Effects of SITS on urate transport by isolated,perfused snake renal tubules

Effects of an inhibitor of membrane anion-exchange transport processes, 4-acetamido-4-isothiocyano-2,2-disulfonic stilbene (SITS), on urate transport by isolated, perfused snake (Thamnophis spp.) proximal renal tubules were studied. SITS (10^–4 mol/l) in the luminal perfusate had absolutely no effect on net urate secretion (J _urate ^net ) or on net fluid absorption (J _v). This observation is compatible with other data that give no support to the concept of a mediated transport step for urate from the cells to the lumen. SITS (10^–4 mol/l) in the bathing medium reversibly inhibitedJ _urate ^net without affectingJ _v. At the time of maximum inhibition ofJ _urate ^net , the concentration of urate in the cell water was increased and the apparent permeability of the luminal membrane to urate was decreased, but the urate efflux across the peritubular membrane and the apparent permeability of the peritubular membrane to urate were unchanged. There was no evidence of significant intracellular binding or trapping of urate. Although an increase in the initial rate of urate transport into the cells across the peritubular membrane could not be demonstrated conclusively in nonperfused tubules, the results still suggest that SITS in the bathing medium may inhibitJ _urate ^net by inhibiting urate movement from the cells to the lumen while actually enhancing transport from the bathing medium into the cells.     

Tetraethylammonium transport by snake renal brush-border membrane vesicles

Transport of tetraethylammonium (TEA) by snake (Thamnophis spp.) renal brush-border membrane vesicles (BBMV) was studied. An outwardly directed proton gradient (pH 6.0 in, pH 7.5 out) stimulated uptake of TEA into BBMV and supported concentrative accumulation. Uptake of radioactively labeled TEA was also stimulated by outwardly directed gradients of unlabeled TEA and choline. The initial rate of TEA uptake was a saturable process that was adequately described by Michaelis-Menten kinetics. TEA uptake was not influenced by changes in the electrical potential difference across the membranes. Although uptake of TEA was stimulated by an outwardly directed Na⁺ gradient and inhibited by an inwardly directed Na⁺ gradient, these effects were probably secondary to the generation of proton gradients via a Na⁺/H⁺ exchanger demonstrated in these same BBMV. In agreement with previous studies with intact snake renal tubules, the present results indicate that TEA transport across the brush-border membrane involves electroneutral countertransport for protons or organic cations.     

Ca2+ and Ba2+ effects on basolateral tetraethylammonium transport in isolated snake renal proximal tubules

 Previous work on snake renal tubules suggested that basolateral transport of tetraethylammonium (TEA) is symmetrical. To examine regulation of this transport step more closely, we determined the effects of (1) reductions in the extracellular Ca²⁺ concentration and basolateral Ca²⁺ entry, and (2) the presence of extracellular Ba²⁺ on TEA uptake and efflux across the basolateral membrane of isolated snake renal proximal tubules. Removal of extracellular Ca²⁺ reduced initial TEA uptake and enhanced TEA efflux. Blocking Ca²⁺ entry also reduced initial TEA uptake. Extracellular Ba²⁺ depolarized the basolateral membrane and reduced both TEA uptake and efflux. Inhibition of basolateral TEA uptake with a reduced membrane potential supports previous data indicating that uptake involves potential-driven facilitated diffusion. Inhibition of basolateral TEA efflux by Ba²⁺ even with a reduced membrane potential not only supports previously obtained data indicating that efflux is not influenced by the potential difference and that basolateral TEA transport is asymmetrical, but also suggests that TEA uptake and efflux may occur by separate pathways. Received: 22 May 1997 / Received after revision: 24 July 1997 / Accepted: 28 July 1997     

Effects of some steroid hormones on Ca2+ transport and oxidative metabolism of isolated mitochondria

Effects of prednisolone, estradiol, and testosterone on the transport of Ca²⁺ and the respiration induced by it in the heart and liver mitochondria of rats were studied. Prednisolone and testosterone were found to reduce the Ca-accumulating capacity of the mitochondria, the rates of ion entry and exit, and the rate of Ca²⁺-induced respiration. Estradiol, while inhibiting Ca²⁺ transport across mitochondrial membrane, did not influence the respiration in the phase of Ca²⁺ absorption, but accelerated it in the phase of ion exit. These data suggest that due to their lipophilic properties, the steroids become incorporated in the mitochondrial membrane, thereby changing its viscosity and permeability and limiting the mobility of transmitter proteins. Translated fromByulleten' Eksperimental'noi Biologii i Meditsiny, Vol. 118, N ^o 12, pp. 616–618, December, 1994     

Properties of two calcium transport systems of isolated rat ileal epithelial cells: effects of Ca2+ channel modulators and membrane potential examined with fluorescent dye,fura-2

Calcium transport systems of isolated ileal epithelial cells were investigated. The concentration of cytosolic free calcium ions, [Ca²⁺]_i, was monitored with a fluorescent Ca²⁺ dye, fura-2. The fluorescence intensity ratio (I ₃₄₀/I ₃₈₀) was used as an index of [Ca²⁺]_i. [Ca²⁺]_i of the cells suspended in the nominally Ca²⁺-free solution was estimated at 52±3 nM. Ca²⁺ uptake was followed for as long as 5 min in the presence of 100–1000 M added CaCl₂. Most of the experiments were performed at 200 M CaCl₂. The Ca²⁺ uptake was abolished by 0.8 mM Ni²⁺ and 50 M Mn²⁺ and partitally antagonized by 50 M verapamil and 50 M diltiazem but not affected by 20 M nifedipine. The Ca²⁺ entry was reduced by increasing concentrations of extracellular K⁺ in the presence of valinomycin, suggesting a voltage-dependent nature of the uptake. On the other hand, the Ca²⁺ transport doubled in the presence of Bay K8644 (8 M), a Ca²⁺ channel agonist. The Bay-K-8644-induced uptake was inhibited by either 10 M nifedipine, 10 M verapamil or 10 M diltiazem and was relatively independent of extracellular K⁺ concentration. These results suggest that there are at least two distinct Ca²⁺ transport systems in the rat ileal epithelial cells, one resistant to organic Ca²⁺ channel blockers but relatively sensitive to membrane potential (basal uptake) and another inducible by Bay K 8644 and sensitive to the channel blockers but relatively independent of membrane potential.     

Effects of parathyroid hormone and N6,O2′-dibutyryl cyclic AMP on Ca2+ transport across the rabbit distal nephron segments perfused in vitro

Effects on Ca²⁺ transport of parathyroid hormone (PTH) and N⁶,O²-dibutyryl adenosine 3,5-cyclic monophosphate (DB-cAMP) were examined in the rabbit distal nephron segments including the cortical thick ascending limb of Henle's loop (CAL), the connecting tubule (CNT) and the cortical collecting tubule (CCT) by the in vitro perfusion technique. When PTH (10^–8 mol·l^–1) was added to the bath, efflux of Ca²⁺ (pmol·mm^–1·min^–1) was increased from 6.29±1.46 to 7.96±1.66 (P<0.02) in the CAL, and from 8.55±1.30 to 13.73±1.24 (P<0.001) in the CNT, respectively, without changes in influx of Ca²⁺. The effect of PTH on Ca²⁺ transport in the CAL, however, was abolished when phosphate concentration in the medium was reduced from 3.0 to 1.0 mmol·l^–1. When DB-cAMP (10^–3 mol·l^–1) was added to the bath, efflux of Ca²⁺ was also increased from 7.01±0.83 to 9.40±0.82, (P<0.05) in the CAL, and from 13.11±0.89 to 19.74±0.52 (P<0.005) in the CNT, respectively. By contrast, neither PTH nor DB-cAMP affected efflux of Ca²⁺ in the CCT. PTH did not affect the transepithelial voltage either in the CAL or in the CNT. But in the CNT, DB-cAMP decreased the voltage from –14.1 to –9.4 mV. The response of adenylate cyclase activity to PTH in the collagenase treated isolated nephron segments was also examined. Significant increases in adenylase cyclase activity were observed in the CAL as well as in the CNT with 10^–6 mol·l^–1 PTH. These data indicate that PTH stimulates Ca²⁺ transport across the CNT probably via activation of the adenylate cyclase-cyclic AMP system. The hormone may also stimulate Ca²⁺ transport across the CAL in a special condition where plasma phosphate concentration is elevated.