Donald F. Tapley

TRANSPORT OF NEUTRAL, DIBASIC AND N-METHYL-SUBSTITUTED AMINO ACIDS BY RAT INTESTINE.

The ability of everted sacs of rat small intestine to transport neutral, dibasic and N-methyl-substituted amino acids has been determined. The data obtained support the suggestion that three separate amino acid-transport systems are present in rat intestine.

Intestinal transport of amino acids studied in vitro with l-[131I]monoiodotyrosine

Abstract The transport of l -[131I]monoiodotyrosine by everted sacs of small intestine is dependent on active accumulation of this amino acid in the gut wall, the terminal ileum being the most active segment. Its transport is stimulated by glucose, which depresses the efflux of monoiodotyrosine from the mucosal surface of the intestine. Certain aminoacids and potassium ion competitively inhibit moniodotyrosine transport. α-amino acids of l -configuration with a free amino group and a long non-polar side-chain are the most effective inhibitors.

Glucuronide formation in the transport of testosterone and androstenedione by rat intestine

Abstract Incubation of [4-14C]testosterone or δ4-androstene-3,17-dione with everted sacs of rat intestine resulted in the net transport of radioactivity to the serosal media. At the end of a 3-h incubation, both the mucosal and serosal media were found to contain radioactivity in compounds with the chromatographic proporties of testosterone, α4-androstene-3,17-dione, androstane-3,17-dione, androsterone, androstarone-3α,17β-diol. Androstane-3α,17β-diol was primarily present in the serosal media as a glucuronide conjugate. This steroid was the only one thus conjugated. Accumulation of this glucuronide conjugate of androstane-3α,17β-diol in the serosal media was responsible for the net transfer of radioactivity across the gut wall.

Glucuronide formation in the transport of estradiol by rat intestine in vitro

Abstract When [14C]-17-β-estradiolis incubated with everted sacs of rat intestine net transport of radioactivity to the serosal medium is accomplished by oxidation of the estradiol to estrone followed by conjugation of the estrone as a glucuronide. The conjugated estrone is accumulated in the serosal medium. Relative impermeability of the mucosa to the conjugate offers a reasonable basis for the accumulation of radioactivity in the serosal fluid.

Glucuronide formation in the transport of thyroxine analogues by rat intestine

Abstract The acetic, propionic and formic acid analogues of thyroxine and the acetic and propionic acid analogues of triiodothyronine are transported by everted sacs of rat small intestine and colon. They accumulate in the serosal fluid in concentrations up to 14 times greater than those in the original medium. In all cases, the transport involves the accumulation in the serosal fluid of a glucuronide which appears to be linked through the phenolic hydroxyl of the transported compound. Similarly, glucuronides of L -thyroxine and L -triiodothyronine accumulate in the serosal fluid, but not in quantities sufficient to result in a concentration gradient. The glucuronides of these thyroxine analogues are not themselves transported by the everted gut sacs. Relative impermeability of the mucosa to these conjugates offers a reasonable explanation for their accumulation in the serosal fluid.

Leucine transport by rat liver mitochondria in vitro

Abstract Leucine accumulation by rat liver mitochondria in vitro has been studied. Uptake was stimulated by ATP and inorganic phosphate and was inhibited by N -ethylmaleimide. With ATP mitochondrial leucine concentration exceeed that of the medium 2–3 fold. Leucine uptake was consistent with saturation kinetics. Certain other amino acids (isoleucine, methionine, valine, and cycloleucine) both significantly inhibited leucine uptake and effected a rapid discharge of leucine from mitochondria pre-loaded with the amino acid. The evidence indicates that rat liver mitochondria contain a leucine transport mechanism, for which several other amino acids are competitive.

The effect of analogues of thyroxine and 2,4-dinitrophenol on the swelling of mitochondria

Abstract Studies with analogues of thyroxine suggest that the ability of thyroxine to cause mitochondrial swelling is dependent upon the presence of the iodo-substituted diphenyl ether, but independent of the phenolic hydroxyl or the composition of the side chain. Three known “peripheral antagonists” of thyroxine have been shown to antagonize thyroxine-induced swelling. The ability of 2,4-dinitrophenol to prevent mitochondrial swelling appears to depend upon the free hydroxyl and the presence of appropriate electro-negative substituents in the ortho- and para-positions.

Stimulation of mitochondrial protein synthesis and oxygen consumption by thyroxine in vitro without deiodination to triiodothyronine

Abstract The abilities of triiodothyronine (T 3 ) and thyroxine (T 4 ) to stimulate protein synthesis and oxygen consumption and to alter ADP/O and respiratory control ratios in normal rat liver mitochondria in vitro have been investigated together with determinations of the extent of conversion of T 4 to T 3 . There was no significant differences in the ability of either hormone to stimulate protein synthesis or oxygen consumption. ADP/O and respiratory control ratios were approximately equally depressed by T 4 and T 3 in concentrations (10 and 50 μM) which increased mitochondrial protein synthesis and oxygen consumption. T 4 was not appreciably deiodinated to T 3 , indicating that the hormones are of equal potency in producing these effects in normal mitochondria in vitro .

The Peripheral Action of Thyroxine

Publisher Summary The mechanism of action of the thyroid hormone, or hormones, has interested investigators since the turn of the century, when the properties of the thyroid secretions first began to be appreciated. The problem was approached initially as it related to the whole animal or to specific organs or tissues; more recently extensive investigation has been conducted at the level of the cell and its constituents, in particular the subcellular particles and certain enzymes. The physiological effects of the thyroid secretions in mammals appear to be well established. There is, however, no general agreement on the structure of the molecule responsible for hormonal activity in the peripheral tissues, nor on the exact mechanism of action at the subcellular level. This chapter concerns itself with the current status of these two problems as they apply to mammals.