamb received infusions of isoprenaline, via the superior vena cava or left atrium, at rates which ranged from 0.005-0.5 microgram kg-1 min-1. 4. Heart rate and left ventricular stroke volume, output and end-diastolic and end-systolic dimensions were measured under control conditions and during various levels of isoprenaline infusion, with and without controlling heart rate. 5. Analysis of variance was done using the mean cell model. Least-square means and standard errors of the least-square means are reported. F ratios were calculated from type III sums-of-squares; P less than 0.05 was considered significant. 6. The mean heart rate increased with isoprenaline (P less than 0.0001) from a mean control level of 169 +/- 8 to 281 +/- 9 beats min-1 (+/- S.E.M.). 7. Mean left ventricular end-diastolic and end-systolic minor axis dimensions decreased significantly with isoprenaline from 16.7 +/- 0.1 mm (control) to 15.7 +/- 0.2 mm (P less than 0.0004) and from 11.7 +/- 0.1 mm (control) to 10.4 +/- 0.2 mm (P less than 0.0001) respectively. When heart rate was controlled with atrial pacing, mean end-diastolic dimension increased significantly at the higher isoprenaline doses from 14.6 +/- 0.1 mm (control) to 15.3 +/- 0.2 mm (control) (P = 0.0002), while mean end-systolic dimension fell significantly from 10.9 +/- 0.1 to 10.5 +/- 0.1 mm (P less than 0.003). Inasmuch as stroke volume increased, the increase in end-diastolic dimension and the fall in end-systolic dimension indicate an increase in venous return to the left ventricle. 8. During spontaneous rhythm, isoprenaline increased stroke volume from 2.45 +/- 0.06 ml (control) to 2.63 +/- 0.09 ml, not statistically significant. When heart rate was controlled, stroke volume increased with isoprenaline dose from 1.68 +/- 0.06 ml (control) to 2.40 +/- 0.08 ml (P less than 0.0001).(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗
PubMed1988

Cardiac muscle following quick-freezing: preservation of in vivo ultrastructure and geometry with special emphasis on intercellular clefts in the intact frog heart.

Intact frog, mouse and finch hearts were quick-frozen on a liquid He-cooled copper block. Adjacent frozen samples from the same heart were processed by freeze-substitution (followed by embedding and thin sectioning), freeze-fracture/etch (followed by platinum/carbon replication) and frozen sectioning (followed by freeze-drying), respectively, and examined with the electron microscope for fidelity of reproducing the in vivo state of heart muscle geometry, especially that of the narrow intercellular clefts between frog cardiac muscle cells. It was concluded that quick-freezing followed by the above procedures accomplishes that and that, therefore, narrow intercellular clefts are an invariant feature of the normal anatomy of frog cardiac muscle, which must be considered in physiological experiments. The methodology showed that quick-freezing through the epicardial surface is capable of producing superb cryopreservation for ultrathin cryosections, as well.

Animals↗
PubMed1987

Developmental changes in the ultrastructure and sarcomere shortening of the isolated rabbit ventricular myocyte.

Sarcomere shortening and ultrastructure of intact isolated myocytes from ventricles of three-week-old and adult rabbits were examined. Cells were fixed and embedded, and after measuring their sarcomere shortening in response to electrical stimulation, they were examined in serial thin sections by electron microscopy. This structure-function analysis showed that adult cells were significantly larger, had longer rest sarcomere lengths, greater amount and velocity of sarcomere shortening, greater velocity of reextension, and shorter contraction duration than immature cells. In immature myocytes, a thin outer shell of myofibrils enveloped a central mass of mitochondria and nuclei, but in adult cells, the cytoskeleton divided the cell into compartments with the mitochondria arranged around and interspersed among the myofibrils. The different arrangement of the organelles and the cytoskeleton at the two ages may account for the shorter rest sarcomere length in the young myocytes and may confer differing internal loads that contribute to their smaller amount and velocity of sarcomere shortening. The corbular and longitudinal sarcoplasmic reticulum were less demarcated in immature than in adult cells. Myocytes from both ages showed postextrasystolic potentiation, suggesting that the sarcoplasmic reticulum modulates calcium at both ages. Restitution of contractility between contractions, obtained by measuring sarcomere shortening of interpolated extrasystoles, was faster in immature than in adult cells and may reflect the structural differences in the sarcoplasmic reticulum. The developmental differentiation in the sarcoplasmic reticulum suggests that changes in compartmentalization of calcium and in the distribution of putative calcium-release sites contribute to the increased contractility of adult myocytes.

Animals↗
PubMed1986

A ventilator for magnetic resonance imaging.

Breathing motion severely degrades the quality of magnetic resonance images (MRI) of the thorax and upper abdomen and interferes with the acquisition of quantitative data. To minimize these motion effects, we built an MRI compatible ventilator for use in animal studies. Solid state circuitry is used for controlling ventilation parameters. The ventilator can be triggered internally at frequencies of 0.1 to 30 Hz or it can be triggered externally such as by the MRI pulse sequence. When triggered by the scanner, ventilation is synchronized to occur between image data acquisitions. Thus, image data are obtained when there is no breathing motion and at a minimum lung volume when hydrogen density is maximum. Since the ventilator can be adjusted to operate at virtually any frequency from conventional to high frequency, ventilation can be synchronized to all commonly used repetition times (100 ms to 2000 ms or more; 600 to 30 breaths/min). Scan synchronous ventilation eliminates breathing motion artifacts from most imaging sequences (single and multiple spin echo and inversion recovery). Best image quality is obtained when scan synchronous ventilation is combined with cardiac gating. These methods are also useful for quantitative research studies of thoracic and abdominal organs.

Abdomen↗
PubMed1986

The quick-freezing of single intact skeletal muscle fibers at known time intervals following electrical stimulation.

Single intact frog skeletal muscle fibers quick-frozen after known time intervals following electrical stimulation are examined electron microscopically in thin sections, after freeze-substitution, in freeze-fracture/etch preparations, and in cryosections prepared for x-ray microprobe analysis. Techniques are described to perform these operations on a single fiber. Notable morphological differences between conventionally fixed and cryopreserved muscle fibers, and between fibers quick-frozen at different post-stimulation intervals are demonstrated.

Animals↗
PubMed1984

Study of the feline and canine populations in the greater Las Vegas area.

Analysis of household dogs and cats, based on age-distribution data and on age-specific birth and survival rates, as well as on pet source, indicated that the dog and cat populations are stable and not increasing in size (lambda congruent to 1). Roaming dogs and cats euthanatized at the pound represented about 5.7% and 8.1% of the estimated dog and cat populations, respectively. The death at the pound seems to be effective in checking pet population growth. Among pets acquired, 84% were less than 1 year of age for dogs as compared with 88% for cats. Breeders and pet shops supplied about 7% of cats and 17% of dogs. About 10% of cats and 10% of dogs were acquired at the pound, while 6.4% of dogs and 14% of cats were acquired as stray. About 45% of dogs and 41% of cats were acquired from pet owners. Some dogs (12.46%) and cats (12%) were imported from outside the Las Vegas area. Of dogs and cats below 2 months of age, 33% and 19.5%, respectively, came from breeders or pet shops or were imported from outside the area. Seventeen percent of unspayed female dogs and 16% of unspayed female cats reproduced. The percentages of spayed females were 77 for dogs and 86 for cats. Forty-five percent of the dogs and 48% of the cats were males. Among dogs at the shelter, 2% were neutered and 26% spayed. At the pound, 24% of dogs were small breeds, 24% medium size breeds, and 52% large breeds.(ABSTRACT TRUNCATED AT 250 WORDS)

Age Factors↗
PubMed1982

Feline population dynamics: a study of the Manhattan, Kansas, feline population.

Analysis of the age-specific birth and survival rates and the age distribution in the pet population of cats in Manhattan, Kansas, revealed that the rate of population change (lambda) was about 1.18. This means that under present birth and death rates, the cat population can increase by about 18% per year. In reality, the increase may not be as high since pet ownership may not increase by as much. The frequency of spayed females of reproductive age in the Manhattan population was about 59%. This may not be sufficient to curb population growth. With the present age-specific survival rates, about 88% of the females should be spayed if the population is to remain stable. The ratio of people to cats in Manhattan was estimated to be 5.2:1. The average number of cats per household was 0.508. The percentage of households with cats was 28 with an average of 1.74 cats per household. In comparison, the average number of dogs per household was 0.43 and the percentage of households with dogs was 43 with an average of 1.36 dogs per household. The methods used in this study are useful for collecting and analyzing data to be used by governmental bodies and veterinarians concerned with proposals to regulate pet population growth and reproduction.

Animals↗
PubMed1980

Canine population dynamics: a study of the Manhattan, Kansas, canine population.

Analysis of the age distribution in the pet population of dogs in the community of Manhattan, Kansas from 1968 to 1979 revealed that the distribution was stationary and the rate of population change (lambda1) was close to 1, which means, that if present conditions do not change, the population probably would remain stable in size in the future. The number of females spayed in the Manhattan population reached a frequency of 66% in 1979. Considering only present age-dependent birth and survival rates, the population could not maintain itself and would decrease rapidly in size. It was found, however, that the population is maintained by a high net rate of immigration, with about 75% of all dogs under 6 months old coming from outside the Manhattan population. The ratio of people to dogs in Manhattan was estimated to be 4.14:1; about 41% of all dogs under 6 months of age were acquired through breeders. Roaming animals in 1 year represented about 12% of the canine population in Manhattan; of those, 36% were stray (not claimed by owner) and the rest were owned and redeemed. The data represented only a portion of the population (dogs with responsible owners) and for a complete understanding of the dynamics of the entire population, one should analyze roaming dogs in a similar way. The methods described in this study should be useful for collecting data to be used by veterinarians and municipal authorities confronted with proposals for regulation dealing with reproduction and pet population.

Age Factors↗
PubMed1979

Genetics of resistance to tetraethyllead.

A Drosophila melanogaster population was exposed for 25 generations to 60 micrograms tetraethyllead per gram of medium. Selection over this period resulted in an increase in fecundity, hatchability and larva-to-adult viability. Chromosome assay showed that response in these traits was generally under additive genetic control in conformity with existing results in the literature on the genetics of resistance to acute environmental stress in D. melanogaster.

Animals↗
PubMed1975

Effect on Mn2+ on permeability properties of frog skin.

Mn2+ added to the inner bathing solution of frog skin caused a transient increase in potential difference (PD) and a decrease in total skin conductance and mannitol influx. Net Na flux and short-circuit current (Is. c.) were also reduced, the isotopic net flux being reduced more than Is. c. This observed discrepancy appears to be the result of Cl- retention in the outer medium since it was not observed when the skin was bathed in a sulfate-substituted chloride-free solution. The effect of Mn2+ on the inner side of the frog skin appears to be due to a reduced permeation of Na+ and Cl- through the outer barrier of the skin. Addition of Mn2+ to the outer solution bathing the frog skin caused an increase in PD and a smaller increase in Is. c. These changes were not associated with alterations in the fluxes of Na+ or mannitol and were observed only when chloride was present in the bathing solutions. The effect of Mn2+ on this side of the frog skin may therefore be due to a net retention of Cl- in the outer solution.

Animals↗