S Acra

Vanderbilt University, Nashville, MI, United States

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Publications (9)25.48 Total impact

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    ABSTRACT: The Na+/H+ exchangers at the brush border membrane (BBM) and the basolateral membrane (BLM) are each distinct with different kinetic and pharmacologic characteristics. At the BBM, Na+/H+ exchange provides an acid microenvironment necessary for nutrient transport. At the BLM, the Na+/H+ exchanger regulates intracellular pH and cell volume which affect cell growth and repair. This study was designed to determine the effect of burn injury on Na+/H+ exchange at the BBM and BLM of the rat enterocyte. Adolescent Sprague-Dawley rats were divided into control (n = 6) and 20% scald burn groups (n = 6). Using differential centrifugation and percoll density gradient techniques, BBM and BLM vesicles were prepared from the jejunum. 22Na+ uptake was measured using a rapid filtration technique. Initial rate uptake studies showed that the slope of Na+ uptake in BBM (y = 0.06x + 0.12, r2 = 0.99) and BLM (y = 0.075x + 0.1, r2 = 0.99) of the control group was higher (P < 0.05) than that in the burn group (y = 0.0345x + 0.06, r2 = 0.98 and y = 0.056x + 0.01, r2 = 0.99). To determine whether the changes in transport are related to the feeding pattern in burn rats, similar experiments were done in pair-fed animals. The initial rate uptake studies of BBM showed a similarily greater slope of Na+ uptake in pair-fed control animals compared to the burn group (y = 0.043x + 0.06, r2 = 0.99 and y = 0.062x + 0.008, r2 = 0.98; P < 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)
    Journal of Surgical Research 03/1995; 58(2):137-42. · 2.02 Impact Factor
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    ABSTRACT: Increased Na+/H+ exchanger activity is associated with cellular hyperplasia. Cellular hyperplasia is an adaptive response to small-intestinal resection. Therefore, we hypothesized that the small-intestinal Na+/H+ exchanger activity increases in response to small-intestinal resection. Twenty-one-d-old, male Sprague-Dawley rats were randomly divided to receive either a 70% small intestinal resection (n = 59), or a mid-small intestinal transection (n = 49). Seven d postoperatively, the animals were killed and the Na+/H+ exchanger activity of the intestinal remnants was studied by a well validated brush border membrane vesicle technique. The initial rate of Na+ uptake in the presence of an outwardly directed pH gradient and the Vmax of the amiloride-sensitive Na+ uptake were significantly increased (p < 0.01 and p < 0.001, respectively) in the resection as compared with the transection remnants and to a greater magnitude in the distal as compared with the proximal remnants. Km values were not significantly different. The amiloride-sensitive Na+ uptake in the setting of various intravesicular pH was significantly greater (p < 0.001) in the distal resection as compared with the distal transection remnants, with points of enhanced Na+/H+ exchanger activity of intravesicular pH 6.62 and 6.87, respectively. The presence and activation of the Na+/H+ exchanger's internal modifier site was confirmed by demonstrating the effect of intravesicular pH on Na+ efflux. The present study demonstrates an up-regulation of intestinal Na+/H+ exchange activity in a small-bowel resection model in the weanling rat.(ABSTRACT TRUNCATED AT 250 WORDS)
    Pediatric Research 03/1993; 33(3):215-20. · 2.67 Impact Factor
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    ABSTRACT: Kinetically distinct Na(+)-H+ exchangers exist on the apical and basolateral membranes of rabbit ileal enterocytes. The apical Na(+)-H+ exchanger appears to function in electroneutral NaCl transport, whereas the basolateral Na(+)-H+ exchanger may function in homeostatic intravesicular pH (pHi) regulation and volume regulation. This study is designed to determine the presence and characteristics of the Na(+)-H+ exchanger in basolateral membrane vesicles (BLMV) prepared from jejunal tissues of human organ donors. A well-validated Percoll-gradient technique was used to prepare BLMV. An outwardly directed H+ gradient [pHi/extravesicular pH (pHo) = 5.2/7.5] resulted in a Na+ uptake overshoot (1.45 +/- 0.21 nmol/mg protein) 2.5-fold above equilibrium values (0.59 +/- 0.13 nmol/mg protein). Na+ uptake at equilibrium represented transport into an osmotically sensitive intravesicular space as validated by an osmolality study. Na+ uptake represented an electroneutral process, as shown by studies in which negative membrane potentials were induced by K+ and the ionophore valinomycin. Na+ uptake was linear for the first 15 s of transport as depicted by y = 0.042x + 0.002, r2 = 0.98. Dixon plot analysis of amiloride sensitivity revealed an ID50 value for amiloride of 29 microM (fourfold lower than ID50 for brush-border Na(+)-H+ exchanger). Kinetic studies of amiloride-sensitive Na+ uptake revealed a maximal velocity = 1.53 +/- 0.19 nmol.mg protein-1.5 s-1 and Michaelis constant = 9.83 +/- 3.5 mM. By varying pHi a sigmoidal effect of internal H+ on Na+ uptake was noted consistent with an internal modifier site for protons. To confirm this finding, the effect of pHi on Na+ efflux and Na(+)-Na+ exchange was studied.(ABSTRACT TRUNCATED AT 250 WORDS)
    The American journal of physiology 02/1993; 264(1 Pt 1):G45-50. · 3.28 Impact Factor
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    ABSTRACT: The present study was designed to investigate Cl- transport across rat ileal basolateral membranes. Basolateral membrane vesicles were prepared by a well-validated technique. The purity of the basolateral membrane vesicles was verified by marker enzyme studies and by studies of d-glucose and calcium uptake. Cl- uptake was studied by a rapid filtration technique. Neither an outwardly directed pH gradient, nor a HCO3- gradient, or their combination could elicit any stimulation of Cl- transport when compared with no gradient. 4,4-Diisothiocyanostilbene-2,2-disulfonic acid at 5 mM concentration did not inhibit Cl- uptake under gradient condition. Similarly, the presence of the combination of outwardly directed Na+ and HCO3- gradients did not stimulate Cl- uptake compared with the combination of K+ and HCO3- gradients or no HCO3- gradient. This is in contrast to our results in the brush border membranes, where an outwardly directed pH gradient caused an increase in Cl- uptake. Cl- uptake was stimulated in the presence of combined Na+ and K+ gradient. Bumetanide at 0.1 mM concentration inhibited the initial rate of Cl- uptake in the presence of combined Na+ and K+ gradients. Kinetic studies of bumetanide-sensitive Cl- uptake showed a Vmax of 5.6 +/- 0.7 nmol/mg protein/5 sec and a Km of 30 +/- 8.7 mM. Cl- uptake was stimulated by an inside positive membrane potential induced by the ionophore valinomycin in the setting of inwardly directed K+ gradient compared with voltage clamp condition. These studies demonstrate two processes for Cl- transport across the rat ileal basolateral membrane: one is driven by an electrogenic diffusive process and the second is a bumetanide-sensitive Na+/K+/2 Cl- process. Cl- uptake is not enhanced by pH gradient, HCO3- gradient, their combination, or outwardly directed HCO3- and Na+ gradients.
    Proceedings of The Society for Experimental Biology and Medicine 01/1993; 201(3):254-60.
  • S A Acra, N Nakagawa, F K Ghishan
    Comprehensive Therapy 11/1991; 17(10):22-6.
  • S Acra, F K Ghishan
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    ABSTRACT: The spontaneously hypertensive rats and their genetically matched controls, Wistar-Kyoto, serve as models of essential hypertension. The present study was undertaken to determine whether brush border membrane vesicles obtained from jejunal enterocytes of spontaneously hypertensive rats show increased Na(+)-H+ exchange as part of a generalized membrane disorder. Brush border membrane vesicles were prepared from the jejunum of adult spontaneously hypertensive rats and Wistar-Kyoto rats using an Mg2+/ethylene glycol tetraacetic acid precipitation method. Uptake of 22Na by these vesicles was found to be into an osmotically sensitive intravesicular space rather than mere binding. Initial Na+ uptake by brush border membrane vesicles was greater in spontaneously hypertensive rats than in Wistar-Kyoto rats (P less than 0.05). Higher total and amiloride-sensitive Na+ uptake in spontaneously hypertensive rats occurred in the presence of an outwardly directed pH gradient, and uptake became statistically similar to that of Wistar-Kyoto rats in the absence of a pH gradient. Moreover, amiloride-insensitive Na+ uptake under an outwardly directed pH gradient did not differ significantly between the two groups. The enhanced Na(+)-H+ activity in spontaneously hypertensive rats is not due to altered membrane permeability to protons, as is shown by acridine orange-quenching studies. Kinetic studies for amiloride-sensitive Na+ uptake showed a greater Vmax in spontaneously hypertensive rats compared with Wistar-Kyoto rats (1.46 +/- 0.05 vs. 1.08 +/- 0.08 nmol.mg protein-1.7 s-1) but the Km values were similar in the two groups. These finding, along with similar findings previously reported in vascular smooth muscle and renal tissue of SHR, strongly suggest that an increased Na(+)-H+ exchange is related to the development of hypertension.
    Gastroenterology 09/1991; 101(2):430-6. · 12.82 Impact Factor
  • M Daher, S Acra, F K Ghishan
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    ABSTRACT: This study was designed to examine the activity of the Na(+)-H+ exchanger across the basolateral membranes of the ileal enterocyte and its developmental pattern. The function of the Na(+)-H+ exchanger was studied using a well validated basolateral membrane vesicle technique. Na+ uptake represented transport into the vesicle rather than binding as validated by initial rate studies. Na+ uptake represented an electroneutral process as shown by studies in which negative membrane potential was induced by the ionophore valinomycin. Various outwardly directed pH gradients significantly stimulated Na+ uptake compared with no pH gradient conditions at all age groups. However, the magnitude of stimulation was significantly different between the age groups with more marked stimulation of amiloride-sensitive Na+ uptake occurring in adolescent rats as compared to weanling or suckling rats. The amiloride sensitivity of the pH stimulated Na+ uptake was investigated using [Amiloride] = 10(-2)-10(-5) M at pHi/pHo = 5.2/7.5. At 10(-2) M amiloride concentration, Na+ uptake was inhibited by 80%, 70%, 77%, in the basolateral membranes of adolescent, weanling and suckling rats, respectively. Dixon plot analysis in both adolescent and weanling rats was consistent with two amiloride binding sites, a low affinity system and a high affinity system. In the suckling rat, on the other hand, the data supported a single high affinity binding site. Kinetic studies revealed a Km for amiloride-sensitive Na+ uptake of 12.6 +/- 6.6, 10.2 +/- 1.77, 9.46 and Vmax of 4.83 +/- 1.22, 4.47 +/- 0.36 and 8.08 +/- 1.92 n.mol.mg.protein-1.7 s-1 in suckling, weanling and adolescent rats, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)
    Journal of developmental physiology 04/1991; 15(3):175-81.
  • S A Acra, F K Ghishan
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    ABSTRACT: This article provides a brief overview of well-established in vivo and in vitro methods that have contributed the most to the understanding of transport processes across the gastrointestinal epithelium. In vivo perfusion techniques in humans revolve around double- and triple-lumen per oral tubes. In animals, in vivo techniques include the single and recirculation perfusion techniques and the double-isotope technique for measurement of net absorption. In vitro methods of studying intestinal transport include the everted gut sac technique, the Ussing chamber, the use of isolated epithelial cells, and the use of brush border and basolateral membranes isolated from enterocytes. The use of fluorescent probes for the measurement of intracellular ionic concentrations is a new and powerful in vivo technique that is now being applied to the gastrointestinal tract.
    Journal of Parenteral and Enteral Nutrition 01/1991; 15(3):93S-98S. · 2.49 Impact Factor
  • S A Acra, F K Ghishan
    Journal of Pediatric Gastroenterology and Nutrition 06/1990; 10(4):421-5. · 2.20 Impact Factor