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Biomedical subjects

S R Taylor

Publications and source records attributed to S R Taylor.

89 records · Page 5Linked to original sources
PubMed1975

Contractures in partially depolarized muscle treated with caffeine or nitrate.

Frog toe muscles partially depolarized with a subcontracture concentration (15 or 20mM) of K+ rapidly developed tension when exposed to 1 mM caffeine, which alone did not cause contracture. This action resulted from a lowering of the mechanical threshold by the caffeine and thus was similar to that caused by replacement of Cl- with NO3-. Maximal tension was the same in the caffeine and NO3- media but took much longer to develop in the former. Equilibrating the muscles with caffeine for 60 s (the time calculated for complete diffusion) did not reduce contraction time significantly. However, prolonging the exposure of muscles to caffeine beyond this time enhanced the drug's capacity to potentiate both the twitch and 30 mM K+ contractures and shortened the contraction time of the contractures. Toe muscles that had undergone a contracture in 100 mM K+ and were recovering in 15 mM K+-Ringer generated tension in the presence of 1 mM caffeine or NO3- but only after enough repolarization had been achieved so that a response to a second challenge with 100 mM K+ could be demonstrated. Although prolonged immersion in the K+-enriched recovery solution caused a disappearance of sensitivity to a test depolarization, addition of 1 mM caffeine or replacement of Cl- with NO3- promptly evoked a contracture. These results imply a reversible step in the recovery process that is both potential and time dependent. Since application of 5 mM caffeine to either normally polarized or depolarized muscles always immediately causes contracture, more than one site of action for caffeine is indicated. After disruption of the transverse tubules, partially depolarized muscles showed no response to NO3- or 1 mM caffeine but contracted vigorously when exposed to 5 mM caffeine.

Action Potentials↗
PubMed1975

Calcium transients in amphibian muscle.

The calcium-sensitive bioluminescent protein aequorin has been microinjected into isolated amphibian twitch muscle fibers in order to detect the intracellular calcium transients associated with excitation-contraction coupling. While it is not yet possible to make quantitative estimates of the changes in calcium concentration involved, it is possible to detect substantial changes in intracellular calcium transients resulting from changes in temperature, stimulation frequency, fiberlength, and the osmotic strength of the bathing medium.

Aequorin↗
PubMed1973

Graded activation in frog muscle fibers.

The membrane potential of frog single muscle fibers in solutions containing tetrodotoxin was controlled with a two-electrode voltage clamp. Local contractions elicited by 100-ms square steps of depolarization were observed microscopically and recorded on cinefilm. The absence of myofibrillar folding with shortening to striation spacings below 1.95 microm served as a criterion for activation of the entire fiber cross section. With depolarizing steps of increasing magnitude, shortening occurred first in the most superficial myofibrils and spread inward to involve axial myofibrils as the depolarization was increased. In contractions in which the entire fiber cross section shortened actively, both the extent of shortening and the velocity of shortening at a given striation spacing could be graded by varying the magnitude of the depolarization step. The results provide evidence that the degree of activation of individual myofibrils can be graded with membrane depolarization.

Animals↗
PubMed1972

Action potential parameters affecting excitation-contraction coupling.

In quantifying type B potentiation effects, given earlier merely qualitatively, it is found that Zn(2+), 1-50 microM, causes increases in action potential duration, twitch tension, and twitch contraction period time, which are all directly proportional to the log of the concentration. Hence, the duration of the action potential, i.e. the magnitude of its mechanically effective period, is a causal factor quantitatively determining the degree of mechanical activation in the isometric twitch. In higher concentrations of Zn(2+) up to 1000 microM, the spike duration and the contraction time continue to increase but the twitch tension is disproportionately smaller, evidently because the high zinc (500-1000 microM) raises the mechanical threshold of excitation-contraction (E-C) coupling and reduces the intrinsic strength of the contractile system. Eserine (1.5 mM) and also high Zn(2+) not only cause type B potentiation effects, but also slow the rise of the spike, thus causing retardation of the very onset of tension production, which is even greater for high Zn(2+) because of the raised mechanical threshold. This retardation is then succeeded by the faster tension output characteristic of type B potentiation resulting from spike prolongation. Thus, the changes in the consecutive, rising and falling phases of the action potential explicitly register their separate effects in the respective very earliest and directly following periods of twitch output; i.e., each phase of the action potential produces its own mechanical "transform." These transforms, and other effects, suggest that the release of activator Ca(2+) from the sarcoplasmic reticulum during E-C coupling can be graded in both the rate and the total amount of the release.

Action Potentials↗
PubMed1970

Striated muscle fibers: inactivation of contraction induced by shortening.

The myofibrils in an isolated muscle fiber remain straight during the early part of a lightly loaded contraction initiated by membrane depolarization, but, as shortening continues, myofibrils in the core of the fiber become wavy, which suggests that their activation has been interrupted by shortening of the fiber. This may be a factor determining the length-tension relation at short muscle lengths.

Animals↗
PubMed1969

Mechanical threshold as a factor in excitation-contraction coupling.

I(-), CH(3)SO(4) (-), and ClO(4) (-), like other previously studied type A twitch potentiators (Br(-), NO(3) (-), SCN(-), and caffeine), lower the mechanical threshold in K depolarization contractures of frog skeletal muscle. In potentiated twitches, I(-), Br(-), CH(3)SO(4) (-), ClO(4), and SCN, as already reported for NO(3) (-) and caffeine, slightly shorten the latent period (L) and considerably increase the rate of tension development (dP/dt) during the first few milliseconds of the contraction period. Divalent cations (8 mM Ca(2+), 0.5-1.0 mM Zn(2+) and Cd(2+)) raise the mechanical threshold of contractures, and correspondingly affect the twitch by depressing the tension output, increasing L, and decreasing the early dP/dt, thus acting oppositely to the type A potentiators. These various results form a broad, consistent pattern indicating that electromechanical coupling in the twitch is conditioned by a mechanical threshold as it is in the contracture, and suggesting that the lower the threshold, in reference to the raised threshold under the action of the divalent cations, the more effective is a given action potential in activating the twitch as regards especially both its early rate and peak magnitude of tension development. The results suggest that the direct action by which the various agents affect the level of the mechanical threshold involves effects on E-C coupling processes of the T tubular and/or the sarcoplasmic reticulum which control the release of Ca for activating contraction.

Action Potentials↗
PubMed1969

The influence of stimulus parameters on contractions of isolated frog muscle fibres.

1. Reversible changes in twitch and tetanus contractions of isolated frog muscle fibres were produced by varying the duration and strength of a transversely applied d.c. stimulus.2. When the stimulus strength was at least 1.1 times threshold the peak force of the contraction elicited was reduced by stimulus durations longer than 1 msec at 20 degrees C, and by durations longer than 3 msec at 2-5 degrees C.3. As the strength of a prolonged stimulus was progressively increased to 3 times rheobase or more, peak twitch and tetanus force declined to 40-70% of the amplitude with short (0.2-0.5 msec) stimuli. The time to peak of the twitch was slightly decreased and the first phase of relaxation was accelerated.4. Cine-micrography of fibres which were permitted to shorten under a light load revealed that the side of the fibre adjacent to the anode was unactivated under conditions which also reduced peak force output.5. With stimulus durations longer than 20 msec, peak force, time to peak, and relaxation time increased as the stimulus strength increased from 2.5 to 3 times rheobase. This might have been produced by a cathodal contracture following the action potential.6. The results are discussed in relation to previous studies of activation in striated muscle.

Action Potentials↗
PubMedDate not supplied

A new variant of type IV glycogenosis: deficiency of branching enzyme activity without apparent progressive liver disease.

Type IV glycogenosis is due to branching enzyme deficiency and is usually manifested clinically by progressive liver disease with cirrhosis and hepatic failure between the second and fourth years of life. We describe a 5-year-old boy who, following an acute febrile illness at 2 years of age, was first noted to have hepatomegaly with mildly elevated serum transaminase levels. Liver biopsy revealed hepatic fibrosis with periodic-acid Schiff-positive, diastase-resistant inclusions in hepatocytes and fibrillar inclusions characteristic of amylopectin by electron microscopy. Enzymatic assay revealed deficient hepatic branching enzyme activity with normal activity of glucose-6-phosphatase, debranching enzyme and phosphorylase activities. During the succeeding 3 years, he grew and developed normally with apparent resolution of any clinical evidence of liver disease and only intermittent elevation in serum transaminase levels associated with fever and prolonged fasting. Repeat liver biopsy at 4 years of age showed persistence of scattered hepatocellular periodic-acid Schiff-positive, diastase-resistant inclusions, but no progression of hepatic fibrosis in spite of persistent deficiency of hepatic branching enzyme activity. Skeletal muscle and skin fibroblasts from the patient also showed deficient enzyme activity. Skin fibroblasts from both parents exhibited half the normal control activity, suggesting a heterozygote state. This is the first documented patient with deficiency of branching enzyme but without evidence of progressive hepatic disease. This patient, coupled with reports of other patients with late onset hepatic or muscle disease with branching enzyme deficiency, suggests that the defect resulting in Type IV glycogen storage disease is more heterogenous and possibly more common than previously suspected.

1,4-alpha-Glucan Branching Enzyme↗
PubMedDate not supplied

Ploidy studies by image analysis on fine needle aspirates of the breast.

Quantitative DNA analysis was performed on preoperative fine needle aspirates of 12 breast cancers and 1 benign lesion using the Cell Analysis Systems (CAS) Model 200 image analysis system. The smears were prepared on CAS slides, routinely processed for Papanicolaou staining, and then destained and restained with Feulgen stain after cytologic evaluation. Four cases were DNA diploid, 3 were tetraploid, 1 was diploid/tetraploid, and 5 were aneuploid. When compared to analysis of samples from the resected tumor, both DNA index and S-phase fraction were constant.

Biopsy, Needle↗