Active sulfate absorption in rabbit ileum: Dependence on sodium and chloride and effects of agents that alter chloride transport

Summary Unidirectional fluxes of³⁵SO₄ across and into rabbit ileal epithelium were measured under short-circuit conditions, mostly at a medium SO₄ concentration of 2.4mm. Unidirectional mucosa (m)-to-serosa (s) ands-to-m fluxes (J _ms,J _sm) were 0.456 and 0.067 moles hr^–1 cm^–2, respectively.J _ms was 2.7 times higher in distal ileum than in mid-jejunum. Ouabain abolished net SO₄ transport (J _net) by reducingJ _ms. Epinephrine, a stimulus of Cl absorption, had no effect on SO₄ fluxes. Theophylline, a stimulus of Cl secretion, reducedJ _ms without affectingJ _sm, causing a 33% reduction inJ _net. Other secretory stimuli (8-Br-cAMP, heat-stable enterotoxin, Ca-ionophore A23187) had similar effects. Replacement of all Cl with gluconate markedly reducedJ _net through both a decrease inJ _ms and an increase inJ _sm. The anion-exchange inhibitor, 4-acetoamido-4-isothiocyano-2,2-sulfonic acid stilbene (SITS), when added to the serosal side, reducedJ _ms by 94%, nearly abolishingJ _net. SITS also decreasedJ _sm by 75%. Mucosal SITS (50 m) was ineffective. 4,4-diisothiocyano-2,2-sulfonic acid stilbene (DIDS) had effects similar to SITS but was less potent. Measurements of initial rates of epithelial uptake from the luminal side (J _me) revealed the following: (1)J _me is a saturable function of medium concentration with aV _max of 0.94 moles hr^–1 cm^–2 and aK _1/2 of 1.3mm; (2) replacing all Na with choline abolishedJ _me; (3) replacing all Cl with gluconate increasedJ _me by 40%; (4) serosal SITS had no effect onJ _me; and (5) stimuli of Cl secretion had no effect onJ _me or increased it slightly. Determination of cell SO₄ with³⁵SO₄ indicated that, at steady-state, the average mucosal concentration is 1.1 mmoles per liter cell water, less than half the medium concentration. Cell SO₄ was increased to 3.0mm by adding SITS to the serosal side. Despite net transport rates greater than 1.4 Eq hr^–1 cm^–2, neither addition of SO₄ to the SO₄-free medium nor addition of SITS to SO₄-containing medium altered short-circuit current. The results suggest that (1) ileal SO₄ absorption consists of Na-coupled influx (symport) across the brush border and Cl-coupled efflux (antiport) across the basolateral membrane; (2) the overall process is electrically neutral; (3) the medium-to-cell Cl concentration difference may provide part of the driving force for net SO₄ absorption; and (4) since agents affecting Cl fluxes (both absorptive and secretory) have little effect on SO₄ fluxes, the mechanisms for their transcellular transports are under separate regulation.     

Localized secretion of acid phosphatase reflects the pattern of cell surface growth in saccharomyces cerevisiae

Secretion of cell wall-bound acid phosphatase by Saccharomyces cerevisiae occurs along a restricted portion of the cell surface. Acid phosphatase activity produced during derepressed synthesis on a phosphate-limited growth medium is detected with an enzyme-specific stain and is localized initially to the bud portion of a dividing cell. After two to three generations of phosphate-limited growth, most of the cells can be stained; if further phosphatase synthesis is repressed by growth in excess phosphate, dividing cells are produced in which the parent but not the bud can be stained. Budding growth is interrupted in α-mating-type cells by a pheromone (α-factor) secreted by the opposite mating type; cell surface growth continues in the presence of α-factor and produces a characteristic cell tip. When acid phosphatase synthesis is initiated during α-factor treatment, only the cell tip can br stained; when phosphate synthesis is repressed during α-factor treatment, the cell body but not the tip can be stained. A mixture of derepressed α cells and phosphatase-negative α cells form zygotes in which mainly one parent cell surface can be stained. The cell cycle mutant, cdc 24 (Hartwell, L.H. 1971. Exp. Cell Res. 69:265-276), fails to bud and, instead, expands symmetrically as a sphere at a nonpermissive temperature (37 degrees C). This mutant does not form a cell tip during α-factor treatment at 37 degrees C, and although acid phosphatade secretion occurs at this temperature, it is not localized. These results suggest that secretion reflects a polar mode of yeast cell- surface growth, and that this organization requires the cdc 24 gene product.     

Effects of hyperthermia on the mouse testis and its response to X-rays, as assayed by weight loss.

The effects of both hyperthermia alone and X-rays combined with hyperthermia on mouse testis have been investigated. Testis weight on heating time was observed for temperatures in the range 39.5 to 43.75 degrees C. The relationship between the reaction rate and the reciprocal of absolute temperature indicated that, over the temperature range considered, the activation energy associated with such thermal damage was (646 +/- 45) x 10(3) J mol-1. No evidence was obtained to indicate a change in slope of the Arrhenius plot over this temperature range. Finally, despite the high sensitivity of the testis to heat and X-rays, no thermal enhancement of the weight loss after irradiation was observed when thermal treatments which, if given alone would produce some observable damage, were administered immediately after irradiation.     

The cardioregulatory system of crayfish: neuroanatomy and physiology.

1. The anatomical arrangement of the cardioregulatory nerves and their physiological activity during cardiac modulation were analysed in Procambarus clarkii. 2. The bilaterally arranged pairs of cardioinhibitors and cardioaccelerator axons, in nerves SN II and SN III respectively, were physiologically identified by correlating spikes in SN II and SN III with the same spikes in the dorsal nerve, which innervates the heart. 3. The cardioinhibitor neurone fired tonically in varied sporadic bursts. During periods of cardiac inhibition, however, this neurone discharged in a long chain of spikes at a characteristic frequency of 40-50Hz. 4. The cardioaccelerator neurone fired tonically at 2-3 Hz but on occasion its activity reached 12 Hz. 5. Three inhibitory cardiac reflexes were analysed. The sensory modalities for the reflexes included (a) stretch of the dorsal pericardial wall, (b) chemical stimulation of coxal hair sensilla with glucose and (c) tactile stimulation of hair sensilla in and below the gill chamber, on the antennae, the antennules and on the anterior cephalothorax. 6. The discharge of both cardioinhibitor neurones showed a weak temporal correlation suggesting a common presynaptic drive, while the pair of cardioaccelerators appeared to have a reciprocal relationship with the cardioinhibitors.     

The temperature dependences of some types of gaseous ionic reactions of astrochemical interest

The rate constants of ion-molecule reactions which are of potential significance in astrochemical systems are found to exhibit significant, and in many cases, negative temperature dependences. The rate constants of fast ion-polar molecule reactions (e.g., XH⁺+B»BH⁺+X) may increase by a factor of 5–10 between 1000 and 10K. Slow reactions that proceed via reaction complexes (e.g., H-transfer and association reactions) often exhibit temperature dependences of the formk=AT ⁻ⁿ ,n=1–5. Both transition state theory considerations and the coupled-oscillator RRK-type model are seen to be able to account qualitatively for the behavior of slow ion-molecule, reactions.     

Electrostatic influence of local cysteine environments on disulfide exchange kinetics

The ionic strength dependence of the bimolecular rate constant for reaction of the negative disulfide 5,5'-dithiobis (2-nitrobenzoic acid) with cysteines in fragments of naturally occurring proteins was determined by stopped-flow spectroscopy. The Debye-Hückel relationship was applied to determine the effective charge at the cysteine and thereby determine the extent to which nearby neighbors in the primary sequence influence the kinetics. Corrections for the secondary salt effect on cysteine pKs were determined by direct spectrometric pH titration of sulfhydryl groups or by observation of the ionic strength dependence of kinetics of cysteine reaction with the neutral disulfide 2,2'-dithiodipyridine. Quantitative expressions was verified by model studies with N-acetyl-cystein. At ionic strengths equal to or greater than 20 mM, the net charge at the polypeptide cysteine site is the sum of the single negative charge of the thiolate anion and the charges of the amino acids immediately preceding and following the cysteine in the primary sequence. At lower ionic strengths, more distant residues influence kinetics. At pH 7.0, 23 degree C, and an ionic strength of 20 mM, rate constants for reaction of the negative disulfide with a cysteine having two positive neighbors, one positive and one neutral neighbor, or two neutral neighbors are 132000, 3350, and 367 s-1 M-1, respectively. This corresponds to a contribution to the activation energy of 0.65- 1.1 kcal/mol per ion pair involved in collision between the cysteine and disulfide regions. The results permit the estimation that cysteine local environments may provide a means of achieving a 10(6)-fold range in rate constants in disulfide exchange reactions in random-coil proteins. This range may prove useful in developing strategies for directing disulfide pairing in synthetic proteins.