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Biomedical subjects

M W Smith

Publications and source records attributed to M W Smith.

At least 325 records · Page 18Linked to original sources
PubMed1973

Influence of cholecalciferol (vitamin D(3)) on the initial kinetics of the uptake of calcium ions by rat small-intestinal mucosa (Short Communication).

The uptake of Ca(2+) by isolated small-intestinal mucosa from vitamin D-depleted and -repleted rats was analysed for the effects of vitamin D on initial kinetics. The rapid association of Ca(2+) with the tissue, which is complete within 1min, was unaffected by the vitamin, whereas the subsequent, linear, uptake was significantly increased. Neither the tissue space accessible to inulin nor the permeability to thiourea was influenced by vitamin D treatment.

Journal Article↗
PubMed1973

Sodium uptake by frog skin and its modification by inhibitors of transepithelial sodium transport.

1. The suitability of inulin and mannitol as markers of the diffusional space that exists between the surface of frog skin and the outer barrier to sodium transport has been compared in experiments designed to measure the rapid uptake of sodium.2. Inulin and mannitol both equilibrate finally with the same space at the outer surface of the frog skin, but the rate at which this equilibration occurs is considerably faster for mannitol.3. The dependence of the rapid uptake of sodium on the concentration of sodium in the external medium, using mannitol to correct for extracellular sodium, can be described by simple saturation kinetics with an apparent K(m) for sodium entry of 24 mM and a maximal rate of uptake of 1.28 muequiv/cm(2). hr.4. The effect of amiloride is to inhibit sodium uptake, the degree of inhibition depending both on the concentration of sodium in the external medium and on the level of transepithelial transport normally maintained by the skin.5. Ouabain inhibits sodium uptake when the tissue has been pre-incubated in sodium containing medium. It has no effect on sodium uptake if the pre-incubation takes place in sodium free medium.6. A short-circuit current and potential difference can be elicited across frog skin in the presence of ouabain, by increasing the concentration of sodium bathing the outside surface. This potential and short-circuit current is abolished by the presence of amiloride.7. These results provide direct evidence that amiloride acts to stop entry of sodium into the transport compartment and suggest that the ouabain inhibition of sodium uptake is mediated primarily through an increase of epithelial sodium concentration.

Amiloride↗
PubMed1973

Uptake of calcium and magnesium by rat duodenal mucosa analysed by means of competing metals.

1. The short-term uptake of Ca and Mg by rat duodenal mucosa consisted of two parts, one apparently complete within 1 min (uptake I) and a second which increased linearly with time from 1 to 12 min (uptake II).2. Uptake II for both Ca and Mg showed a curvilinear dependence on concentration. Uptake I was related linearly to concentration up to the highest concentration tested.3. Mg caused a significant inhibition of Ca uptake II. Ca was without effect on Mg uptake.4. Mn reduced significantly both uptakes I and II for Ca; Ba and Sr were without effect on Ca uptake.5. Two lanthanides, La and Pr, both inhibited uptakes I and II for Ca. Inhibition by Pr could be reversed by increasing the concentration of Ca. Some uptake II for Ca still persisted when uptake I was completely abolished by 3 mM-La.6. It is suggested that a small part of Ca uptake I may be associated with the subsequent mediated entry into the mucosa, since inhibition of Ca uptake consistently involves both forms of uptake. However, since some Ca still enters the mucosa when uptake I is completely inhibited, it appears that some entry of Ca can occur by diffusion.

Animals↗
PubMed1973

The effect of salt adaptation on the permeability and cation selectivity of the goldfish intestinal epithelium.

1. The short-term uptake of Na by the goldfish mucosa was compared using both inulin and choline as markers of extracellular space. The results were virtually identical, the distribution of both choline and inulin increasing rapidly to measure a space at 1 min which then remained nearly constant during a following 4 min incubation.2. Using inulin as space marker, the uptake of various alkali metal cations was determined from a 1 min contact with the mucosa. The relative rates of uptake were Tl > K > Rb > Cs > Na > Li, with a low selectivity ratio, the range of permeabilities being no greater than 5.3. The selectivity sequence was the same in both salt and fresh-water adapted fish. Of the alkali metal cations tested, only Na showed a significantly decreased uptake on adaptation to salt.4. Isolated intestinal preparations from salt-adapted fish showed a reduced short-circuit current compared with fish adapted to fresh water, the values being 12.3 +/- 1.2 and 35.7 +/- 1.5 muA cm(-2) respectively. In both cases the short-circuit current was equivalent to the net transport of Na measured isotopically.5. In Krebs-Henseleit medium, the measured tissue resistance was approximately 100 Omega cm(-2) for both salt and fresh-water adapted fish.6. It is concluded that regulation of cation transport in goldfish intestinal epithelium is specific for Na and mediated primarily through cellular rather than extracellular pathways.

Adaptation, Physiological↗
PubMed1972

Effect of environmental temperature on the kinetic properties of goldfish brain choline acetyltransferase.

1. Michaelis constants of goldfish brain choline acetyltransferase were found to depend on the concentration of the second substrate present and on the temperature to which the fish had been adapted. 2. Primary plots constructed from results obtained with enzyme prepared from cold-adapted or warm-adapted fish indicated that synthesis of acetylcholine took place by a sequential mechanism. 3. The affinity of choline acetyltransferase for acetyl-CoA was about 100 times that for choline irrespective of whether the enzyme had been prepared from warm-adapted or cold-adapted fish. 4. The maximum rate at which choline acetyltransferase synthesized acetylcholine and the energy of activation for this synthesis remained independent of the previous environmental temperature of the fish. 5. The affinity of choline acetyltransferase for choline and acetyl-CoA showed a complex dependence on temperature. The affinity of the enzyme from cold-adapted fish for substrates increased as the incubation temperature was lowered, whereas that of the enzyme from warm-adapted fish first increased and then decreased. 6. The maximum affinity of choline acetyltransferase for both substrates, from both cold-adapted and warm-adapted fish, occurred at temperatures that corresponded approximately to the respective environmental temperatures of the fish. 7. These changes in enzyme affinity for substrates are not thought to be due to the presence of isoenzymes. Their adaptive significance is unknown, but it could be connected with the maintenance of the enzyme in a stable form.

Acetyltransferases↗
PubMed1972

Changes in the intestinal transport of sodium induced by exposure of goldfish to a saline environment.

1. The uptake and transflux of sodium by goldfish intestines has been compared under different experimental conditions. Uptake was measured from a 1 min contact of the intestinal mucosa with radioactive sodium chloride solution. Transflux was measured over a period of 2 hr using the everted sac technique.2. Keeping goldfish in saline reduced the transflux of sodium to one quarter the value found for fresh-water fish. The uptake of sodium was halved by this treatment. Cortisol injected previously into saline-adapted fish changed neither the transflux nor the uptake of sodium measured subsequently.3. In fresh-water fish hypophysectomy reduced sodium transflux while leaving the uptake of sodium unchanged. Injection of cortisol restored sodium transflux to control levels without producing any additional effects on the uptake of sodium.4. It is suggested that adaptation to saline involves regulation of sodium movement across the microvillar membrane of the mucosal cell. Cortisol would appear to play no part in this type of regulation.5. The presence of cortisol, or possibly other steroids with similar actions, has however been shown to be essential for the normal operation of sodium transport in this tissue. It is not clear exactly how cortisol exerts this effect. What evidence there is suggests that cortisol exerts a metabolic control rather than changing directly the membrane permeability of the cell.

Adaptation, Physiological↗