Toxin pharmacology of the ATP-induced hyperpolarization in Madin-Darby canine kidney cells

ATP causes glomus cell [Ca2+]c increase without corresponding increases in CSN activity

The hypothesis that an increase in intracellular calcium [Ca(2+)](c) in carotid body (CB) glomus cells will cause enhanced afferent carotid sinus nerve (CSN) activities was tested in the rat CB in-vitro with the use of extracellular ATP. ATP caused a dose dependent [Ca(2+)](c) increase in identified glomus cells. A major part of total [Ca(2+)](c) increase (2/3) was due to the [Ca(2+)] influx. The rest of [Ca(2+)](c) increase (1/3) was due to the release of [Ca(2+)] from the endoplasmic reticulum (ER) [Ca(2+)] stores, and it was inhibited by the pretreatment of cells with cyclopiazonic acid (CPA), an intracellular Ca(2+)-ATPase blocker. Suramin, a purinergic P(2) receptor membrane blocker, blocked [Ca(2+)] influx due to ATP in the presence of extracellular [Ca(2+)]. Perfusion with 5 and 10 microM ATP stimulated CSN activities in both normoxia (Nx) and hypoxia (Hx). Above that level, 100 microM ATP induced slight initial stimulation in CSN activities which were subsided subsequently in Nx and partly diminished in Hx, while 500 microM ATP completely inhibited CSN activities in Nx and Hx after a slight initial stimulation. Electrophysiological measurements of the glomus cell membrane potential in the presence of ATP (100 microM) during Nx indicated cellular enhanced outward K(+) current and hyperpolarization, suggesting potential mechanism for the inhibition of CSN activities. Thus, ATP dependent linear increases in [Ca(2+)](c) did not give rise to a corresponding increase in CSN activities, contravening the normally expected increase in CSN activities following [Ca(2+)](c) rise.

Characterization of Zn(2+) transport in Madin-Darby canine kidney cells

The aim of this study was to characterize the mechanism implicated in Zn(2+) transport in MDCK cells. Trace elements such as Zn(2+), Cd(2+) or Cu(2+) induced MDCK cell depolarization at the micromolar level as demonstrated by bis-oxonol fluorescence and whole-cell patch experiments. This depolarization was inhibited by La(3+) and Gd(3+) and was not related to the activation of Na(+) or Cl(-) channels. Uptake of 65Zn was assessed under initial rate conditions. The kinetic parameters obtained at 37 degrees C were a K(m) of 18.9 microM and a V(max) of 0.48 nmol min(-1) (mg protein(-1)). Intracellular pH measurements using BCECF probe demonstrated that Zn(2+) transport induced a cytoplasmic acidification. The cytoplasmic acidification resulting from Zn(2+) uptake activated Na(+)/H(+) antiporter, which allowed for the recycling of protons. These data suggest that Zn(2+) enters MDCK cells through a proton-coupled metal-ion transporter, the characteristics of which are slightly different from those described for the metal transporter DCT1. This mechanism could be in part responsible of the metal transport evidenced in the distal parts of the renal tubule.

Activation of calcium influx by ATP and store depletion in primary cultures of renal proximal cells

Cytoplasmic calcium changes and calcium influx evoked by adenosine triphosphate (ATP) were investigated in primary cultures of rabbit proximal convoluted tubule cells. Extracellular ATP (50 microM) induced a biphasic increase of [Ca2+]i measured with the calcium probe fura-2. In the early phase, the mobilization of intracellular pools resulted in a transient increase of [Ca2+]i from 106 +/- 11 nM (n = 36) to 1059 +/- 115% (n = 29) of the resting level within 10 s. In the presence of external calcium, [Ca2+]i then decreased within 3 min to a sustained level (398 +/- 38%, n = 8). Measurements of fura-2 quenching by external manganese revealed that this phase was the result of an increased Ca2+ uptake, blocked by lanthanum (10 microM) and verapamil (100 microM) but not by the nifedipin (25 microM). Internal calcium store depletion by ATP induced an increased calcium influx through lanthanum- and verapamil-sensitive, nifedipin-insensitive calcium channels, located on the apical membrane of the cells. As indicated by 86Rb+ efflux measurements, ATP activated a potassium efflux that was blocked by barium and Leiurus quinquestriatus hebraeus (LQH) venom (containing charybdotoxin) indicating the involvement of Ca(2+)-sensitive K+ channels. Moreover, in the presence of the LQH venom, the internal calcium stores were not replenished after being depleted by ATP. Our results indicate that an ATP-evoked hyperpolarization of the plasma membrane leads to increased Ca2+ influx, which facilitates the replenishment of the internal stores.

Synthesis of charybdotoxin and of two N-terminal truncated analogues. Structural and functional characterisation

Charybdotoxin and two N-terminal truncated peptides, corresponding to the 2-37 and 7-37 sequences, were obtained by stepwise solid-phase synthesis using N alpha-t-butyloxycarbonyl and benzyltype side-chain protection. While this strategy was generally useful, the S-acetamidomethyl protecting group used for the six cysteines was not completely stable under HF treatment and its subsequent removal by mercury(II) treatment was neither complete nor devoid of side reactions. The completely deprotected native and truncated sequences were folded efficiently in the presence of glutathione and were finally purified by high-pressure liquid chromatography with overall yields of 4.0-5.0%. Each protein was characterised chemically, structurally and functionally. 1H-NMR spectroscopy was used and a complete assignment of all the protons of the three synthetic proteins was achieved. NMR data show that synthetic charybdotoxin is indistinguishable from the natural protein. The two truncated proteins contain the same elements of secondary structure and a similar overall three-dimensional structure, in agreement with circular dichroic measurements. The shortest analogue, however, may have local structural perturbations and/or higher flexibility. Biological activity on dog epithelial Ca(2+)-activated K+ channels and on rat brain synaptosomal voltage-dependent K+ channels show that synthetic charybdotoxin was as potent as the natural toxin on both channels. For both channels, deletion of the first amino acid, 5-oxoproline (pyroglutamic acid) decreased only slightly the potency of the inhibitor, while deletion of the entire 1-6 segment reduced potency much more. We conclude that the N-terminal region of charybdotoxin plays a functional role in tuning the toxin's biological activity but is not essential for the folding and stability of the structure. The structure of the shortest analogue represents an interesting example of how a well organised and stable alpha/beta fold can be engineered with only 31 amino acid residues.

Toxin pharmacology of the large-conductance Ca(2+)-activated K+ channel in the apical membrane of rabbit proximal convoluted tubule in primary culture

The patch-clamp technique was used to study the toxin pharmacology of the large-conductance Ca(2+)-activated K+ channel (BKCa) present in the apical membrane of rabbit proximal convoluted tubules (PCT) in primary culture. Experiments were performed with the inside-out configuration. This channel was very selective for K+ against Na+ and had a conductance of 180 pS with 140 mmol/l in the pipette and the bath. The action of toxins was studied on the extracellular side of the channel by using the pipette perfusion technique. Experimental conditions were 140 mmol/l KCl in the pipette and 140 mmol/l NaCl in the bath. Pipette potential was maintained at 0 mV. Perfusion of crude venom from Leiurus quinquestriatus hebraeus inhibited reversibly the open probability (Po) in a concentration-dependent fashion (IC50 = 0.8 mg/l; n = 3). The following synthetic or purified toxins were tested: synthetic charybdotoxin (ChTX) IC50 = 7.3 x 10(-9) M (n = 5); iberiotoxin (IbTX) IC50 = 5.5 x 10(-7) mol/l (n = 3); and kaliotoxin (KTX) IC50 = 4.8 x 10(-7) mol/l (n = 3). The suppression of the six first N-terminal amino-acids slightly reduced the affinity of ChTX (IC50 = 1.2 x 10(-8) mol/l, n = 4). Neither Dendroaspis polylepis venom nor purified alpha dendrotoxin modified Po even at high concentrations (20 mg/l and 10(-6) mol/l respectively). Apamin, which blocked the small-conductance K+ channel in cultured PCT, did not act on BKCa. These results indicate that ChTX is the most efficient known toxin against the epithelial BKCa in primary cultures of PCT.(ABSTRACT TRUNCATED AT 250 WORDS)

Nucleotides mobilize intracellular calcium stores of renal proximal cells in primary culture: existence of a suramin-sensitive mechanism

Changes of intracellular calcium concentrations [Ca2+]i were measured in primary cultured rabbit proximal convoluted tubules (PCT). A dual-excitation, digital-imaging inverted microscope was used to monitor the fura-2 fluorescence. The basal calcium level was 106 +/- 11 nM (n = 36). The stimulatory effects of adenosine triphosphate (ATP), adenosine diphosphate (ADP) and adenosine were studied. ATP and ADP induced transient increases of [Ca2+]i (1059 +/- 115% of the resting level (n = 29), and 659 +/- 134% (n = 10), respectively) by releasing calcium from cytoplasmic stores. Adenosine had less effect (279 +/- 48% of the resting level, n = 3). In the same conditions the ATP antagonist suramin (100 microM) inhibited the action of ATP and ADP to 231 +/- 52% (n = 3), and 308 +/- 29% (n = 4) of the resting level, respectively, but did not modify that of adenosine (281 +/- 72%, n = 3). A pretreatment (500 ng/ml for 2 h at 37 degrees C) of the culture with the toxin of Bordetella pertussis completely blocked the ATP response. Our results are evidence for the presence of a functional suramin-sensitive ATP and ADP puriceptor in cultured renal proximal cells. A pertussis-toxin-sensitive G protein is linked to the transduction mechanism. This receptor is distinct from an adenosine puriceptor also found in the proximal monolayer.