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Biomedical subjects

D G Allen

Publications and source records attributed to D G Allen.

At least 181 records · Page 10Linked to original sources
PubMed1985

Artificial insemination by donor at Groote Schuur Hospital.

An artificial insemination by donor (AID) service was started in the Infertility Clinic at Groote Schuur Hospital, Cape Town, in response to repeated requests by patients, doctors and social workers. The methods, results of treatment, and reasons for reorganizing the work of the clinic are presented. Future management at the clinic is also outlined. The clinic has been reorganized and work on semen cryopreservation is being carried out by the Andrology Service at Groote Schuur Hospital. A semen bank has also been established. The ethical, religious, legal and genetic aspects of AID are reviewed.

Adult↗
PubMed1985

High-time-resolution 31P NMR studies of the perfused ferret heart.

A cell is described that has enabled isolated Langendorff-perfused ferret hearts to be studied in a Bruker WM200 widebore superconducting nuclear magnetic resonance (NMR) spectrometer. Left ventricular pressure was monitored with a latex balloon catheter, and the hearts were paced with a stimulator triggered from the spectrometer's central computer, enabling gated studies to be performed. Suitable radiofrequency filtering for the pacing leads is described. Phosphorus (31P) NMR was used to determine internal pH and the concentration of phosphorylated metabolites under resting conditions. The perfusion rate is shown to affect the phosphocreatine/ATP ratio at low flow rates, but the removal of phosphate from the perfusate is shown not to affect metabolite levels or the internal pH. The time resolution of the method is assessed and its potential for monitoring transient effects illustrated by studies of the effects of acetylcholine and cyanide-induced anoxia. The cardiac gated 31P NMR experiment is discussed and four spectra, corresponding to mid- and end systole and mid- and end diastolic are presented. No effects of cycling of high-energy phosphates are evident in these results.

Animals↗
PubMed1985

The cellular basis of the length-tension relation in cardiac muscle.

The relation between muscle length or sarcomere length and developed tension for lengths up to the optimal for contraction (Lmax) is much steeper in cardiac muscle than in skeletal muscle. The steepness of the cardiac length--tension relation arises because the degree of activation of the cardiac myofibrils by calcium increases as muscle length is increased. Two processes contribute to this length-dependence of activation: (i) the calcium sensitivity of the myofibrils increases with muscle length and (ii) the amount of calcium supplied to the myofibrils during systole increases with muscle length. Of these two, the change in calcium sensitivity is the most clearly defined and is responsible for a large part of the rapid change in developed tension when muscle length is altered. It is likely that this change in calcium sensitivity is due to a change in the affinity of troponin for calcium but the underlying mechanism has not been identified. There is good evidence that changes in the calcium supply to the myofibrils can account for the slow changes in tension that follow an alteration in length; there may also be rapid changes in calcium supply but this is less clearly established at present.

Actin Cytoskeleton↗
PubMed1985

A nuclear magnetic resonance study of metabolism in the ferret heart during hypoxia and inhibition of glycolysis.

31P nuclear magnetic resonance was used to measure the relative concentrations of phosphorus-containing metabolites in Langendorff-perfused ferret hearts. Intracellular concentrations of inorganic phosphate ([Pi]i), phosphocreatine ([PCr]i), ATP ([ATP]i) and H+ (pHi) were monitored under control conditions and while oxidative phosphorylation and/or glycolysis were prevented. Mechanical performance was assessed by recording the pressure developed in a balloon placed in the left ventricle. Oxidative phosphorylation was prevented either by replacement of O2 with N2 or by addition of cyanide. When the rate of oxidative phosphorylation was reduced by either method, developed pressure fell to a stable level of about 35% of control after 5 min. The pHi (control value 6.98) first increased to a peak of 7.07 after 2 min but then decreased to give a stable acidosis (pH 6.85). [PCr]i decreased rapidly to about 15% of the control value after 5 min whereas [ATP]i declined very slowly, reaching about 90% of the control value after 10 min. Reduction in the rate of glycolysis was achieved either (i) by removal of external glucose and depletion of glycogen stores by a long (1-2 h) period of stimulation or (ii) by removal of glucose and application of 2-deoxyglucose (1 mM) for 30-60 min. These procedures had only a small effect on pressure development, [ATP]i, [PCr]i and pHi. Measurements of lactate production showed that these procedures reduced the rate of glycolysis by a factor of about 10. When oxidative phosphorylation was prevented during periods when the rate of glycolysis was reduced, developed pressure fell to less than 5% of control after 5 min and there was a subsequent increase in resting pressure (hypoxic contracture). pHi (control value 7.03) first increased to a peak of 7.12 and then declined to about pH 7.00, but there was no subsequent acidosis. [PCr]i fell rapidly to about 10% of control after about 5 min while [ATP]i declined to about half of its control value over 10 min. It is concluded that (i) when oxidative phosphorylation alone is prevented, the changes in pHi can account for a substantial part of the changes in developed pressure. The increase in [Pi]i probably also contributes to the decline of developed pressure. (ii) When oxidative phosphorylation was prevented under conditions in which the rate of glycolysis was also reduced, the more pronounced decline in developed pressure which occurs within 5 min cannot be accounted for by pHi changes and is probably not explained by the rise in [Pi]i or by the moderate fall of [ATP]i.(ABSTRACT TRUNCATED AT 400 WORDS)

Adenosine Triphosphate↗
PubMed1985

The relationship between intracellular calcium and contraction in calcium-overloaded ferret papillary muscles.

The photoprotein aequorin was microinjected into cells on the surface of ferret papillary muscles. Tension and aequorin light, a function of intracellular Ca concentration ([Ca2+]i), were monitored. The preparations were exposed to increased concentrations of extracellular Ca ([Ca2+]o). Small increases in [Ca2+]o led to the usual increase in both the systolic light signal and the developed tension. However, high [Ca2+]o led to a fall in developed tension known as Ca overload. This fall of tension was seen at lower [Ca2+]o if the experiment was performed in the presence of strophanthidin. In conditions of Ca overload, the systolic light signal was either similar in amplitude or larger than the systolic light observed under conditions which did not lead to Ca overload. Oscillations of diastolic light were invariably present under conditions of Ca overload. These oscillations were accompanied by after-contractions which were small in relation to the magnitude of the aequorin light oscillations. During Ca overload, the variance of the amplitude of the systolic light signal was greater than could be accounted for by the random nature of the arrival of photons. Small systolic light signals occurred when there had been an oscillation of light in the diastolic period immediately preceding the systolic light signal. Large systolic light signals occurred when the preceding period (approximately 1 s) was free of oscillations of light. These observations suggest that if the sarcoplasmic reticulum (s.r.) has spontaneously released its contents of Ca as shown by a diastolic [Ca2+]i oscillation, then a stimulated systolic Ca signal occurring within about the next second is smaller, possibly because it takes longer than this period for the s.r. to reload with Ca. If this process occurs randomly in the various cells of the preparation, developed tension will be reduced because those cells which have a small Ca release will act as a compliance in series with cells which have a large Ca release.

Aequorin↗
PubMed1985

The role of intracellular [Ca2+] and [H+] in contractile failure of the hypoxic heart.

When oxidative metabolism is inhibited in heart muscle, developed tension often increases slightly before decreasing below control. We have examined the possible mechanisms underlying these changes in developed tension in two series of experiments. In the first series of experiments, the photoprotein aequorin was used to monitor intracellular free [Ca2+] [( Ca2+]i) in papillary muscles during inhibition of oxidative phosphorylation, using either cyanide or hypoxia. The observed changes of developed tension were independent of changes in [Ca2+]i. It was therefore possible that these changes of tension were due to changes of intracellular pH (pHi). We tested this idea in a second series of experiments, using 31P nuclear magnetic resonance to monitor pHi, [ATP], and phosphocreatine concentration [( PCr]) in Langendorff-perfused ferret hearts. During the application of cyanide, pHi increased transiently before decreasing to below control. [PCr] decreased throughout this period, but [ATP] did not change. It is concluded that the observed changes of pHi could account for most of the observed changes of developed tension. It is suggested that the initial increase of pHi is due to PCr breakdown and the subsequent decrease of pHi to accelerated anaerobic glycolysis.

Acid-Base Equilibrium↗
PubMed1985

Effects of aspirin and propranolol alone and in combination on hemostatic determinants in the healthy cat.

The effects of aspirin (75 mg orally/average-size cat) and propranolol (5 mg orally every 8 hours/average-size cat) alone and in combination on hemostatic determinants in healthy cats were studied. In cats, aspirin alone did not cause a significant effect in platelet numbers, plasma fibrinogen, activated partial thromboblastin time, prothrombin time, thrombin time, or platelet aggregation response to adenosine diphosphate. Aspirin did, however, significantly reduce the degree of aggregation induced by acid soluble collagen. Propranolol alone or in combination with aspirin did not cause a significant effect on platelet numbers, plasma fibrinogen, activated partial thromboblastin time, prothrombin time, thrombin time, or platelet aggregation in response to acid soluble collagen, adenosine diphosphate, or adrenaline. It was concluded that aspirin alone at the recommended dosage of one-quarter of a 5-grain tablet (1.25 grains or 75 mg) every other day will significantly affect platelet function and may be of value in the prevention of thromboembolic disease in the cat.

Animals↗
PubMed1985

Comparison of three methods for cardiac output determination in cats.

Cardiac output (CO) was measured in sodium pentobarbital-anesthetized cats over a wide range of blood flow rates. In 10 cats, CO was measured simultaneously, using Fick determination and thermodilution techniques. Echocardiography was used to estimate contractility of the heart by measuring percentage change in minor diameter and velocity of circumferential fiber shortening. These indices were compared with CO by the other techniques. Echocardiographic equations used for CO determination in man were evaluated for reliability in the cat. Thermodilution and Fick determination correlated best with low CO (r = 0.89) and less with intermediate (r = 0.69) and high (r = 0.75) CO. Percentage change in minor diameter and velocity of circumferential fiber shortening correlated with thermodilution measurements of the cardiac index (r = 0.71 and r = 0.84, respectively). The value of echocardiography for CO estimation was questionable, using existing equations. Fick determination of CO was more inconsistent and was more prone to technical error than was thermodilution.

Anesthesia↗
PubMed1984

Management of patients with a diastolic blood pressure of 90 mmHg in the third trimester of pregnancy.

A retrospective study of 50 patients admitted to the Peninsula Maternity Hospital, Cape Town, with a diastolic blood pressure of 90 mmHg was carried out. Proteinuria and a serum urate level of 0,36 mmol/l or more were found to be risk factors in this group. It is suggested that these patients can be managed as outpatients if no proteinuria is present and the serum urate level is less than 0,36 mmol/l; they should be trained to test urine at home and should be followed up on a weekly basis. A small prospective study showed that home urine testing was both workable and satisfactory.

Blood Pressure↗
PubMed1984

Model of calcium movements during activation in the sarcomere of frog skeletal muscle.

A model of calcium movement during activation of frog skeletal muscle is described. The model was based on the half sarcomere of a myofibril and included compartments representing the terminal cisternae, the longitudinal sarcoplasmic reticulum, the extramyofibrillar space, and the myofibrillar space. The calcium-binding proteins troponin, parvalbumin, and calsequestrin were present in appropriate locations and with realistic binding kinetics. During activation a time-dependent permeability in the terminal cisternal wall led to calcium release into the myoplasm and its diffusion through the myoplasm longitudinally and radially was computed. After adjustment of three parameters, the model produced a myoplasmic free-calcium concentration that was very similar to those recorded experimentally with calcium indicators. The model has been used to demonstrate the importance of parvalbumin in the relaxation of skeletal muscle, to describe the time course and magnitude of calcium gradients associated with diffusion across the sarcomere, and to estimate the errors associated with the use of aequorin as an intracellular calcium indicator in muscle.

Aequorin↗
PubMed1984

Measurements of intracellular calcium concentration in heart muscle: the effects of inotropic interventions and hypoxia.

The use of aequorin as an intracellular calcium indicator in ventricular muscle is described. If the increase of intracellular calcium concentration associated with each contraction (the calcium transient) is measured during inotropic interventions, it is possible to distinguish two classes of inotropic intervention. One class leads to changes in the calcium transient which parallel the changes in tension. The second class leads to changes in the calcium transient and tension which are different in magnitude or direction. In this latter class, changes in the sensitivity of the contractile proteins to calcium are occurring and represent an important part of the inotropic mechanism. When oxidative phosphorylation is inhibited in an isolated mammalian papillary muscle, tension declines but the amplitude of the calcium transients is unaffected. Intracellular acidosis caused by lactate production associated with the increased rate of glycolysis is the probable mechanism of this decline in tension. When both oxidative phosphorylation and glycolysis are inhibited, both the calcium transient and the developed tension decline rapidly to zero. This profound contractile failure may be a consequence of a decline in the free energy of hydrolysis of ATP so that the sarcoplasmic reticulum can no longer accumulate calcium. Hypoxic contractures occur in the absence of significant increases in resting [Ca2+]i and are probably due to rigor produced by the low [ATP].

Acidosis↗
PubMed1984

Control of intracellular ionized calcium concentration by sarcolemmal and intracellular mechanisms.

The regulation of the resting intracellular ionized calcium concentration [( Ca2+]i) has been studied in ferret papillary muscle using the photoprotein aequorin to measure [Ca2+]i. Elevating [Ca2+]o produced an initial rapid increase of [Ca2+]i and tension which then decayed to a steady level. This secondary fall of [Ca2+]i is attributed to a secondary decrease of Ca entry on Na-Ca exchange produced by the known fall of [Na+]i. Replacing external Na by K produced a large transient increase of both [Ca2+]i and tension which then decayed spontaneously to near the resting level. If Na was removed after metabolic inhibition with cyanide and deoxyglucose then neither tension nor [Ca2+]i recovered. The addition of the mitochondrial uncoupler FCCP to a muscle in Na-free solution produced a gradual rise of tension but only elevated [Ca2+]i after a delay of many minutes. Similarly caffeine did not elevate [Ca2+]i. These experiments do not support the hypothesis that the regulation of resting [Ca2+]i in Na-free solutions depends solely on intracellular sequestration of [Ca2+]i. The first twitch elicited in Na-containing solutions after exposure to Na-free solution was much larger than control and was associated with a large Ca transient attributed to increased loading of the sarcoplasmic reticulum with Ca in the Na-free solution. The elevation of [Ca2+]i in Na-free solutions was accompanied by spontaneous fluctuations of both [Ca2+]i and tension with a frequency of about 3 Hz. These fluctuations were abolished by drugs such as caffeine or ryanodine which interfere with sarcoplasmic reticulum function. These results provide direct evidence for the spontaneous release of Ca from the sarcoplasmic reticulum inferred from previous, less direct, work.

Animals↗
PubMed1984

Factors influencing free intracellular calcium concentration in quiescent ferret ventricular muscle.

The photoprotein aequorin was injected into cells of ferret papillary muscles to monitor the resting intracellular free Ca concentration [( Ca2+]i). Increasing the external Ca concentration [( Ca2+]o) increased both resting [Ca2+]i and resting tension. The tension and [Ca2+]i both rose to a peak and then declined to a steady-state level which was higher than the control. Qualitatively similar, but larger, effects were observed if [Ca2+]i was first elevated with strophanthidin. The increase of [Ca2+]i was accompanied by the development of spontaneous oscillations of [Ca2+]i. When a steady level of [Ca2+]i had been reached in high [Ca2+]o, [Ca2+]o was reduced back to the control level for a brief period. A subsequent increase of [Ca2+]o produced a rise of [Ca2+]i to the same steady level as that previously found in the high [Ca2+]o but the initial peak and subsequent decline were absent. It is suggested that the decline of [Ca2+]i from the initial peak is mediated by a fall of intracellular Na concentration [( Na+]i) limiting Ca entry on a Na-Ca exchange. Increasing external K concentration [( K+]o) from 5 to 30 mmol/l had no detectable effect on [Ca2+]i under control conditions. However, if [Ca2+]i was first increased either by applying strophanthidin or by increasing [Ca2+]o, increasing [K+]o produced a transient rise of [Ca2+]i and tension. This rise was unaffected by D600. It is suggested that the secondary decline of [Ca2+]i after the initial rise may, again, be produced by a fall of [Na+]i acting on an Na-Ca exchange. Acidification produced by increasing [CO2] had no detectable effect on [Ca2+]i under control conditions. However, if [Ca2+]i was increased by strophanthidin, acidification produced a rise of [Ca2+]i. This rise of [Ca2+]i was partly transient even when the intracellular acidification was presumably maintained (raising CO2 at constant [HCO3-]). Acidification in Na-free solutions had qualitatively similar effects to those in Na-containing solutions. In Na-free solutions (Na replaced by K) the [Ca2+]i could be maintained at a low level for at least several hours. Increases of [Ca2+]o in Na-free solutions led to a decrease of [Ca2+]i, and similarly decreasing [Ca2+]o led to an increase in [Ca2+]i. These anomalous effects of [Ca2+]o on [Ca2+]i could be abolished by Mn ions or D600. It is suggested that changes in [Ca2+]o may have reciprocal effects on Ca permeability and hence on [Ca2+]i. The application of the mitochondrial uncoupler FCCP in Na-free solutions led to an increase of resting tension followed, after a substantial delay, by an increase of [Ca2+]i.(ABSTRACT TRUNCATED AT 400 WORDS)

Aequorin↗
PubMed1984

Characterization of oscillations of intracellular calcium concentration in ferret ventricular muscle.

The photoprotein aequorin was injected into superficial cells of ferret papillary muscles. Tension and aequorin light (a function of intracellular [Ca2+]) were monitored. Increasing intracellular Ca concentration ([Ca2+]i), either by decreasing extracellular Na, or by inhibiting the Na pump with strophanthidin, produced spontaneous oscillations of [Ca2+]i and tension. Fourier analysis showed that these oscillations had frequencies of up to 3-4 Hz. If the muscle was stimulated in these conditions the Ca transient associated with the twitch was followed by a series of damped oscillations of [Ca2+]i which were accompanied by after-contractions. Under a given set of conditions the frequency of the stimulated oscillations was similar to that of the spontaneous oscillations. Manoeuvres which increase [Ca2+]i increased the frequency of both spontaneous and stimulated oscillations. Drugs which inhibit the function of the sarcoplasmic reticulum (caffeine and ryanodine) abolished both stimulated and spontaneous oscillations. The spontaneous oscillations during a Na-free contracture were unaffected by the Ca channel blocker D-600. When repetitive stimulation was begun the frequency and magnitude of the stimulated oscillations increased over several minutes. Increasing the frequency of stimulation increased the magnitude of the stimulated oscillations. It is concluded that the spontaneous oscillations of [Ca2+]i may be due to oscillatory Ca release from the sarcoplasmic reticulum. The similar properties of the spontaneous and stimulated oscillations suggest that the latter may be due to a synchronization of the former.

Aequorin↗
PubMed1984

Influence of deuterium oxide on calcium transients and myofibrillar responses of frog skeletal muscle.

The influence of substituting D2O for H2O on calcium transients and on contraction was studied in intact single skeletal muscle fibres injected with aequorin and in mechanically skinned fibres from frogs. Most experiments were carried out at 10 degrees C. Experiments performed in vitro established that the calcium concentration-effect curve for aequorin is depressed and the rate of change of aequorin light emission after sudden changes of [Ca2+] is decreased when D2O is substituted for H2O. D2O substitution greatly reduces the amplitude of the aequorin signals of intact fibres both in twitches and in tetani. This is partly the result of the influence of D2O on aequorin, but the amplitude of the calcium transients is reduced as well. In both H2O and D2O Ringer solutions, the amplitude and time course of the sarcoplasmic calcium transient during a tetanus vary greatly with the stimulus frequency. In H2O, frequencies high enough to produce mechanical fusion normally produce cytoplasmic calcium concentrations high enough to saturate the myofibrils, with the result that stimulus frequency has very little influence on the mechanical response. Saturation does not occur in D2O, and the amplitude and form of fully fused tetani are greatly influenced by the frequency of stimulation. Aequorin was used as a calcium indicator to estimate the influence of D2O substitution on the apparent equilibrium association constant for the calcium-EGTA complex. The constant was reduced by more than one order of magnitude in D2O at pD = pH = 7.0. Experiments on mechanically skinned muscle fibres showed that D2O substitution decreased the apparent sensitivity of the myofibrils to calcium, but increased the force of contraction at saturating calcium concentrations. The latter effect is probably responsible for the potentiation of tetanic tension that is observed in certain D2O/H2O mixtures or in H2O solutions immediately after a switch from D2O. In intact muscle fibres the potentiation of twitch or tetanic tension after a switch from D2O to H2O declines with a half-time of about 25 s; this probably reflects the time course of exchange of intracellular D2O for H2O. The effects of D2O on the intracellular calcium transient appeared and disappeared with a half-time of less than 1 s; this time is of the same order of magnitude as that calculated for the exchange of H2O and D2O in the T-tubules. We conclude that D2O affects calcium release at a site in the T-tubule which is more accessible to the extracellular space than to the intracellular space.

Action Potentials↗
PubMed1984

The effects of shortening on myoplasmic calcium concentration and on the action potential in mammalian ventricular muscle.

When cardiac muscle shortens during a contraction, the duration of mechanical activity is abbreviated (shortening deactivation), but the duration of the action potential is prolonged. Neither of these phenomena is fully understood, but both may be related to changes in the myoplasmic free calcium concentration. In these experiments, isolated papillary muscles from cats and ferrets were allowed to contract under various mechanical conditions while myoplasmic calcium was monitored with aequorin, or in parallel experiments the membrane potential was recorded with microelectrodes or a sucrose gap. When shortening occurred, myoplasmic calcium was increased and the membrane potential was more positive than in isometric contractions. The changes in calcium apparently precede the depolarization. We propose that muscle shortening reduces calcium binding to the contractile proteins and leads to a rise in myoplasmic calcium, and that this rise in myoplasmic calcium activates an inward current leading to the observed changes in the action potential. These processes may be important contributory factors in some arrhythmias.

Action Potentials↗