Biomedical subjects

M G Brunette

Publications and source records attributed to M G Brunette.

At least 37 records · Page 2Linked to original sources

PubMed1994

Two vitamin D3-dependent calcium binding proteins increase calcium reabsorption by different mechanisms. II. Effect of CaBP 9K.

The distal tubule is a major site of hormone regulated Ca2+ reabsorption. We have previously shown that in the rabbit, vitamin D depletion decreases both the Ca2+ uptake by the luminal membrane of nephron distal segments and the ATP-dependent Ca2+ transport by the basolateral membrane of these same segments. We also reported that in in vitro experiments, the vitamin D-dependent calcium binding protein (CaBP) 28K enhances Ca2+ uptake by the high-affinity Ca2+ transport system of the distal luminal membranes. The purpose of the present study was to investigate the role of CaBP 9K in Ca2+ transport by the two polar membranes of the proximal and distal nephron segments. Preloading the luminal membranes of the two series of tubules with 10 microM CaBP 9K did not modify 0.5 mM Ca2+ uptake by these membranes. In contrast, preincubation of distal tubule basolateral membranes with 10 microM CaBP 9K strongly enhanced ATP-dependent Ca2+ transport by these membranes. A similar although more modest effect was observed in proximal basolateral membranes. This effect was present only in calmodulin depleted membranes. Calmodulin also increased the ATP-dependent Ca2+ uptake by the depleted membranes. The two actions were not additive. A dose-response curve showed that the maximal CaBP 9K effect was obtained at 10(-5) M. CaBP 9K increased the Vmax of ATP-dependent uptake from 0.43 +/- 0.03 to 0.79 +/- 0.06 nmol/mg/10 sec (P < 0.005), leaving the Km unchanged. It is concluded that CaBP 9K activates the Ca2+ pump in the basolateral membrane of both the proximal and distal tubule.(ABSTRACT TRUNCATED AT 250 WORDS)

Absorption↗

PubMed1994

An unusual case of an aldosterone-producing adenoma in a prepubertal girl with severe post-surgical adrenal suppression.

An 11-year-old girl was referred with hypertension, hypokalemia, low renin activity, metabolic alkalosis and hyperaldosteronism. Adrenal computed tomography revealed the presence of a large left adrenal adenoma of 4 x 4.5 cm in size. Following adrenalectomy, the child presented an unusual acute salt-loosing syndrome which necessitated administration of a large quantity of sodium chloride and corticoids. The cells from the tumor were plated. They rapidly proliferated to a monolayer. After incubation for 24 h, basal production of aldosterone (55.4 +/- 9.07 pmol/ml) was observed. This production was stimulated by the presence of ACTH 10(-8) M and KCl 55 mM; angiotensin II 10(-8) M failed to enhance aldosterone secretion. In the four experimental conditions, however (control, ACTH, angiotensin II and KCl), ANF 10(-8) M decreased aldosterone secretion. We conclude that hyperaldosterone secretion by an adrenal adenoma is not due to resistance to the inhibitory effect of ANF but rather to cell multiplication itself. In this patient, the post-surgical salt-loosing syndrome was attributed to the combined effect of the chronic state of high ANF, low potassium and low ACTH in the contralateral gland on the one hand and a certain degree of transitory renal unresponsiveness of the renal distal tubule to aldosterone on the other.

Adenoma↗

PubMed1994

Parathyroid hormone and hydrochlorothiazide increase calcium transport by the luminal membrane of rabbit distal nephron segments through different pathways.

The present study was designed to investigate the effect of PTH on calcium (Ca2+) transport through the luminal membrane of proximal and distal rabbit tubule segments. Proximal tubule and distal tubule segment suspensions were incubated with the hormone, and the luminal membranes were subsequently purified. Incubation with 10(-8) M human PTH(1-34) strongly increased initial Ca2+ uptake by the distal membranes. The effect of PTH was dose dependent, with an apparent ED50 of 8.2 +/- 1.0 nM. We recently reported the presence of two kinetics of Ca2+ uptake by the distal luminal membranes. PTH affected exclusively the high affinity component, increasing the maximum velocity from 0.31 +/- 0.02 to 0.76 +/- 0.07 pmol/micrograms.10 sec (P < 0.001), and leaving the Michaelis-Menten constant Ca2+ unchanged. The addition of 500 microM hydrochlorothiazide (HCTZ) to the luminal membranes of distal tubules incubated with PTH further enhanced Ca2+ uptake. The effect of HCTZ was on the low affinity system. HCTZ (100 microM) enhanced the maximum velocity from 2.5 +/- 0.3 to 3.7 +/- 0.6 pmol/micrograms protein.10 sec (P < 0.01) without affecting the Michaelis-Menten constant. Whereas 1 microM nitrendipine alone did not affect Ca2+ transport by the distal tubule luminal membranes, the Ca2+ channel inhibitor completely abolished the effect of PTH. Conversely, 1 microM Bay K 8644 increased Ca2+ uptake by membranes from PTH-treated distal tubules but was ineffective in membranes from control tubules. Neither PTH nor nitrendipine nor Bay K 8644 had any effect on the luminal membranes from proximal tubules. These results suggest that: 1) the high affinity, low velocity Ca2+ transport system in the luminal membrane from distal cortical segments is sensitive to PTH; 2) the effects of nitrendipine and Bay K 8644 on Ca2+ uptake were observed only in membranes from tubules incubated with PTH; 3) this uptake is distinct from the thiazide-sensitive Ca2+ transport system; and 4) PTH does not influence Ca2+ transport by the luminal membrane of proximal tubules.

3-Pyridinecarboxylic acid, 1,4-dihydro-2,6-dimethy↗

PubMed1993

Effect of (1-34) parathyroid hormone-related peptide on the composition and turnover of phospholipids in syncytiotrophoblast brush border and basal plasma membranes of human placenta.

The effect of parathyroid hormone-related peptide on the lipid composition and the turnover of phosphoinositides was studied in brush border and basal plasma membranes of human placenta syncytiotrophoblasts. Lipid composition of the two polar membranes differed markedly with respect to the cholesterol/phospholipid ratio (0.57 +/- 0.04 and 0.91 +/- 0.05 in basal plasma membranes and brush border membranes, respectively). Sphingomyelin was the major phospholipid in both membranes. Except for the phosphoinositide-phosphatidylserine complex which was higher in basal plasma membranes, the phospholipid composition was comparable in the brush border membrane and basal plasma membranes. Incubation of the tissue with 10(-8) M parathyroid hormone-related peptide (1-34) resulted in a significant increase in the phosphatidylinositol phosphate content of the two membranes and in the phosphatidylinositol biphosphate concentration in the basal plasma membranes. Finally, when the tissue was preincubated with [3H]myo-inositol in the presence of 10(-8) M parathyroid hormone-related peptide (1-34), the hormone significantly stimulated the inositol phosphate release by the two membranes. These results demonstrate that: (1) in the placental syncytiotrophoblast, as found in other transport epithelia, the lipid composition of the polar membranes is different; (2) parathyroid hormone-related peptide stimulates the phosphoinositide turnover in both membranes.

Cell Membrane↗

PubMed1993

Effect of vitamin D depletion on calcium transport by the luminal and basolateral membranes of the proximal and distal nephrons.

To study the effect of vitamin D on calcium (Ca2+) reabsorption by the kidney, we measured Ca2+ uptake by the basolateral and luminal membranes of proximal and distal tubules obtained from rabbits fed a vitamin D-deficient diet for 3 weeks. Results were compared to those obtained with a group of control animals fed a normal diet. Serum Ca2+ concentrations were comparable in both groups. In the control group, serum PTH, 25-hydroxyvitamin D3, and 1,25-dihydroxyvitamin D3 remained relatively stable. In the vitamin D-deficient animals, serum PTH levels slightly, but not significantly, increased, and the levels of vitamin D metabolites abruptly fell. Vitamin D depletion produced a 40% decrease in ATP-dependent Ca2+ uptake by the basolateral membrane of the distal tubule. There was no change in the activity of the Na+/Ca2+ exchanger. A very significant effect was also observed in the luminal membrane of the distal tubule, where a 50% decrease in Ca2+ uptake was observed after the third week of vitamin depletion. Administration of 0.1 microgram 1,25-dihydroxyvitamin D3 16 and 2 h before death partially reestablished normal uptake. In contrast, no change in Ca2+ uptake could be detected in the basolateral or luminal membranes of the proximal tubule. These observations provide the first evidence of an effect of vitamin D on Ca2+ transport at both the basolateral and luminal membranes of the distal segment of the nephron.

Adenosine Triphosphate↗

PubMed1992

Calcium transport through the luminal membrane of the distal tubule. I. Interrelationship with sodium.

Calcium (Ca2+) transport by isolated luminal membranes from rabbit renal distal tubule has been characterized. Ca2+ uptake by these membrane vesicles exhibited saturation kinetics. In the absence of sodium (Na+) in the incubation medium, a low affinity system was observed with a KmCa2+ of 2.83 +/- 0.64 mM and Vmax of 3.03 +/- 0.48 pmol/microgram/10 sec. A second type of kinetics was also detected with a high affinity and a low velocity (KmCa2+ 0.04 +/- 0.01 mM, Vmax 1.18 +/- 0.22 pmol/micrograms/10 sec). The luminal membranes from proximal tubules showed a single system with a KmCa2+ of 0.49 +/- 0.20 mM and Vmax of 1.26 +/- 0.17 pmol/micrograms/10 sec. The presence of Na+ sharply decreased Ca2+ uptake by the high affinity system of the membranes from distal tubules, increasing the KmCa2+ to 0.07 mM +/- 0.01 (P less than 0.01) and decreasing the Vmax to 0.27 pmol/microgram/10 sec (P less than 0.005). This effect of Na+ was concentration-dependent, with a half-maximal effect at 38 mM Na+ and a Hill coefficient of 0.9. In contrast, Na+ had no effect on Ca2+ transport through the luminal membranes of proximal tubules nor on the low affinity system of the distal tubule. The composition of the intravascular medium also influenced Ca2+ uptake by the membranes from distal tubules. Compared to mannitol, trans-Na+ or K+ significantly reduced Ca2+ transport. Finally, cis-K+ induced an increase in this transport. As found with Na+, the effect of K+ was concentration-dependent, with a Hill coefficient of 0.42.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗

PubMed1992

Calcium transport by the luminal membrane of distal tubule: II. Effect of pH, electrical potential and calcium channel inhibitors.

We studied the effect of pH, electrical potential and calcium channel inhibitors on calcium (Ca2+) uptake by the luminal membranes of distal nephron from rabbit kidney. Ca2+ uptake was measured using 45Ca and the rapid filtration technique. Uptake by the luminal membranes prepared from distal tubule suspensions were compared to the corresponding values obtained with membranes from proximal tubules. In the distal tubule experiments, Ca2+ transport was measured in the presence and the absence of Na+ in the incubation medium. As previously reported, Na+ inhibited Ca2+ uptake by the distal membrane vesicles. Increasing the pH of either the extravesicular or intravesicular media, or both, enhanced Ca2+ uptake by the distal membrane vesicles. We previously described two kinetics of Ca2+ transport by the luminal membrane of the distal nephron, with high and low affinities, respectively. In both the presence and absence of Na+, alkaline pH stimulated the low affinity-high velocity system by decreasing the Km Ca2+. In the presence of Na+, alkaline pH also increased the Vmax Ca2+ of the high affinity system with no significant changes in Km. These effects of pH were not related to a H+/Ca2+ exchange mechanism. In contrast, pH did not significantly influence Ca2+ transport through the proximal membrane vesicles. When an electrical potential gradient (inside negative) across the vesicles was created by either various anion gradients through the membranes, or by inducing K+ efflux with valinomycin, no relation could be detected between the electrical gradient and 0.5 mM Ca2+ uptake by the proximal or distal membranes.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗

PubMed1992

Ca2+ transport through the brush border membrane of human placenta syncytiotrophoblasts.

The calcium (Ca2+) uptake by brush border membrane vesicles isolated from fresh human placentas has been characterized. This process was saturable and time- and concentration-dependent. It exhibited a double Michaelis-Menten kinetics, with apparent Km values of 0.17 +/- 0.03 and 2.98 +/- 0.17 mM Ca2+, and Vmax values of 0.9 +/- 0.13 and 2.51 +/- 0.45 pmol.micrograms-1.5 s-1. It was not influenced by the presence of Na+ or Mg2+ in the incubation medium. It was not increased by K+ or anion diffusion potentials, inside negative. At a steady state of 1 mM Ca2+ uptake, a large proportion (approximately 94%) of the Ca2+ was bound to the internal surface of the membranes. Preincubation of these membrane vesicles with voltage-dependent Ca2+ channel blockers (nifedipine and verapamil) had no influence on Ca2+ uptake. However, this uptake was very sensitive to pH. In the absence of a pH gradient, the Ca2+ uptake increased with alkalinity. When the intravesicular pH was kept constant while the pH of the incubation medium was increased, Ca2+ uptake was also stimulated by alkaline pH. In contrast, when the pH of the incubation medium was kept constant and the intravesicular pH was progressively increased, Ca2+ uptake was diminished with alkaline pH. Therefore, H+ gradient (H+ in trans-position greater than H+ in cis-position) favored Ca2+ transport, suggesting a H+/Ca2+ exchange mechanism. Finally, in contrast to the basal plasma membrane, the brush border membrane did not show any ATP-dependent Ca2+ transport activity.

Adenosine Triphosphate↗

PubMed1991

Characterization of calcium transport by basal plasma membranes from human placental syncytiotrophoblast.

We have studied the mechanisms involved in calcium (Ca2+) transport through the basal plasma membranes (BPM) of the syncytiotrophoblast cells from full-term human placenta. These purified membranes were enriched 25-fold in Na+/K(+)-adenosine triphosphate (ATPase), 37-fold in [3H] dihydroalprenolol binding sites, and fivefold in alkaline phosphatase activity compared with the placenta homogenates. In the absence of ATP and Mg2+, a basal Ca2+ uptake was observed, which followed Michaelis-Menten kinetics, with a Km Ca2+ of 0.18 +/- 0.05 microM and Vmax of 0.93 +/- 0.11 nmol/mg/min. The addition of Mg2+ to the incubation medium significantly decreased this uptake in a concentration-dependent manner, with a maximal inhibition at 3 mM Mg2+ and above. The Lineweaver-Burk plots of Ca2+ uptake in the absence and in the presence of 1 mM Mg2+ suggest a noncompetitive type of inhibition. Preloading the BPM vesicles with 5 mM Mg2+ had no significant effect on Ca2+ uptake, eliminating the hypothesis of a Ca2+/Mg2+ exchange mechanism. This ATP-independent Ca2+ uptake was not sensitive to 10(-6) M nitrendipine nor to 10(-4) M verapamil. An ATP-dependent Ca2+ transport was also detected in these BPM, whose Km Ca2+ was 0.09 +/- 0.02 microM and Vmax 3.4 +/- 0.2 nmoles/mg/3 min. This Ca2+ transport requires Mg2+, the optimal concentration of Mg2+ being approximately 1 mM. Preincubation of the membrane with 10(-6) M calmodulin strongly enhanced the initial ATP-dependent Ca2+ uptake. Finally, no Na+/Ca2+ exchange process could be demonstrated.

Adenosine Triphosphate↗

PubMed1991

The hypocalciuric effect of thiazides: subcellular localization of the action.

The acute administration of thiazides results in a decrease in the urinary Ca2+/Na+ ratio, whereas chronic administration of these diuretics decreases calciuria. In both situations, Ca2+ transport is enhanced in the early part of the distal tubule. The purpose of our study was to determine whether the hypocalciuric action of thiazides was due to a change in the active transport of Ca2+ through the basolateral membrane of the nephron or to an effect (direct or indirect) on the permeability of the distal tubule luminal membrane to calcium. In order to detect intrinsic differences between membranes of the proximal and distal tubules, the effect of the diuretic was examined in proximal and distal tubule preparations, and in basolateral and luminal membranes from the two segments separately. Preincubation of microdissected distal tubules in hypotonic solution containing 500 microM hydrochlorothiazide (HCTZ) did not influence the Ca2(+)-dependent ATP hydrolysis (Ca2+ = 1 microM) nor the Mg2(+)-dependent ATP hydrolysis (Mg2+ = 100 microM). Similarly 100 microM HCTZ did not change the Ca2+ ATPase activity in intact proximal and distal tubule suspensions, at Ca2+ concentrations ranging from 0.05 microM to 1 microM. ATP-dependent Ca2+ transport was present in basolateral membrane vesicles from proximal and distal tubule suspensions. Preincubation of the membranes with 100 microM HCTZ did not influence this transport. A Na+/Ca2+ exchanger, present in the basolateral membranes from the distal tubule, was also insensitive to HCTZ. In contrast, preincubation of luminal membranes from the distal tubules (but not proximal tubules) with 500 microM HCTZ significantly increased the Ca2+ uptake by these membranes.(ABSTRACT TRUNCATED AT 250 WORDS)

Adenosine Triphosphate↗

PubMed1991

The mechanism of parathyroid hormone action on calcium reabsorption by the distal tubule.

PTH increases calcium reabsorption exclusively in the distal nephron. Two mechanisms of Ca++ transport through the basolateral membrane (BLM) have been described: the ATP-dependent and the sodium gradient-dependent transport. In the present study, we investigated the effect of PTH and (Bu)2cAMP on these two mechanisms. We recently reported, using 100 microM Ca++ as the substrate, that whereas the ATP-dependent system was present in the proximal and distal tubule (DCT), the Na+/Ca++ exchanger was located only in the DCT. Using 2 microM Ca++ as the substrate, the Na+/Ca++ exchanger was again found to be present only in the DCT. Incubation of DCT suspension with 10(-8) M bovine PTH (1-34) resulted in a significant increase in the Na(+)-dependent Ca++ uptake by the corresponding BLM vesicles. This effect was dose dependent. The half-maximal stimulation was obtained with 1 X 10(-8) M PTH. At this concentration, PTH increased the maximum velocity (Vmax) from 0.34 +/- 0.06 t 0.54 +/- 0.02 nmol/mg/10 s (P less than 0.05) without influencing the Michaelis-Menten constant (Kms). Incubation of DCT suspensions with (Bu)2cAMP mimicked this effect. The dose-response curve showed a peak action at 1 mM (Bu)2cAMP. In contrast, neither (Bu)2cAMP nor PTH influenced the ATP-dependent Ca++ transport through BLM from proximal tubule nor DCT. It is proposed that PTH influences Ca++ reabsorption by the DCT because the target molecule, the Na+/Ca++ exchanger, is located exclusively at this site.

Animals↗

PubMed1990

Effect of insulin on D-glucose transport by human placental brush border membranes.

We studied the effect of insulin on the uptake of D-glucose by human placental brush border membranes (BBM) in vitro. D-glucose transport through placental BBM is a Na(+)-independent transport, inhibited by 0.5 mM phloretin. Increasing the substrate concentration from 1 to 50 mM resulted in an increase in glucose uptake according to an S-shaped relationship. Hill plot analysis suggests that at least two molecules of D-glucose are transported at the same time by the carrier. Preincubation of the placental tissue with insulin for 45 min at 22 degrees C significantly enhanced the D-glucose influx into the membrane vesicles, without influencing the slope of the Hill plot. A dose-response curve of the effect of insulin revealed that although the effect was already significant at 10(-9) M, the maximal activity was reached at 10(-8) M. The influence of insulin on D-glucose uptake was present only when preincubation of the placental tissue with the hormone was performed in the presence of Mn2+. Incubation of placental tissue with 10(-8) M insulin did not influence D-glucose efflux from the BBM vesicles. Finally, direct incubation of the membranes with insulin had no effect on the glucose influx into these membrane vesicles. We conclude that insulin, at physiological concentrations, enhances glucose uptake by the BBM, and that such a regulation might contribute to the glucose homeostasis in the fetal circulation, independent of the maternal variations in glycemia.

Biological Transport↗

PubMed1990

Characterization of the high and low affinity components of the renal Ca2(+)-Mg2+ ATPase.

The purpose of this study was to characterize the interrelationship between free calcium (Ca2+) and magnesium (Mg2+) in the Ca2+ ATPase enzyme cycle of kidney membranes. Experiments were performed with basolateral membranes from rat renal cortex and microdissected proximal and distal tubules from mice. Results were similar in the three types of preparations. We first investigated the effect of ATP concentration on Ca2(+)- and Mg2(+)-dependent ATP hydrolysis. With 0.2 microM Ca2+, the enzyme activity, as a function of ATP concentration, showed two saturable components: a high affinity component with a Km of 33 microM ATP and a low affinity component with a Km of 0.63 mM ATP. These components may represent either two distinct sites of ATP binding or two forms of the same site. For the sake of simplicity, it was assumed that the two components correspond to a high affinity and a low affinity substrate site. At the high affinity site (ATP = 50 microM), the Ca2+ dependence of ATP hydrolysis followed a single Michaelis-Menten kinetics with Km for Ca2+ of 0.08 microM. The addition of 1 mM Mg2+ resulted in a relatively constant increase in ATP hydrolysis at all Ca2+ concentrations, indicating that the effects of the two cations were additive. With high ATP concentration (ATP = 3 mM), Ca2+ also induced an ATP hydrolysis according to a saturable process, with a Km for Ca2+ of 0.2 microM. In contrast with what occurred with low concentrations of ATP, addition of millimolar Mg2+ completely curtailed the sensitivity of the enzyme to Ca2+.(ABSTRACT TRUNCATED AT 250 WORDS)

Adenosine Triphosphate↗

PubMed1989

The renal Na+/Ca2+ exchange system is located exclusively in the distal tubule.

The movement of Ca2+ across the basolateral plasma membrane was determined in purified preparations of this membrane isolated from rabbit proximal and distal convoluted tubules. The ATP-dependent Ca2+ uptake was present in basolateral membranes from both these tubular segments, but the activity was higher in the distal tubules. A very active Na+/Ca2+ exchange system was also demonstrated in the distal-tubular membranes, but in proximal-tubular membranes this exchange system was not demonstrable. The presence of Na+ outside the vesicles gradually inhibited the ATP-dependent Ca2+ uptake in the distal-tubular-membrane preparations, but remained without effect in those from the proximal tubules. The activity of the Na+/Ca2+ exchange system in the distal-tubular membranes was a function of the imposed Na+ gradient. These results suggest that the major differences in the characteristics of Ca2+ transport in the proximal and in the distal tubules are due to the high activity of a Na+/Ca2+ exchange system in the distal tubule and its virtual absence in the proximal tubule.

Animals↗

PubMed1989

Influence of insulin on phosphate uptake by brush border membranes from human placenta.

Regulation of phosphate transport by insulin was investigated in brush border membranes from human placenta at term. At 22 degrees C, a 45 min incubation of the total tissue with 10(-6) M insulin significantly decreased both the initial rate and the peak of sodium-dependent phosphate uptake by the corresponding brush border membranes. In contrast, Na+ transport was not influenced by the hormone. Increasing the insulin concentration from 0 to 10(-5) M resulted in a dose-dependent inhibition of phosphate uptake with half-maximal effect at 1.1 x 10(-9) M. The hormone decreased PO4 transport by decreasing the affinity of the carrier for the substrate (Km = 0.180 +/- 0.010 mM and 0.215 +/- 0.015 mM in absence and presence of 10(-6) M insulin respectively, P less than 0.05). The inhibitory effect of insulin required the presence of Mn2+ whereas neither Mn2+ nor insulin alone had any influence on PO4 uptake. It is therefore assumed that receptor phosphorylation, which needs the presence of Mn2+, is an intermediate step of insulin action on PO4 uptake by the subsequently isolated brush border membranes. In contrast, insulin had no effect on PO4 uptake when the membranes were directly incubated with the hormone prior to the transport measurement, suggesting that an intracellular messenger is needed for the inhibitory effect. This messenger is not cAMP since insulin at 10(-6) M concentration has no effect on cAMP content of the total placental tissue.(ABSTRACT TRUNCATED AT 250 WORDS)

Cyclic AMP↗

PubMed1989

Effect of parathyroid hormone on PO4 transport through the human placenta microvilli.

The transport of phosphate (PO4) through the placenta is a secondary active phenomenon whose control mechanisms are unknown. In this study, we investigated whether PTH, the main hormone regulating PO4 transport in the kidney and gut, has a similar role in the placenta. Incubation of normal term human placenta fragments for 1 min with PTH increased the cAMP content of the tissue by 285%. A dose-response curve of the effect of the hormone showed that the cAMP accumulation reached a maximal level with 3.5 x 10(-8) M PTH. Incubation of the placenta fragments with 10(-4) M di-butyryl cAMP resulted in a significant decrease in the PO4 uptake by the brush border membranes prepared from these fragments. Increasing concentrations of di-butyryl cAMP from 0 to 10(-3) M significantly decreased the PO4 uptake from 0.29 +/- 0.02 to 0.22 +/- 0.01 pmol/micrograms/20 s. Similarly, incubation of the placental tissue with PTH resulted in a comparable decrease in the PO4 uptake by the corresponding brush border membrane vesicles. In contrast, direct incubation of brush border membranes with the hormone did not influence PO4 uptake. It is concluded that PTH probably regulates the PO4 transport through the placenta syncytio-trophoblast cell through cAMP mediation. Because adenylate cyclase is located in the basal plasma membrane, it is likely that only the fetal hormone is implicated in this process.

Biological Transport, Active↗

PubMed1988

Calcium transport through the placenta.

The calcium content of the foetus increases exponentially during gestation. The transfer of calcium from the mother to the foetus is believed to occur via an active mechanism, based on observations that the calcium concentration is higher in foetal than in maternal blood, and that perfusion in situ of the umbilical artery of the placenta results in a net increase in the calcium concentration of the perfusate even when the calcium content of the perfusate is threefold higher than in the maternal blood. The calcium concentration in the foetal blood is mainly regulated by foetal parathyroid hormone and the plasma concentration of 1.25-(OH)2 vitamin D3. Several experiments suggest that this vitamin D metabolite plays a key role in calcium transport through the syncytial cell. A vitamin D dependent calcium binding protein has been detected in the placenta, which might intervene in the hormonal control of this transport. The cellular mechanisms of calcium transport through the placenta are not clearly defined. An ATP-dependent calcium uptake by microsomal membrane vesicles from human placenta has been described by several investigators. This calcium transport is saturable, magnesium dependent, and displays Michaelis-Menten kinetics with a Km of approximately 70 nM ionized calcium. Two Ca2+ ATPases with low and high affinities for calcium are described. Since there are discrepancies concerning the exact localization of these enzymes in the syncytial cell, it has not been established whether they intervene in calcium transport through the placenta.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗

PubMed1988

Parathyroid hormone receptor in human placental syncytiotrophoblast brush border and basal plasma membranes.

The syncytiotrophoblast of the placenta is the site of exchange of nutrients and minerals between the mother and fetus. We have recently demonstrated that PTH influences, in vitro, phosphate transport through the placenta brush border membranes (BBM) and increases cAMP accumulation in placental tissue. To demonstrate the site of binding of PTH in the cytoplasmic membrane, we have purified two polar membranes: the first located on the apical side, the BBM, and the second, on the fetal side, the basal plasma membrane (BPM). BBM were enriched 24-fold in alkaline phosphatase (marker for BBM), and the BPM was enriched 37-fold in binding of [3H] dihydroalprenolol (marker for BPM) compared to homogenate. Both placental membranes contain binding sites (maximum binding = 0.550 +/- 0.032 and 0.298 +/- 0.065 pmol/mg protein for BBM and BPM, respectively) with similar affinities (Kd = 2.05 +/- 0.23 and 1.78 +/- 0.19 nM, respectively) for 125I-[Nle8,Nle18,Tyr34] bovine (b) PTH-(1-34) amide. The three bovine preparations [bPTH-(1-34), its analog [Nle8,Nle18,Try34]bPTH-(1-34) amide, and the antagonist bPTH-(3-34)] were equipotent in binding to both placental membranes. In contrast, human PTH-(1-84) was more effective in displacing the bovine radioligand in BBM. Thyrocalcitonin and insulin, two non-PTH peptides, did not significantly displace the radioligand in BBM and BPM. Adenylate cyclase activity, located exclusively in BPM, was stimulated by PTH. Since the enzyme is absent from BBM, it is probable that the binding of the hormone to this membrane activates another system of messengers.

Adenylyl Cyclases↗