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H Velázquez

Publications and source records attributed to H Velázquez.

At least 19 recordsLinked to original sources
PubMed1995

Expression of the thiazide-sensitive Na-Cl cotransporter by rabbit distal convoluted tubule cells.

A thiazide-sensitive Na-Cl cotransporter contributes importantly to mammalian salt homeostasis by mediating Na-Cl transport along the renal distal tubule. Although it has been accepted that thiazide-sensitive Na-Cl cotransport occurs predominantly along the distal convoluted tubule in rats and mice, sites of expression in the rabbit have been controversial. A commonly accepted model of rabbit distal nephron transport pathways identifies the connecting tubule, not the distal convoluted tubule, as the predominant site of thiazide-sensitive Na-Cl cotransport. The thiazide-sensitive Na-Cl cotransporter has been cloned recently. The present experiments were designed to localize sites of thiazide-sensitive Na-Cl cotransporter mRNA expression along the rabbit distal nephron. Nonradioactive in situ hybridization with a thiazide-sensitive Na-Cl cotransporter probe was combined with immunocytochemistry with an antibody that recognizes distal convoluted tubule cells and with a Na+/Ca2+ exchanger antibody that recognizes only connecting tubule cells. The results indicate that thiazide-sensitive Na-Cl cotransporter mRNA is highly expressed by cells of the distal convoluted tubule and not by connecting tubule cells. Segments that stain with the Na+/Ca2+ exchanger antibody (connecting tubules) do not demonstrate thiazide-sensitive Na-Cl cotransporter mRNA expression. We conclude that the predominant site of thiazide-sensitive Na-Cl cotransporter mRNA expression in rabbit distal nephron is the distal convoluted tubule and that sites of mRNA expression of electroneutral Na and Cl transport are similar in rabbits, rats, and mice.

Animals↗
PubMed1993

Identification of a novel K-channel gene (KC22) that is highly expressed in distal tubule of rabbit kidney.

The Shaker gene family encodes voltage-gated K channels. Five partial-length Shaker-like cDNAs (KC2, 4, 10, 19, and 22) were previously isolated from rabbit kidney using polymerase chain reaction (PCR) [G. V. Desir, E. Hamlin, A.H. Puente, R.F. Reilly, F. Hiledebrandt, and P. Igarashi. Am. J. Physiol. 262 (Renal Fluid Electrolyte Physiol. 31): F151-F157, 1992]. We now report the cloning of another Shaker-like cDNA (KC6) from rabbit kidney and the identification of one isoform that is highly expressed in rabbit distal tubule cells grown in culture. A partial-length cDNA (859 bp) for KC6 was isolated by PCR amplification of rabbit kidney cDNA using Shaker-specific degenerate primers. KC6 was most similar to the rat brain clone RBK2 (77% amino acid identity) and to the rabbit clone KC19 (78% amino acid identity). Transcript levels for KC2, 4, 6, 10, 19, and 22 were quantified using the ribonuclease protection assay. Transcripts for all six isoforms were detected in renal tissues. KC22 was the most abundant isoform in kidney cortex and medulla (20- to 40-fold greater than the other isoforms). Furthermore, KC22 expression levels were fivefold higher in primary cultures of rabbit distal convoluted tubules and connecting tubules than in whole kidney cortex. Although the partial-length sequence for KC22 represents the most conserved regions in the Shaker gene family it only has 35-88% amino acid identity with other Shaker channels, suggesting that KC22 represents a novel isoform. In contrast, KC4 and KC19 (less abundant in kidney than KC22) are highly homologous to the rat brain clones RBK1 and RBK2, respectively (97% amino acid identity).(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1993

[Nutritional status of vitamin D in mothers and neonates of Ushuaia and Buenos Aires].

Serum levels of calcium, phosphorus, total alkaline phosphatase (AP) and 25 hydroxyvitamin D (250HD) were measured at the end of the winter in Group 1 (Ushuaia, latitude 55 degrees S): 16 women (24-48 hs postpartum serum blood) and 20 neonates (cord blood) and in Group 2 (Buenos Aires, latitude 34 degrees S) 21 women (24-48 hs postpartum serum blood) and their 21 neonates (cord blood). The neonatal serum calcium and phosphorus were higher and the neonatal serum AP and 250HD level were lower than maternal levels in both groups (Table 1 and 2). Serum levels of 250HD were diminished (< 8 ng/ml) in 62% of the mothers and 81% of the neonates of Ushuaia and in 24% of the mothers and 16% of the neonates of Buenos Aires (figure 1). Neonatal serum 250HD levels correlate with maternal serum 250HD levels in the paired group of Buenos Aires (r = 0.65, p < 0.003) (Figure 2). In Ushuaia the serum 250HD levels (X +/- SD) in neonates (3.9 +/- 2.7 ng/ml) and in mothers (6.3 +/- 4.8 ng/ml) were lower than in Buenos Aires (neonates: 11.3 +/- 6.0 ng/ml and mothers: 14.4 +/- 8.4 ng/ml, p < 0.001). Maternal serum calcium levels were lower in Ushuaia (8.7 +/- 0.8 mg/dl) than in Buenos Aires (9.2 +/- 0.4 mg/dl) (p < 0.05). In conclusion, 1) In Ushuaia pregnant women and their neonates had a deficient nutritional state of vitamin D. Preventive administration of vitamin D would probably be beneficial.(ABSTRACT TRUNCATED AT 250 WORDS)

Adult↗
PubMed1992

Active potassium absorption by the renal distal tubule.

Maintenance of potassium homeostasis during potassium depletion appears to involve an active potassium absorptive mechanism in the distal nephron. Direct demonstration of such a pathway in the distal tubule of the rat has been lacking. The purpose of the current study was to examine the hypothesis that an ATP-dependent active transport mechanism plays a role in potassium absorption by the rat distal tubule. We utilized in vivo microperfusion techniques in Sprague-Dawley rats maintained on a regular diet of low-potassium diet for 3-4 wk. The effect of a selective inhibitor of the gastric H-K-adenosinetriphosphatase (ATPase) (Sch 28080, 0.1 mM) was tested in distal tubules of both groups of rats. Distal tubules of normal rats secreted potassium. Sch 28080 had no effect on this net potassium flux. In contrast, distal tubules of potassium-deficient rats absorbed potassium. Sch 28080 abolished this potassium absorption and produced a small hyperpolarization of the lumen-negative transepithelial voltage (VTE). The change in VTE can be explained by a concomitant increase in potassium concentration in the late distal tubule. These results are consistent with the presence of an H-K-ATPase in the distal tubule of potassium-deficient rats.

Absorption↗
PubMed1992

Luminal influences on potassium secretion: chloride, sodium, and thiazide diuretics.

In the presence of Cl-, K+ secretion by the distal tubule saturates with increasing luminal Na+ concentration. Apparent maximal K+ secretion is attained with luminal Na+ concentrations of 40 mM. The results of the present study show that lowering the Cl- concentration of luminal fluid can increase the level of Na(+)-stimulated K+ secretion beyond the maximal level attained in the presence of Cl-. The effect of lowering luminal Cl- concentration to less than 10 mM on K+ secretion is greater with higher Na+ concentration. Under these conditions, chlorothiazide decreases K+ secretion. When chlorothiazide is present, changing the Na+ concentration does not affect K+ secretion. Because in rats a thiazide effect is attributed primarily to the distal convoluted tubule (DCT), we postulate that it is primarily DCT cells that increase K+ secretion when Na+ concentration is raised in the presence of low luminal Cl- concentration. We propose that the rat DCT cells have both an absorptive Na(+)-Cl- cotransport mechanism and a secretory K(+)-Cl- cotransport mechanism in the luminal membrane that can mediate the apparent exchange of Na+ for K+.

Animals↗
PubMed1991

Effect of Na-channel blockers and lumen Ca on K secretion by rat renal distal tubule.

In previous studies the effectiveness of amiloride (AML) in reducing K secretion has been variable. Based on studies by Cuthbert and Wong (Mol. Pharmacol. 8: 222-229, 1972) in which the Na-channel-blocking action of AML in frog skin was found to require the availability of Ca ions in extracellular fluid, we postulated that the ability of AML and its analogue, benzamil (BZA), to inhibit distal tubule K secretion depends on the presence of Ca in luminal fluid. We found that addition of Ca to a perfusion solution containing 50 microM BZA did reduce K secretion more than BZA alone. Maximal inhibition was observed with 2.5 mM free ionic Ca. Graded increases in luminal Ca in presence of AML or BZA reduced K transport in a dose-dependent manner. The decrease in K secretion with increasing luminal Ca was paralleled by a decrease in transepithelial voltage. These results support our hypothesis that the effectiveness of Na-channel blockers to reduce K secretion by the rat distal tubule depends on presence of luminal Ca and suggest an interaction between luminal Ca and Na-channel blockers on the Na channel.

Amiloride↗
PubMed1991

Adaptation of distal convoluted tubule of rats. II. Effects of chronic thiazide infusion.

Mammalian distal tubules adapt structurally and functionally when NaCl concentration in tubule fluid is altered chronically. These experiments were designed to test the hypothesis that chronic administration of hydrochlorothiazide (HCTZ), a drug that blocks Na and Cl uptake across apical membranes of rat distal tubule cells, would reduce intrinsic transport capacity of distal tubules and reduce the number of thiazide-sensitive transporters. Osmotic pumps were implanted into rats to deliver 3.75 mg/day HCTZ or vehicle for 10-14 days. All animals were offered a solution containing 0.8% NaCl and 0.1% KCl as drinking fluid. Free-flow micropuncture after 10-14 days indicated that Na and Cl delivery to distal tubule was not significantly different in HCTZ- and vehicle-treated animals. Microperfusion in vivo with an artificial interstitial solution, with no thiazide, indicated that 10-14 days of HCTZ infusion did reduce Na transport capacity of distal tubules from 390 +/- 32 to 203 +/- 24 pmol/min (P less than 0.01). In contrast, the number of thiazide-sensitive NaCl transporters, determined as high-affinity receptors for [3H]metolazone in renal cortical membranes, was higher in HCTZ group than in controls (2.2 +/- 0.4 vs. 1.0 +/- 0.1 pmol/mg protein, P less than 0.01). These data support the hypothesis that chronic blockade of NaCl entry across apical membranes of distal tubule cells reduces NaCl transport capacity, an effect that occurs despite an increase in the number of thiazide receptors. They indicate that thiazide receptor binding studies should be interpreted in combination with direct functional measurements.

Adaptation, Physiological↗
PubMed1990

Luminal calcium regulates potassium transport by the renal distal tubule.

We examined the effect of changes in lumen calcium concentration on net potassium transport by distal tubules in anesthetized rats. Tubules were perfused with a control solution that resembled interstitial fluid but lacked calcium. Experimental solutions were prepared by adding varying amounts of CaCl2 to the control solution to produce solutions with free ionic calcium concentration ([Ca2+]) of 0.2, 0.4, and 0.8 mM. In paired comparisons 0.2 mM Ca2+ did not affect net potassium transport, whereas 0.4 and 0.8 mM Ca2+ each reduced potassium secretion by approximately 30%. Unidirectional potassium fluxes using 86Rb as a tracer for potassium and transepithelial voltage (VTE) were measured to characterize further the effect of calcium on potassium transport. Presence of 0.8 mM Ca2+ in the lumen did not affect unidirectional absorptive potassium flux; therefore, the decrease in net potassium flux was accounted for entirely by a decrease in unidirectional secretory potassium flux. The lumen negative VTE measured in the late distal tubule decreased during perfusion with 0.8 mM Ca2+. These results are consistent with the hypothesis that increases in lumen (extracellular) calcium concentration in the range normally present in the distal tubule reduce net potassium secretion by decreasing the electrochemical gradient for potassium secretion.

Animals↗
PubMed1989

Adaptation of the distal convoluted tubule of the rat. Structural and functional effects of dietary salt intake and chronic diuretic infusion.

We studied the effects of dietary NaCl intake on the renal distal tubule by feeding rats high or low NaCl chow or by chronically infusing furosemide. Furosemide-treated animals were offered saline as drinking fluid to replace urinary losses. Effects of naCl intake were evaluated using free-flow micropuncture, in vivo microperfusion, and morphometric techniques. Dietary NaCl restriction did not affect NaCl delivery to the early distal tubule but markedly increased the capacity of the distal convoluted tubule to transport Na and Cl. Chronic furosemide infusion increased NaCl delivery to the early distal tubule and also increased the rates of Na and Cl transport above the rates observed in low NaCl diet rats. When compared with high NaCl intake alone, chronic furosemide infusion with saline ingestion increased the fractional volume of distal convoluted tubule cells by nearly 100%, whereas dietary NaCl restriction had no effect. The results are consistent with the hypotheses that (a) chronic NaCl restriction increases the transport ability of the distal convoluted tubule independent of changes in tubule structure, (b) high rates of ion delivery to the distal nephron cause tubule hypertrophy, and (c) tubule hypertrophy is associated with increases in ion transport capacity. They indicate that the distal tubule adapts functionally and structurally to perturbations in dietary Na and Cl intake.

Adaptation, Physiological↗
PubMed1987

Thiazide-sensitive sodium chloride cotransport in early distal tubule.

At least two pathways mediate sodium absorption across the luminal membrane of the renal distal tubule. One pathway is a conductive channel and the other appears to be a coupled Na-Cl cotransport pathway. The distal tubule comprises three segments: the distal convoluted tubule, the connecting tubule, and the initial collecting duct. To provide information about cellular locations of the proposed sodium transport pathways, we perfused early (14-38% of whole distal length) and late (61-83% of whole distal length) segments of whole distal tubules separately in vivo in anesthetized rats. When perfused with a solution that resembles fluid normally arriving at the distal tubule (75 mM Na, 68 mM Cl), rates of sodium absorption were similar in early and late segments (early 68 +/- 29.6, late 67 +/- 27.5 pmol X min-1 X mm-1). When perfused with a solution that resembles interstitial fluid (148 mM Na, 110 mM Cl), sodium transport was significantly higher in early than in late segments (276 +/- 28.4 vs. 113 +/- 29.7 pmol X min-1 X mm-1). Chlorothiazide (10(-3) M), which blocks sodium and chloride absorption in whole distal tubules, reduced sodium and chloride transport to zero in early distal tubules but had no significant effect in late distal tubules. Removing all chloride from perfusion solutions reduced sodium transport in early but not late distal segments.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1987

Chloride-dependent potassium secretion in early and late renal distal tubules.

Potassium transport by subsegments of the rat surface distal tubule was studied using a modified in vivo microperfusion method. The nephron segments between 14 and 38% and between 62 and 83% of total distal length distance between macula densa region and confluence of tubule with another) were perfused separately. The first of these two segments is composed primarily of distal convoluted tubule (DCT) cells; the more distal segment is made up primarily by initial collecting tubule (ICT) epithelium. Experiments were performed to measure potassium secretion via two pathways: a diffusion mechanism driven by a favorable electrochemical gradient for potassium, and a cotransport mechanism activated when lumen chloride concentration is low. In a first series of experiments, both the DCT and the ICT secreted potassium when perfused with an artificial control solution resembling fluid normally present at the beginning of the distal tubule. Absolute rates of potassium secretion were higher in the ICT than in the DCT. Decreasing lumen Cl concentration stimulated potassium secretion more in the ICT than in the DCT. In a second series of experiments, the subsegments were perfused with a solution in which ion concentrations were raised to levels found in interstitial fluid. Under these circumstances, potassium secretion was lower in both segments. Decreasing lumen Cl concentration resulted in higher rates of potassium secretion in the DCT than those seen in the first series with low chloride; rates of potassium secretion in the ICT were as high as in the first series.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1987

Mechanisms of sodium, potassium and chloride transport by the renal distal tubule.

The distal tubule contributes importantly to renal sodium and chloride reabsorption and potassium secretion. Changes in various factors acting from the luminal or basolateral side of the distal tubule can alter rates of net ion transport. Experiments were designed to examine some of the factors of luminal fluid that can alter sodium, chloride and potassium transport rates. This was accomplished by employing in vivo microperfusion techniques to change luminal sodium, potassium and chloride concentrations and fluid flow rate independent of any effects on systemic homeostatic mechanisms. Results of these experiments indicate that increasing the rate of fluid flow in the distal tubule, within the physiologic range, stimulates potassium secretion, even when sodium concentration does not change. Low sodium concentrations inhibit net potassium secretion only when they are below those that have been measured in this segment under physiologic conditions. Potassium secretion rates increase significantly when the luminal chloride concentration is reduced, an effect that can be dissociated from changes in transepithelial voltage. Sodium absorption is limited by low luminal chloride concentration and chloride absorption is limited by low luminal sodium concentration. These results and others have led us to postulate that at least four different pathways mediate Na, K and Cl transport across the luminal membrane of distal tubule cells. Two pathways permit diffusive movement of cations, one an amiloride inhibitable channel for sodium, the other a barium inhibitable channel for potassium. Two other pathways appear to mediate coupled K-Cl secretion and Na-Cl absorption across the apical membrane of distal tubule cells. A new model of apical membrane transport pathways in the distal tubule is presented.

Absorption↗
PubMed1987

Thiazide diuretics.

Thiazide diuretics increase salt and water excretion primarily by inhibiting mechanisms for electroneutral sodium and chloride transport by distal convoluted tubule cells. This might be termed the 'specific' effect of this class of diuretics and accounts for the 'chlorouretic' effectiveness of the drug. Secondary to this inhibition of sodium and chloride absorption, potassium secretion is stimulated most likely because of the resultant increase in distal tubule fluid flow rate, and calcium absorption is stimulated possibly via a decrease in distal convoluted tubule cell sodium activity and an increase in basolateral sodium/calcium exchange. To a varying degree, thiazides also inhibit carbonic anhydrase. This effect can contribute to the diuresis, but is largely buffered by the reserve transport capacity of the loop of Henle. To the extent that the effects of transport inhibition in the proximal tubule are transmitted to the distal tubule, tubuloglomerular feedback may be activated and effect a reduction in the glomerular filtration rate.

Animals↗
PubMed1986

Control by drugs of renal potassium handling.

This review has focused on the influence of several diuretic drugs on potassium handling by the kidney. One class of drugs (loop diuretics) acts by directly inhibiting a potassium absorptive mechanism in the luminal membrane of cells of the thick ascending limb of Henle's loop. Two other groups of diuretics affect potassium transport indirectly by inhibiting salt and water absorption upstream from the potassium secretory site in the late distal tubule: carbonic anhydrase inhibitors act in the proximal tubule; thiazides act in the early distal tubule. The subsequent increase in lumen flow rate then stimulates net potassium secretion by the distal tubule. A fourth class of drugs (spironolactone) acts by antagonizing the response of the distal tubule to aldosterone. These drugs decrease the ability of aldosterone to stimulate distal potassium secretion. Finally, a fifth group of drugs (potassium-sparing diuretics) decreases potassium secretion by increasing the luminal membrane voltage and thus decreasing the electrochemical gradient for potassium exit from the cell.

Acetazolamide↗
PubMed1986

Unidirectional potassium fluxes in renal distal tubule: effects of chloride and barium.

Low luminal concentrations of chloride stimulate net potassium secretion by the renal distal tubule, independent of changes in transepithelial voltage. These effects are not prevented by the luminal application of the potassium channel blocking agent barium. Because net potassium secretion comprises secretory and absorptive components, we sought to evaluate the effects of chloride and barium on unidirectional potassium fluxes in the renal distal tubule. In vivo microperfusion methods were used in anesthetized Sprague-Dawley rats. Perfusion solutions contained either 42K or 86Rb as tracers for potassium. Tracer efflux coefficients, indicating apparent potassium permeability, were similar when measured using either isotope. Net potassium flux was determined as the difference between perfusion and collected rate, and unidirectional absorptive potassium flux was calculated as the product of the mean luminal potassium concentration and the tracer efflux coefficient. During perfusion with a solution that resembled fluid normally arriving at the early distal tubule, the absorptive potassium flux was approximately 25% of the unidirectional secretory flux. Reducing lumen chloride concentration increased net potassium secretion, because blood-to-lumen potassium flux increased from 61 +/- 12.7 to 96 +/- 14.6 pmol/min. Barium reduced both absorptive and secretory fluxes but did not prevent the stimulation of net potassium secretion that occurs when luminal chloride concentration is reduced. Apparent potassium permeability during perfusion with a solution that resembled fluid normally arriving at the early distal tubule was 800 nm/s when corrected for voltage. Together with the results of previous experiments, these results are consistent with the presence of a secretory pathway linking potassium with chloride in the luminal membrane of cells of the distal tubule.

Animals↗