Ca2+-dependent ATPases in the basolateral membrane of rat kidney cortex

High-affinity Ca2+,Mg(2+)-ATPase in plasma membrane-rich preparations from olfactory epithelium of Atlantic salmon

High-affinity Ca2+,Mg(2+)-ATPase was identified in a plasma membrane-rich fraction of olfactory epithelium from Atlantic salmon (Salmo salar). The enzyme required both Ca2+ and Mg2+ for activation. The apparent Km for Ca2+ was 9.5 nM and Vmax was 0.85 mumol Pi/mg of protein per min. Stimulation by Ca2+ was optimal at 5-100 microM MgCl2. Bovine brain calmodulin had no effect on Ca2+,Mg(2+)-ATPase, even after multiple washes of the membrane preparation with EDTA or EGTA. Endogenous calmodulin was somewhat resistant to removal and could be detected with immunoblotting after multiple washes of the membrane preparation with EDTA or EGTA. This endogenous calmodulin may regulate Ca2+,Mg(2+)-ATPase activity because the activity was inhibited by calmidazolium. Vanadate inhibited Ca2+,Mg(2)-ATPase activity and thapsigargin, a specific inhibitor for Ca2+,Mg(2+)-ATPase of endoplasmic reticulum, had no effect on the enzyme activity. High affinity Ca2+,Mg(2+)-ATPase exists in both ciliary and nonciliary membranes with a similar Km for Ca2+. Ca2+,Mg(2+)-ATPase activity is greater in cilia preparations than in membranes from the deciliated olfactory epithelium. As a putative plasma membrane Ca2+ pump, this high-affinity Ca2+,Mg(2+)-ATPase may play an important role in the regulation of intracellular Ca2+ in olfactory epithelia. In particular, the ciliary membrane may play a prominent role in the removal of Ca2+ from ciliated olfactory receptor cells after odorant stimulation.

Renal handling of Ca2+ in diabetes

Ca2+ transport in kidney has gained considerable attention in the recent past. Our laboratory has been involved in understanding the regulatory mechanisms underlying Ca2+ transport in the kidney across the renal basolateral membrane. We have shown that ANP, a cardiac hormone, mediates its biological functions by acting on its receptors in the kidney basolateral membrane. Furthermore, it has been established that ANP receptors are coupled with Ca2+ ATPase, the enzyme that participates in the vectorial translocation of Ca2+ from the tubular lumen to the plasma. It is possible that a defect in the ANP-receptor-effector system in diabetes (under certain conditions such as hypertension) may be associated with abnormal Ca2+ homeostasis and the development of nephropathy. Accordingly, future studies are needed to establish this hypothesis.

ATP independent calcium transport and binding by basal plasma membrane of human placenta

The transport of large amounts of Ca2+ by the plasma membranes of human placental syncytiotrophoblast is essential to the mineralization of the growing fetal skeleton. We have investigated transport by the basal (fetal-facing) plasma membrane (BPM). Ca2+ was taken up by purified BPM vesicles in a time-dependent manner and equilibrium attained in approximately 60 min. The apparent equilibrium space was many fold higher than that determined using other substrates (e.g. leucine), suggesting that Ca2+ is concentrated or bound within the vesicles. The more rapid uptake and exit in the presence of A23187 indicates that membrane transport is rate limiting and that Ca2+ is internalized within the membrane space. The initial rate of uptake was approximately by measurement during the first 2 s of incubation. Concentration dependence data were fit to a Michaelis-Menten model with one saturable site and diffusion (Km = 12 microM; Vmax = 4 nmol/min/mg; KD = 39 nmol/min/mg/mM). Saturable Ca2+ binding (Kd = 16 microM; Bmax = 3.4 nmol/mg) was of lower capacity than previously observed for microvillous membrane.

Mechanisms of calcium transport across the basolateral membrane of the rabbit cortical thick ascending limb of Henle's loop

Although net Ca2+ absorption takes place in the thick ascending limb of Henle's loop, detailed mechanisms are unknown. Because it has been reported that the Ca2+ entry step across the luminal membrane is mediated by Ca2+ channels inserted by stimulation with parathyroid hormone, we studied the mechanism of Ca2+ transport across the basolateral membrane of rabbit cortical thick ascending limb (CTAL) perfused in vitro by using microscopic fluorometry of cytosolic Ca2+ ([Ca2+]i) with fura-2. The resting [Ca2+]i in this segment was 49.8 +/- 4.5 nmol/l. Neither Na+ removal from the bathing solution nor addition of ouabain (0.1 mmol/l) to the bath increased [Ca2+]i, indicating that a Na+/Ca2+ exchanger in the basolateral membrane may not contribute in any major way to [Ca2+]i of CTAL. To confirm our technical accuracy, similar protocols were conducted in the connecting tubule, where the existence of a Na+/Ca2+ exchanger has been reported. In this segment, Na+ removal from the bath increased cell Ca2+ from 148.6 +/- 6.4 nmol/l to 647.6 +/- 132.0 nmol/l, confirming the documented fact. [Ca2+]i in the CTAL was markedly increased when 1 mmol/l NaCN was added to the bath in the absence of glucose. Calmodulin inhibitors (trifluoperazine or W-7) increased [Ca2+]i. When the bath pH was made alkaline, [Ca2+]i was also increased. This response was abolished when Ca2+ was eliminated from the bath, indicating that the Ca2+ entry across the basolateral membrane is dependent on bath pH. Increase in [Ca2+]i induced by an alkaline bath was inhibited by increased the bath K+ from 5 nmol/l to 50 mmol/l, suggesting that the Ca2+ entry system is voltage-dependent.(ABSTRACT TRUNCATED AT 250 WORDS)

Parathyroid hormone stimulates ATP-dependent calcium pump activity by a different mode in proximal and distal tubules of the rat

A new technique was developed to isolate basolateral membrane vesicles individually from proximal and distal tubules of the rat cortex. This new technique enabled us to study differences in their kinetics and mechanisms of hormonal regulation of Ca pump between proximal and distal tubules. The Ca pump in distal tubule has very high affinity (42.6 nM Ca2+) and the one in proximal tubule has relatively low affinity (75.6 nM Ca2+). Parathyroidectomy (PTX) decreased the Vmax of Ca pump activity in proximal tubule (4.68 +/- 0.99 vs. 9.08 +/- 2.21 nmol 45Ca2+/min per mg protein BLMV, P less than 0.05), while it increased Km in distal tubule (93.1 +/- 11.0 vs. 35.1 +/- 16.1 nM Ca2+, P less than 0.05). Restoration of serum Ca2+ concentration by 1,25(OH)2D3 supplement could not reverse these changes by PTX in Ca pump activity in either the proximal or the distal tubule. In conclusion, this study strongly suggested that parathyroid hormone stimulated Ca pump activity by increasing the Vmax in proximal tubule and by increasing the affinity in distal tubule. 1,25(OH)2D3 does not have a direct effect on the basolateral membrane Ca pump activity.

Glucose inhibits the high-affinity (Ca2+ + Mg2+)-ATPase in the plasma membrane of a glucose-responsive insulinoma

(Ca2+ + Mg2+)-ATPase enzyme activity of a purified plasma membrane preparation from a glucose responsive rat insulinoma, was characterized as Ca2(+)-dependent dephosphorylation of [gamma-32P]ATP. A high-affinity enzyme with a Km(ATP) ranging from 20 to 30 microM and a Km(Ca2+) of 1 microM was identified. Glucose inhibited this high-affinity enzyme in a dose-dependent manner, with no significant inhibition at a concentration between 0 and 5 mM, 50% inhibition at 13.3 mM and 94.5% inhibition at 30 mM. The inhibitory effect of glucose was immediate and rapidly reversible. The effect was stereospecific for the alpha-anomer. These findings support the concept that glucose acts directly at the beta-cell plasma membrane and is involved in the maintenance of elevated intracellular free calcium concentrations associated with insulin release by directly or indirectly inhibiting energy-dependent calcium efflux. Glyceraldehyde (20 mM) increased enzyme activity 3-fold, while other metabolic fuels had no effect. This suggests that inhibition of the enzyme is not an obligatory requirement for insulin release. Calmodulin stimulated the enzyme activity in calmodulin-depleted but not in undepleted membranes. Trifluoperazine (30-100 microM) inhibited (Ca2+ + Mg2+)-ATPase in a dose-dependent manner (14-61% activity) and the activity was also inhibited by vanadate (0.1-1.0 mM) and NaCl (150 mM).

Stimulation of ATP-driven Ca2+ pump in the basal-lateral plasma membranes of kidney cortex during compensatory renal growth

During compensatory renal growth 45Ca2+ transport in basal-lateral plasma membrane vesicles isolated from the rat renal cortex have been investigated. Stimulation of Ca2(+)-ATPase activity was observed, without an effect of compensatory renal growth on Na+/Ca2+ exchanger activity and on passive Ca2+ permeability of the vesicles. Twelve hours following unilateral nephrectomy about 40% increase of Ca2(+)-ATPase activity above control value was observed and this effect was present until the end of the experimental period (7 days). When kinetic parameters for Ca2(+)-ATPase were studied in native membranes, an increase of Vmax was observed, whereas the Km for Ca2+ was similar in control vesicles and vesicles isolated from the remnant kidney. Depletion of endogenous calmodulin resulted in a decrease of Vmax and an increase of Km (Ca2+), while its addition reversed these parameters and increased the Hill coefficient from about 1 to about 2. Once again, only a significant increase of Vmax in vesicles isolated from the remnant kidney above the control value was observed. Finally, increase of Ca2(+)-ATPase activity during compensatory renal growth could be abolished by actinomycin D, indicating that its stimulation is due to protein synthesis.

ATP-driven Ca2+ pump in the basolateral membrane of rat kidney cortex catalyzes an electroneutral Ca2+/H+ antiport

An ATP-driven Ca2+ pump in the basolateral membrane of rat kidney cortex pumps Ca2+ out of the cell at the expense of MgATP (Km = 0.191 mM). This pump has a high affinity for free Ca2+ (26 nM). Vanadate, lanthanum, N-ethylmaleimide and calmodulin inhibitor R24571 inhibited this pump activity. Dimethyl[2-14C]oxazolidine-2,4-dione [( 14C]DMO) was entrapped in the vesicles in association with the ATP-driven Ca2+ influx. The ATP-driven Ca2+ influx was stimulated by the intravesicular acid pH and an upper convex Lineweaver-Burk reciprocal plot suggested two possible kinetics; one is that this Ca2+ pump is an allosteric enzyme with more than 1.72 H+ binding sites and another is the presence of two Ca2+ pumps with different affinities for H+. Valinomycin study indicated that the ATP-dependent Ca2+ transport by the BLMV was electroneutral and voltage independent. These results strongly suggest that the ATP-driven Ca2+ pump in the renal basolateral membrane catalyzes an electroneutral Ca2+/H+ antiport.

Ca2+-dependent ATP hydrolysis of the porcine intestinal brush-border membranes

The brush-border membrane from the porcine small intestine possesses Ca2+-dependent ATPase activity. The Ca2+ stimulation of ATP hydrolysis by the membranes is biphasic with a high affinity (Km = 0.38 microM) and a low affinity (Km = 98.3 microM). Treatment of the membrane vesicles with n-heptylthioglucoside did not cause further increase of the Ca2+-ATPase activity. Mg2+ also stimulates the ATP hydrolysis in the absence of Ca2+ but decreases the Ca2+-ATPase activities at 0.59 and 200 microM free Ca2+. The Ca2+-ATPase activities are not inhibited by addition of vanadate, ouabain, sodium azide and alkaline phosphatase inhibitors (theophylline and L-phenylalanine), irrespective of the Ca2+ concentrations in medium. A specific calmodulin-inhibitor W-7 (up to 30 microM) also did not influence on the Ca2+-ATPase activities at 0.59 and 200 microM free Ca2+. The Ca2+-ATPase activities at 0.59 and 200 microM free Ca2+ show no specificity for ATP. ADP, GTP and CTP could also be used as substrates. From these results, it is suggested that the porcine intestinal brush-border membrane possesses Mg2+-independent Ca2+-ATPase activity and that the Ca2+-ATPase activities with biphasic responses for Ca2+ stimulation observed in the present study reside on the same protein. The physiological functions of the Ca2+-ATPase in the membranes, however, remain unknown at present.