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Biomedical subjects

A F Brading

Publications and source records attributed to A F Brading.

At least 127 records · Page 7Linked to original sources
PubMed1980

Evidence for multiple sources of calcium for activation of the contractile mechanism of guinea-pig taenia coli on stimulation with carbachol.

1 The evidence presented suggests there are three sources of Ca available for contraction of the smooth muscle of the guinea-pig taenia coli on stimulation with carbachol; the inward Ca current of the spike, a second voltage-dependent Ca channel and an internal Ca store. 2 The initial increment of tension in response to carbachol is thought to be due to an increase in spike frequency which is probably the main source of Ca at low carbachol concentrations (< 10(-6) M). 3 The maintained tension in the continuous presence of high concentrations of carbachol seems to involve continuous influx of membrane-bound Ca by a potential-dependent mechanism which can be very quickly deactivated, resulting in rapid relaxation. This mechanism can be blocked by 2 X 10(-7) M methoxyverapamil (D600). 4 An internal Ca store can be released by high concentrations of carbachol (< 10(-6) M) and is probably responsible for the initial peak of tension, of about 5 min duration seen on continuous application of high concentrations of carbachol and for the tension increase in response to carbachol in tissues depolarized in high-K. 5 Investigation of the properties of the store indicates that it; (i) is very rapidly filled by application of high extracellular Ca; (ii) empties after a few minutes in zero-Ca EGTA Krebs solution; (iii) can be refilled in depolarized tissues in the presence of low concentrations of D600 and Mn, but does not refill during application of carbachol at concentrations greater than 10(-6) M; (iv) contains enough Ca for one near-maximal contraction and once emptied can assist relaxation by Ca re-uptake.

Animals↗
PubMed1980

The effect of sodium removal on the contractile response of the guinea-pig taenia coli to carbachol.

1. The effects of Na-free solutions (using Li, Tris, sucrose or Mg as Na substitutes) on the contractile responses, membrane depolarization and 42K efflux produced by carbachol in the smooth muscle of the guinea-pig taenia coli have been investigated. The effect of these Na-free solutions on intracellular ion content of the muscle has also been studied. 2. Na removal induced a pattern of changes in the tone of the muscle characteristic of the substitute used, probably reflecting changes in transmembrane Ca fluxes involving Na. 3. Contractile responses to 10 sec application of 5 x 10(-5) M-carbachol were greatly reduced in Na-free solutions with all the Na substitutes used. This did not correlate with reduction in membrane depolarization or 42K efflux produced by the drug in the various Na-free medial used. 4. Intracellular Na seems important for maintaining the contractile response, since in Na-free solutions cellular Na levels and contractile responses were better maintained at 13 degrees than 34 degrees C and in tissues stimulated with carbachol every 10 min the final magnitude of the response was related to cellular Na content. If, however, the tissues was left unstimulated in Na-free Mg or sucrose solution a large response could still be obtained when cellular Na content was very low. A model is described which could account for these results in terms of an intracellular Ca store released by carbachol and requiring intracellular Na. 5. In tissues continuously exposed to 10(-4) M-carbachol Na removal, even for only 10 sec, produced rapid relaxation, probably secondary to changes in electrical properties of the membrane caused by removal of external Na.

Animals↗
PubMed1978

Calcium-induced increase in membrane permeability in the guinea-pig taenia coli: evidence for involvement of a sodium-calcium exchange mechanism.

1. High-Na tissues exposed to a Na-free solution with dimethyldiethanol ammonium chloride (DDA) or sucrose replacing Na, develop an increase in membrane permeability to small ions and molecules such as Na, K, sucrose and CoEDTA. 2. The increase in permeability only occurs when the Na gradient across the cell membrane is reversed, and is not due to damaging effects of the Na-free solution. It does not occur in normal or high-K tissues, and 15 mM-[Na]0 is enough to prevent the permeability change in high-Na tissues. 3. Tissues with increased permeability maintain high levels of Ca and the increased permeability does not occur in Ca-free solutions, or in solutions containing 5 mM-La3+. The rate of development of membrane leakiness depends on the level of extracellular Ca. 4. Tissues exposed to iodoacetic acid (IAA) and dinitrophenol (DNP) also develop a membrane leakiness, dependent on extracellular Ca and blocked by La3+. 5. The time taken for development of the increase in membrane permeability in metabolically inhibited tissues can be affected by the Na gradient. With no gradient, or a slightly reversed gradient the membrane break-down occurs more rapidly. 6. It is concluded that the increase in permeability is caused by an increase in internal Ca ions, and that the Na gradient as well as the levels of ATP are important in controlling Ca movements. 7. Tension recordings also support the some form of Na--Ca exchange mechanism operating in the taenia, and this mechanism is not completely blocked by La3+ ions, although they suppress the break-down in membrane permeability.

Animals↗
PubMed1977

The use of lanthanum to estimate the numbers of extracellular cation-exchanging sites in the guinea-pig's taenia coli, and its effects on transmembrane monovalent ion movements.

1. Tissues were allowed to equilibrate in a Tris-buffered Krebs solution and were then exposed to similar solutions containing up to 5 mM-La. La caused shrinkage and significant losses of tissue K, Na, Mg and Ca. The shrinkage was exactly accountable for by a reduction in the extracellular space (e.c.s.) as measured by [14C]sucrose. No significant change was seen in tissue Cl content. Exposure for 1 hr to 5 mM-La led to a total cation loss of 24-3 +/- 1-6 m-equiv/kg or, correcting for the small change in Cl content, a loss of positive charge of 23-8 +/- 2-2 m-equiv/kg fresh wt. 2. Using the radioisotope 140La it was shown that this loss of cation was balanced by an uptake of La3+. 3. Subtraction of the ions in the measured [14C]sucrose space from the total tissue ion contents led to estimates of the "cellular" ion contents. The effects of 1 hr exposure to 5 mM-La on these were a loss of 12-9 +/- 2-4 m-equiv/kg of cation and a gain of 10-0 +/- 2-6 m-equiv/kg of Cl. 4. Similar changes in ion content were produced by La on "Na-loaded" and "K-loaded" tissues, these being tissues which by exposure to K-free or Na-free (high K) solutions had replaced all their K with Na or vice versa. 5. The uptakes of 24Na and 36Cl by Na-loaded tissues were both describable as the sum of two exponentail processes: a fast component (t 1/2 congruent to 1/2 min), which was presumed to be extracellular and a slower, presumed transmembrane, component. La reduced the rapid component of uptake of 24Na by an amount greater than that predicted by the reduction in the e.c.s., the extra amount lost being some 10--15 m-equiv/kg. La also reduced the amount of rapidly exchanging 36Cl, bwt this reduction was entirely accounted for by the change in the e.c.s. La reduced the rate constant of the slow component of 24Na uptake. 6. La reduced the rapidly exchanging component of 42K uptake by normal tissues by an amount equivalent to about 0-5 m-mole/kg fresh wt. of K in excess of the change in the extracellular space. 7. La had little effect on the effluxes of 36Cl and 42K from normal tissues. However, it reduced the size of the fastest component of exchange of 42K efflux from K-loaded tissues by an amount equal to some 10-15 m-equiv/kg in excess of the reduction in the e.c.s. A similar reduction in the rapidly exchanging component of 24Na efflux from normal tissues was also seen. La slowed the efflux of 24Na from Na-loaded tissues at times when the tracer lost could safely be regarded as intracellular. 8. The taenia coli when exposed to K-free solutions gains Na and loses K. In the presence of La the gain in Na was completely blocked. K was still lost, however, being accompanied by Cl and increased shrinkage. La also prevented the uptake of Na from high Na media by ion-depleted tissues (produced by exposure to sucrose media), while having little effect on the uptake of K from high K media by such tissues. 9 Tedia by such tissues. 9. The cation displaced by La in excess of that lost due to the reduction of the e.c.s...

Animals↗
PubMed1975

Sodium/sodium exchange in the smooth muscle of the guinea-pig taenia coli.

External Na has been shown to initiate a loss of 24 Na from high-Na smooth muscle of the guinea-pig taenia coli. This is an ouabain-insensitive effect, and there appears to be a 1:1 exchange of Na ions, suggesting a classical Na/Na exchange mechanism. 2. The Na/Na exchange has many properties in common with a similar exchange studied in high-Na beef erythrocytes by Motais (1973), and Motais & Sola (1973). It is very temperature-sensitive, it is partially inhibited by the sulphydryl reagents studied and it has a fairly low external affinity for Na. 3. The affinity of the external site for alkali metal cations is Na greater than Li greater than K greater than Rb greater than Cs. 4. It has proved impossible to estimate the affinity of the intracellular sites for Na. The curve relating intracellular Na content with the stimulated efflux reaches a maximum and then declines slightly. 5. Another unexpected finding was that after the rate of loss of Na has been reduced in a Na-free medium, reintroducing Na causes an overshoot in the rate, it increases to a value beyond the original one, and then slowly declines to it. 6. Unlike the classical Na/Na mechanism, the process is reduced, but not abolished by metabolic inhibition that depletes the tissue of ATP. 7. The results are interpreted to suggest that the Na/Na exchange is occurring from a cellular compartment of limited volume, which is itself exchanging with the main cell compartment. It is suggested that this small compartment is the sarcoplasmic reticulum, and the effect of metabolic inhibition is to interfere in some way with the relationship between this compartment and the cell membrane.

Adenosine Triphosphate↗
PubMed1974

An estimate of sodium-potassium pump activity and the number of pump sites in the smooth muscle of the guinea-pig taenia coli, using (3H)ouabain.

1. Binding of tritiated ouabain to the smooth muscle of the guinea-pig taenia coli showed two components, one saturable at lower glycoside concentrations and the other linear with increasing concentrations.2. The saturable component alone was affected by extracellular potassium concentrations. This component seems to be bound to sodium pumping sites, and when completely saturated binds 1.1 x 10(11) molecules per mg fresh wt. of tissue, or 250-300 molecules per square micron of membrane, assuming a volume:surface area ratio of 1.5 mum.3. Only a fraction of (42)K uptake by the cells can be blocked by ouabain at maximal concentrations. In normal Krebs solution two thirds can be blocked. The remaining one third is presumably passive uptake. The fraction blocked is reduced as the extracellular potassium concentration, and thus passive uptake, is increased.4. The amount of potassium pumped into the cells at various concentrations of extracellular potassium has been calculated. In normal Krebs solution the amount pumped in 45 min was 20.0 m-mole/kg fresh wt., and this was increased at higher potassium concentrations.5. On the assumption of a stoichiometry of 3Na: 2K, the pump sites in normal Krebs solution have a turnover rate of 1320 min(-1).6. Indirect calculations of sodium movements suggest that the sodium permeability may be about 0.9 x 10(-8) cm sec(-1) and the pump may generate a current of 0.9 x 10(-7) A cm(-2). This crossing an Ohmic membrane resistance of 30-60 kOmega cm(2) would be equivalent to a potential difference of 3-5 mV.

Animals↗
PubMed1971

Analysis of the effluxes of sodium, potassium and chloride ions from smooth muscle in normal and hypertonic solutions.

1. Efflux curves of (24)Na, (42)K and (36)Cl from the guinea-pig taenia coli were obtained in normal Krebs solution, and in hypertonic Krebs solution in which the osmolarity had been doubled by the addition of sucrose.2. The efflux curves were complex, and in order to get average curves each was analysed as the sum of three exponential terms, and average curves were constructed from the means of the constants found.3. In order to estimate the membrane permeability to the ions, it was necessary to make assumptions as to the distribution of ions in the tissue. Several models have been examined and the predictions of the models with respect to the membrane properties were compared with the data obtained by using electrophysiological methods by other workers.4. It was found that reasonable predictions of membrane properties could only be made using models in which the majority of the rapidly exchanging sodium is considered to be extracellular. This amount of sodium is more than can be accounted for as freely dissolved in the extracellular water.5. A possible interpretation of the ion exchange and distribution would be to suppose that some proportion of the three ions is contained in an extracellular compartment not available to the normal extracellular markers, and limited in its exchange by the rate of diffusion in the extracellular phase, and that the truly intracellular fractions of the tissue ions do not exchange with the external solution in a simple exponential manner, but in a manner described by an aggregate of exponential terms due to inherent variation in the permeabilities of the individual cell membranes.6. There is no evidence for any change in the membrane permeabilities to sodium and potassium in hypertonic solution, but there is evidence for a decrease in chloride permeability in this solution.

Animals↗
PubMed1971

The resting membrane potential of the somatic muscle cells of Ascaris lumbricoides.

1. The resting membrane potential of Ascaris muscle fibres, which is normally relatively insensitive to ion changes in the medium, has been measured under a wide variety of conditions.2. The results have been interpreted in terms of a form of the constant field equation containing additional terms for the contribution of ions and charged groups other than potassium, sodium and chloride.3. Normally the contribution of the additional terms is large and tends to outweigh the contributions of potassium, sodium and chloride.4. The contribution of the additional terms is considerably reduced in the absence of sodium and in the presence of gamma-amino butyric acid and acetylcholine.5. It is suggested that the additional terms may represent the contribution of an electrogenic active transport mechanism to the factors determining the membrane potential.

Acetylcholine↗
PubMed1969

Distribution and kinetics of CoEDTA in smooth muscle, and its use as an extracellular marker.

1. [(60)Co]EDTA has been evaluated as an extracellular marker in guinea-pig taenia coli and rabbit myometrium.2. The complex was found to be stable, non-toxic, convenient to use and had the advantage of being readily measured without interfering with the analysis of water and electrolyte contents.3. In both smooth muscles [(60)Co]EDTA distributed rapidly into a volume equivalent to that available to [(14)C]sorbitol and slightly greater than that available to inulin.4. The optimum concentrations of carrier required to effectively saturate any adsorption sites were 0.1 mM-CoEDTA for taenia coli and 1.0 mM for myometrium.5. [(60)Co]EDTA was distributed into a volume equivalent to the total water content of rabbit tendon, indicating that it was not excluded from connective tissue water.6. About 5% of the [(60)Co]EDTA taken up by smooth muscle in 15 minutes exhibited slow efflux kinetics. This property was also exhibited by [(14)C]sorbitol and has been reported for other extracellular markers.7. The efflux of 95% of [(60)Co]EDTA followed bulk diffusion kinetics for both taenia coli and myometrium.8. The simultaneous efflux of (24)Na indicated that 1-2% exchanged slowly while 98-99% of the (24)Na exchange could be described by bulk diffusion kinetics, similar to [(60)Co]EDTA. About 15% of this (24)Na was contained outside of the [(60)Co]EDTA space.9. More detailed studies involving the analysis of simultaneous efflux of [(60)Co]EDTA and (24)Na may help to locate and characterize the properties of the excess rapidly exchanging sodium (12-15 m-equiv/kg. wet wt.) in smooth muscle.

Animals↗
PubMed1968

The effect of hypo- and hypertonic solutions on volume and ion distribution of smooth muscle of guinea-pig taenia coli.

1. The intra- and extracellular spaces and ionic content of the taenia coli of the guinea-pig have been measured in a series of bathing solutions in which the tonicity varied from 0.5 to 3 times the tonicity of the normal Krebs solution.2. Equilibrium of the tissue in the experimental solution is reached within about 30 min.3. The absolute values of the parameters measured have been shown to depend on the blotting technique used. These differences were eliminated by expressing the results as a percentage of the values found in the normal Krebs solution.4. In hypertonic solutions the cell behaves as a perfect osmometer, the cell volume changing in proportion to the tonicity of the bathing medium. Only a small amount of cations is lost from the cells. A considerable amount of chloride is lost, making the postulation of its replacement by some other anions necessary in order to maintain electroneutrality of the intracellular solution.5. In hypotonic solutions the cells do not behave as predicted for a perfect osmometer. In 0.5 hypotonic solution an actual decrease in cell volume was observed associated with an increase of the extracellular space probably due to penetration of [(14)C]sorbitol into the cell. The intracellular ionic concentration was decreased. These findings suggest damage of the cell membrane.6. The observed hyperpolarization of the membrane in hypertonic solution can be explained by the increased intracellular potassium concentration.

Animals↗