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Biomedical subjects

P J Butler

Publications and source records attributed to P J Butler.

At least 127 records · Page 7Linked to original sources
PubMed1984

A defined structure of the 30 nm chromatin fibre which accommodates different nucleosomal repeat lengths.

Earlier work on the condensation of chromatins of different repeat lengths into the 30 nm fibre has been surveyed and it is shown that the external geometry of the fibre must be the same for all the chromatins. This can only be fitted by a helical coiling of nucleosomes into a solenoid with the linker DNA disposed internally. On this basis, various models were calculated and compared with published electric dichroism data. The only good fit is found with a 'reverse-loop' model, where the linker DNA forms a complete turn into the hole of the solenoid, of opposite hand to the nucleosomal DNA superhelix. This gives a topological linking number of one per nucleosome and would resolve the 'linking number paradox' if the DNA screw is the same in chromatin as in solution. The feasibility of a reverse-loop for short linkers (down to 15 base pairs) was investigated by model building and kinks of approximately 120 degrees into both DNA grooves are described, which will allow such packing. There will, however, be a 'forbidden' range for the linker DNA length, between approximately 1 and 14 bp, corresponding to nucleosomal repeats of 163 and 176 bp.

Animals↗
PubMed1984

Changes in activity and ventilation in response to hypoxia in unrestrained, unoperated dogfish (Scyliorhinus canicula L.).

daily activity cycles, together with changes in activity and ventilation frequency in response to hypoxia (PO2 about 8 kPa), have been measured in unrestrained, unoperated dogfish. Continuous recording of activity over 48 h reveals that dogfish are essentially nocturnal, being three to four times more active at night than during the day. In relatively inactive, diurnal dogfish, rapid reduction of environmental PO2 does not cause any significant increase in swimming activity, whereas prolonged hypoxia actually appears to suppress activity. In more active, nocturnal dogfish, rapid reduction of environmental PO2 causes an immediate reduction in activity which remains suppressed throughout the hypoxic period. It is concluded therefore that increases in circulating catecholamines in response to hypoxia are the result of hypoxia alone, rather than of any increase in locomotory activity. In resting diurnal dogfish, ventilation frequency is lower than has previously been reported for this species and, contrary to previous reports, increases markedly by 49% in response to hypoxia. It appears that in previous studies on confined dogfish, respiratory frequency, and probably ventilation volume, may have been elevated to near maximum levels even in 'resting' normoxic fish. This may have profound effects on so-called resting values for oxygen transfer in this species.

Animals↗
PubMed1984

Heart rate and aerobic metabolism in Humboldt penguins, Spheniscus humboldti, during voluntary dives.

Heart rate and aerobic metabolism have been recorded from three Humboldt penguins, Spheniscus humboldti, freely diving on a freshwater pond (9 X 4.6 X 2.7 m deep), using an implanted radiotransmitter and an open circuit respirometer. Oxygen uptake at mean dive duration (46.2s) was 26% greater than the resting value, but the difference was not statistically significant. Heart rate was also similar to the resting value. It is concluded that voluntary dives of penguins are completely aerobic and that oxygen stores are sufficient to allow metabolism to continue at the rate estimated in the present study for 2.27 min during voluntary submersion. This is longer than that calculated for tufted ducks, probably because the penguins are more efficient at underwater locomotion and because they are almost neutrally buoyant.

Animals↗
PubMed1983

Higher-order structure of nucleosome oligomers from short-repeat chromatin.

Sedimentation measurements and electron microscopy at a series of ionic strengths suggest that chromatin from neurons of the cerebral cortex is able to form condensed structures in vitro that are probably several turns of a solenoid with about six nucleosomes per turn. Since neuronal chromatin has a short nucleosomal repeat (approximately 165 bp) allowing virtually no linker DNA between nucleosomes, and yet forms apparently 'normal' elements of solenoid, the packing of nucleosomes in the solenoid must be highly constrained. This permits only a limited number of possible models, and enables tentative suggestions to be made about the location of the linker DNA in the typical solenoid.

Animals↗
PubMed1983

Factors affecting the respiratory and cardiovascular responses to hypercapnic hypoxia, in mallard ducks.

Experiments were performed to determine the factors responsible for the differences in heart rate and blood flow to the leg between ducks after 60 sec head submersion and those spontaneously breathing a hypercapnic hypoxic gas mixture; blood gases were similar in both cases. It is concluded that, in forcibly submerged ducks, full development of the reduction in heart rate and of the accompanying cardiovascular adjustments is dependent upon the cessation of central respiratory activity and of respiratory movements. The CO2-sensitive receptors in the lungs account for approximately one third of the antagonism to these changes in ducks spontaneously breathing a hypoxic hypercapnic gas mixture. Other contributions are from central inspiratory neurons (a quarter of total), musculo-skeletal and cardiovascular mechanoreceptors (a quarter of total) and pulmonary mechanoreceptors (one sixth of total).

Animals↗
PubMed1982

Nucleotide sequence of tobacco mosaic virus RNA.

Oligonucleotide primers have been used to generate a cDNA library covering the entire tobacco mosaic virus (TMV) RNA sequence. Analysis of these clones has enabled us to complete the viral RNA sequence and to study its variability within a viral population. The positive strand coding sequence starts 69 nucleotides from the 5' end with a reading frame for a protein of Mr 125,941 and terminates with UAG. Readthrough of this terminator would give rise to a protein of Mr 183,253. Overlapping the terminal five codons of this readthrough reading frame is a second reading frame coding for a protein of Mr 29,987. This gene terminates two nucleotides before the initiator codon of the coat protein gene. Potential signal sequences responsible for the capping and synthesis of the coat protein and Mr 29,987 protein mRNAs have been identified. Similar sequences within these reading frames may be used in the expression of sets of proteins that share COOH-terminal sequences.

Amino Acid Sequence↗
PubMed1982

Nervous control of heart rate: activity in the cardiac vagus of the dogfish.

In the absence of any sympathetic innervation to the heart, nervous control of heart rate in the dogfish is solely attributable to inhibitory parasympathetic input from the vagus nerve. Action potentials can be recorded from the cardiac vagus of the dogfish following its exposure in the anterior cardinal sinus. The rates of heartbeat and ventilation, blood pressure, hematocrit, and responses to external stimuli such as hypoxia, which include a bradycardia, remained typical of fish with their nervous and circulatory systems virtually intact. The recordings included sporadically active units that accelerated during hypoxia, possibly inducing the bradycardia, and regular bursts of action potentials synchronous with ventilatory movements that appeared to arise reflexly from stimulation of pharyngeal proprioceptors. These bursts may loosely couple the respiratory and cardiac pumps, increasing the probability of concurrence between periods of maximum water and blood flow. The preparation enables detailed study of the temporal relationships between the pumps and its neurophysiological basis.

Action Potentials↗
PubMed1982

Control of heart rate by carotid body chemoreceptors during diving in tufted ducks.

Previous work has shown that during forcible submersion of domestic ducks there is a gradual reduction in heart rate to 10-20% of its predive value after 45-60 s. Bilateral denervation of the carotid body chemoreceptors abolishes most of this bradycardia. By use of implanted radio transmitters it has been shown that in free-swimming tufted ducks, Aythya fuligula, there is an immediate reduction in heart rate, on spontaneous diving, from an elevated predive level. It then increases for a few seconds before it stabilizes at a level similar to that recorded when the duck is swimming fairly vigorously. The present study has shown that, following bilateral denervation of the carotid bodies, there is a significant increase in mean dive duration but no effect on the immediate reduction in heart rate on submersion. Heart rate is, however, significantly higher toward the end of spontaneous dives after carotid body denervation. Unlike the situation in mallards and their domesticated varieties, carotid body denervation has no effect on heart rate in tufted ducks during the first 40 s of forced dives. The carotid bodies therefore do not play the dominant role in cardiac control during submersion of diving ducks that has been suggested by work involving the forcible submersion of the mallard duck and its domesticated varieties.

Animals↗
PubMed1982

Respiratory and cardiovascular control during diving in birds and mammals.

Recent studies on freely diving birds and mammals indicate that, contrary to the classical hypothesis, the majority of dives are aerobic with minimal cardiovascular adjustments (i.e. bradycardia and selective vasoconstriction). It is postulated that during these aerobic dives the cardiovascular adjustments result from the opposing influences of exercise and the classical diving response, with the bias towards the exercise response. It is envisaged that the active muscles, as well as the brain and heart, are adequately supplied with blood to enable them to metabolize aerobically. Intense mental activity, particularly in carnivores seeking their prey, may also attenuate the classical response. Aerobic dives are usually terminated well before the oxygen stores are depleted, and another dive follows once they have been replenished. In this way a series of dives is performed. Prolonged dives are endured as a result of a shift towards the classical response of bradycardia, presumably more intense vasoconstriction, and anaerobiosis. This may be a form of alarm response, particularly in small animals such as ducks and coypus, or it may be a means of allowing the marine birds and mammals that dive deeply for their food to engage in unusually long hunting expeditions. For those that dive under ice, it may also allow long periods of underwater exploration as well as being a safety mechanism should the animal become disoriented.

Afferent Pathways↗
PubMed1982

Differences between directly measured and calculated values for cardiac output in the dogfish: a criticism of the Fick method.

Cardiac output has been measured directly, and calculated by the Fick method, during normoxia and hypoxia in six artificially perfused dogfish (Scyliorhinus canicula) in an attempt to estimate the accuracy of this method in fish. The construction and operation of a simple extra-corporeal cardiac bypass pump is described. This pump closely mimics the flow pulse profiles of the fish's own heart and allows complete control of both cardiac stroke volume and systolic and diastolic periods. During normoxia (PO2 = 21 kPa) there was no significant difference between directly measured and calculated values for cardiac output. However, some shunting of blood past the respiratory surface of the gills may have been obscured by cutaneous oxygen uptake. In response to hypoxia (PO2 = 8.6 kPa) there is either a decrease in the amount of blood being shunted past the respiratory surface of the gills and/or an increase in cutaneous oxygen uptake such that the Fick calculated value for cardiac output is on average 38% greater than the measured value. It is proposed that the increase in the levels of circulating catecholamines that is reported to occur in response to hypoxia in this species may play an important role in the observed response to hypoxia. The results are discussed in terms of their implications for the calculation of cardiac output by the Fick principle in fish.

Animals↗