Characterization and photoaffinity labeling of receptor sites for the Ca2+ channel inhibitors d-cis-diltiazem, (+/-)-bepridil, desmethoxyverapamil, and (+)-PN 200-110 in skeletal muscle transverse tubule membranes

In order to further understand the molecular nature of the voltage-sensitive Ca2+ channel in skeletal muscle, we have performed classical radioligand binding studies and photoaffinity labeling with different types of tritiated inhibitors of the Ca2+ channel. The equilibrium dissociation constants (KD) for (-)-[3H]desmethoxyverapamil, d-cis-[3H]diltiazem, and (+/-)-[3H]bepridil at their receptor sites in skeletal muscle transverse tubule membranes are: 1.5 +/- 0.5, 50 +/- 5, and 20 +/- 5 nM, respectively. Maximum binding capacities in picomoles/milligram of protein were: 70 +/- 10 for (-)-[3H]desmethoxyverapamil, 50 +/- 15 for d-cis-[3H]diltiazem, and 75 +/- 15 for (+/-)-[3H]bepridil. The kinetics of association at 10 degrees C for the three types of tritiated compounds were relatively slow (3 X 10(5) M-1 S-1 for (-)-[3H]desmethoxyverapamil, 8 X 10(3) M-1 S-1 for d-cis-[3H]diltiazem, and 4.2 X 10(5) M-1 S-1 for (+/-)-[3H]bepridil). The dissociation of (-)-[3H]desmethoxyverapamil and d-cis-[3H]diltiazem from their receptor sites was also a slow process with half-lives of dissociation of 33 and 36 min, respectively. Competition studies using the three tritiated ligands suggest that they bind to the same receptor site which appears to be in a 1:1 stoichiometry with the dihydropyridine receptor. Photoaffinity labeling with high intensity ultraviolet light in the presence of (+/-)-[3H]bepridil or d-cis[3H]diltiazem resulted in the specific covalent incorporation of radioactivity into a polypeptide of Mr 170,000 +/- 10,000. A polypeptide of Mr 170,000 was also specifically labeled in photoaffinity labeling experiments using the high affinity dihydropyridine derivative (+)-[3H]PN 200-100.

Transmembrane ionic transport systems and hypertension

Various systems contribute to the regulation of intracellular calcium. The most important are: (1) the voltage-dependent calcium channel; (2) sodium/calcium exchange, resulting in a correlation between intracellular sodium and calcium; (3) plasma membrane calcium ATPase; and (4) the inositol triphosphate-regulated channel, releasing calcium from the endoplasmic and sarcoplasmic reticulum stores. The essential properties of these different systems, their pharmacologic features, and their regulation by various agonists (beta- and alpha-adrenergic angiotensin-type peptides, etc.) must be considered both for an understanding of the pathophysiology of hypertension and for its treatment.

Purification and characterization of the dihydropyridine-sensitive voltage-dependent calcium channel from cardiac tissue

The dihydropyridine-sensitive voltage-dependent Ca2+ channel from cardiac tissue was purified 900-fold using DEAE-Sephadex A-25, concanavalin A-Sepharose, and wheat germ agglutinin-Sepharose. The purified preparation was highly enriched in a peptide of 140,000 daltons when electrophoresed on sodium dodecyl sulfate gels in the presence of 2-mercaptoethanol, or 170,000 when electrophoresed in the presence of iodoacetamide. Polyclonal antibodies raised against the purified subunits of the rabbit skeletal muscle Ca2+ channel recognized the 170-kDa protein in preparations electrophoresed under nonreducing conditions, and the large peptide of 140 kDa and smaller peptides of 29-32 kDa in preparations analyzed under reducing conditions. Monoclonal antibodies, which were raised against the native Ca2+ channel from skeletal muscle, immunoprecipitated [3H]PN 200-110 binding activity from solubilized cardiac membranes and immunoprecipitated 125I-labeled peptides (from the purified cardiac Ca2+ channel preparation) which migrated as a single species of 170 kDa under nonreducing conditions, or as 140, 32, and 29 kDa under reducing conditions. The results show that the purified cardiac Ca2+ channel, like that previously purified from skeletal muscle, consists of a major component of 170 kDa which is comprised of a 140-kDa peptide linked by disulfide bonds to smaller peptides of 32-29 kDa. Peptide maps of the 140-kDa peptide purified from cardiac and skeletal muscle preparations were strikingly similar, suggesting a high degree of homology in their primary sequence.

Antidiabetic sulfonylureas control action potential properties in heart cells via high affinity receptors that are linked to ATP-dependent K+ channels

Both avian and mammalian heart cells have high affinity receptors for antidiabetic sulfonylureas. The biochemical identification of these receptors has been carried out with [3H]glibenclamide. The Kd values for the most potent sulfonylureas, such as glibenclamide itself, are in the nanomolar range. Comparative studies of structure-function relationships indicate high similarities of binding properties between the sulfonylurea receptors in cardiac cells and insulinoma cells, respectively. The duration of the action potential of guinea pig cardiac cells was drastically reduced by decreasing intracellular ATP concentrations by perfusion or by blockade of oxidative phosphorylation. Glibenclamide was found to restore normal or nearly normal action potential properties in [ATP]in-depleted cardiac cells. Single channel recording using the patch-clamp technique has shown that this effect is associated with high affinity blockade of ATP-sensitive K+ channels by sulfonylureas.

Leiurotoxin I (scyllatoxin), a peptide ligand for Ca2(+)-activated K+ channels. Chemical synthesis, radiolabeling, and receptor characterization

Leiurotoxin I (scyllatoxin) is a 31-amino acid polypeptide from the venom of the scorpion Leiurus quinquestriatus hebraeus which has been previously isolated and sequenced by others. This paper reports (i) the total synthesis of this scorpion neurotoxin as well as some aspects of its structure-function relationships; (ii) the synthesis of the analog [Tyr2]leiurotoxin I (scyllatoxin) that has been monoiodinated at high specific radioactivity (2000 Ci/mmol) and has served for the characterization of the properties of 125I-[Tyr2]leiurotoxin I binding sites (Kd = 80 pM, molecular mass of 27 and 57 kDa for two polypeptides in the leiurotoxin I binding protein); (iii) the similarity of physiological actions between leiurotoxin I and apamin. Both toxins contract Taenia coli previously relaxed with epinephrine, both toxins block the after-hyperpolarization due to Ca2(+)-activated K+ channel activity in muscle cells in culture; (iv) the probable identity of binding sites for apamin and leiurotoxin I. In spite of a different chemical structure apamin competitively inhibits 125I-[Tyr2] leiurotoxin I binding and vice versa. Moreover, the peculiar effects of K+ on 125I-[Tyr2]leiurotoxin I binding are identical to those previously observed for 125I-apamin binding.

Multiple mRNA isoforms encoding the mouse cardiac Kv1-5 delayed rectifier K+ channel

The mouse Kv1-5 K+ channel cDNA has been cloned from heart. This channel was highly expressed in heart and, to a lesser extent, in other tissues, including brain and thymus. Two alternatively spliced isoforms were found. The longer form encoded a 602-amino acid protein, while in the short form (Kv1-5 delta 5'), the first 200 amino acids lying upstream the transmembrane segment S1 were deleted. RNase protection experiments showed that both Kv1-5 mRNA isoforms are present in the mouse tissues examined, the longer form being predominant. The short mRNA (Kv1-5 delta 5') arose by an unusual splicing event within the exonic sequence. An additional short cDNA clone (Kv1-5 delta 3') that codes for a carboxyl-terminal truncated protein has been isolated. The gene coding sequence contained a single exon and has been mapped on human chromosome 12 (p13) and on mouse chromosome 6 (band F). Expression in Xenopus oocytes revealed that the long (Kv1-5) and the amino-terminal deleted (Kv1-5 delta 5') isoforms elicited similar K+ currents with a drastically decreased efficacy for Kv1-5 delta 5'. The carboxyl-terminal truncated Kv1-5 delta 3' clone was not functional but inhibited the expression of the long isoform.

Effects of a new class of calcium antagonists, SR33557 (fantofarone) and SR33805, on neuronal voltage-activated Ca++ channels

SR33557 (fantofarone) and SR33805 are structurally novel calcium antagonists that bind selectively to the alpha 1-subunit of the L-type Ca++ channel at a site distinct from the classical 1,4-dihydrophyridine, phenylalkylamine and benzothiazepine sites but in allosteric interactions with them. Blocking effects of fantofarone and SR33805 on the different types of voltage-activated Ca++ currents have been investigated with the whole-cell patch-clamp method in chick dorsal root ganglion neurons (for T-, L- and N-type currents) and in rat cerebellar Purkinje neurons (for P-type current) in primary culture. Neuronal L-type Ca++ channels are blocked totally by fantofarone and SR33805 in the microM range of concentration as in skeletal muscle and cardiac cells at a holding membrane potential of -80 mV. The sequence of efficacy is SR33805 (IC50 = 26 nM) > fantofarone (IC50 = 0.35 microM). N- and P-type channels are not very sensitive to fanto-farone and SR33805 (IC50 approximately 5 microM). The T-type channel is not affected by these drugs.

Diamine oxidase is the amiloride-binding protein and is inhibited by amiloride analogues

Diamine oxidase (histaminase), an enzyme that oxidatively deaminates putrescine and histamine, was purified from human placenta and from pig kidney. Both NH2-terminal sequences are highly homologous to the human kidney amiloride-binding protein, previously thought to be a component of the amiloride-sensitive Na+ channel. Monoclonal antibodies raised against the pig kidney amiloride-binding protein immunoprecipitate a polypeptide with the same M(r) (105,000) as that of pig kidney diamine oxidase. That polypeptide has both diamine oxidase activity and the capacity to bind [3H]phenamil, a tritiated amiloride derivative. Cells stably transfected with human kidney amiloride-binding protein cDNA express a high diamine oxidase activity. In transfected cells as well as with the purified enzyme, this activity was inhibited by amiloride and by some of its derivatives, such as phenamil and ethylpropylamiloride. Amiloride inhibition seems to be due to drug binding at the active site of the enzyme. These data indicate that human placental diamine oxidase is identical to the human kidney amiloride-binding protein and that amiloride analogues may have wider physiological effects besides those on epithelial ion transport.

The Na+ channel in mammalian cardiac cells. Two kinds of tetrodotoxin receptors in rat heart membranes

The properties of interaction of both tetrodotoxin (TTX) and tritiated ethylenediamine tetrodotoxin [3H] en-TTX) were studied in rat heart membranes at different stages of development and in cultured cells. Studies by electrophysiology and by 22Na+ flux measurements on cardiac cultured cells indicate that the functional form of the Na+ channel is of low affinity for TTX (250-700 nM). Binding experiments (bioassay and [3H]en-TTX binding) on cultured cardiac cells from newborn rats indicate the presence of both high and low affinity binding sites for TTX with dissociation constants (Kd) of 1.6 and 135 nM, respectively. On homogenates of hearts taken just after birth, [3H]en-TTX binding reveals no high affinity binding site for TTX but the presence of a low affinity binding site with a Kd of 125 nM. This result was confirmed by kinetic studies and competition experiments. Conversely, binding studies on homogenates and extensively purified membranes from adult ventricles reveal the presence of both high and low affinity binding sites for TTX with Kd values of 1.5 and 170 nM, respectively. The maximum binding capacity for the low affinity binding sites is 45 times higher than that of the high affinity binding sites. High affinity sites do not exist at the fetal stage or at birth, but after 5 days their number gradually increases to reach a maximum level around 45 days after birth. Conversely, the number of low affinity binding sites is essentially invariant between birth and adulthood. Monolayers of cardiac cells from hearts at 2 days after birth which have no high affinity TTX-binding sites in vivo develop both high and low affinity binding sites for TTX in vitro. The results presented here are the first direct demonstration of the coexistence in rat heart plasma membrane of two families of binding sites for TTX.

HERG and KvLQT1/IsK, the cardiac K+ channels involved in long QT syndromes, are targets for calcium channel blockers

We examined the effects of the calcium channel blockers nitrendipine, diltiazem, verapamil, bepridil, and mibefradil on the cloned HERG and KvLQT1/IsK K+ channels. These channels generate the rapid and slow components of the cardiac delayed rectifier K+ current, and mutations can affect them, which leads to long QT syndromes. When expressed in transfected COS cells, HERG is blocked in a concentration-dependent manner by bepridil (EC50 = 0.55 microM), verapamil (EC50 = 0.83 microM), and mibefradil (EC50 = 1.43 microM), whereas nitrendipine and diltiazem have negligible effects. Steady state activation and inactivation parameters are shifted to more negative values in the presence of the blockers. Similarly, KvLQT1/IsK is inhibited by bepridil (EC50 = 10.0 microM) and mibefradil (EC50 = 11.8 microM), while being insensitive to nitrendipine, diltiazem, or verapamil. These results demonstrate that both cloned K+ channels HERG and KvLQT1/IsK, which represent together the cardiac delayed rectifier K+ current, are sensitive targets to calcium channel blockers. This work may help in understanding the mechanisms of action of verapamil in certain ventricular tachycardia, as well as some of the deleterious adverse cardiac events associated with bepridil.

Riluzole improves functional recovery after ischemia in the rat retina

PURPOSE

Retinal ischemia leads to neuronal death. The effects of riluzole, a drug that protects against the deleterious effect of cerebral ischemia by acting on several types of ion channels and blocking glutamatergic neurotransmission, were investigated in a rat model of retinal ischemic injury.

METHODS

Retinal ischemia was induced by increasing intraocular pressure above systolic blood pressure for 30 minutes. Electroretinograms were recorded before ischemia and at different periods of reperfusion. Riluzole was injected or topically applied to the eye before or after ischemia and twice daily during the reperfusion period. Retinas were harvested for histopathology (toluidine blue and silver-impregnation stainings, Tdt-dUTP terminal nick-end labeling [TUNEL] method) and immunohistochemistry for cytoskeletal glial fibrillary acid protein and c-jun NH2-terminal kinase (p-JNK).

RESULTS

Ischemia for 30 minutes caused a reduction of a- and b-waves of the electroretinogram. Systemic and topical treatments with riluzole significantly enhanced the recovery of the reduced a- and b-waves after defined reperfusion times. Riluzole also prevented or attenuated ischemia-induced retinal cell death (necrosis and apoptosis) and reduced the activation of p-JNK, c-jun phosphorylation, and the increase of cytoskeletal proteins induced by ischemic injury.

CONCLUSIONS

Riluzole acted in vivo as a potent neuroprotective agent against pressure-induced ischemia. Therefore, riluzole may be a major drug for use in protection against retinal injury.

[3H]Ethylpropylamiloride, a ligand to analyze the properties of the Na+/H+ exchange system in the membranes of normal and hypertrophied kidneys

[3H]Ethylpropylamiloride is a useful radioactive label to identify the Na+/H+ exchange system (Vigne, P., Frelin, C., Audinot, M., Borsotto, M., Cragoe, E. J., and Lazdunski, M. (1984) EMBO J. 3, 2647-2651). This paper extends the analysis of the properties of interaction of [3H]ethylpropylamiloride with the exchanger and describes its use with hypertrophied kidneys. [3H]Ethylpropylamiloride-binding sites copurify with the luminal membrane marker alkaline phosphatase but not with the basolateral membrane marker (Na+,K+)ATPase, thus indicating an asymmetric distribution of the Na+/H+ exchanger. Specific [3H]ethylpropylamiloride binding is dependent on pH. The pH dependency indicates that an ionizable function with a pKapp of 7.0 is essential in the association of the amiloride derivative. H+ acts competitively on [3H]ethylpropylamiloride binding; Na+, Li+, or cholinium ions have no effect on the association. Compensatory adaptation of the kidney to chronic reduction of renal mass is accompanied by a 1.7-fold increase in the activity of the Na+/H+ exchange system. Properties of interaction of internal and external pH with the Na+/H+ exchanger of normal and hypertrophied kidneys are identical. Titration of [3H]ethylpropylamiloride-binding sites in normal and hypertrophied kidneys suggests that the increased activity of the Na+/H+ exchange system is not accompanied by an increased concentration of exchangers.

Molecular size of different neurotoxin receptors on the voltage-sensitive Na+ channel

Measurements were made of the molecular sizes of two distinct receptors on the Na+ channel in rat brain synaptosomes that are specific for different neurotoxins. Radiation inactivation of the binding of radiolabeled derivatives of the toxins was consistent with Mr = 260,000 for the tetrodotoxin receptor and Mr = 266,000 for the receptor specific for two scorpion toxins, toxin II from Centruroides suffusus suffusus and toxin gamma from Tityus serrulatus serrulatus. Covalent cross-linking of the latter to its receptor similarly indicated Mr = 270,000. It seems most likely that these two distinct receptors reside on the same molecule.

Na+ channels with high and low affinity tetrodotoxin binding sites in the mammalian skeletal muscle cell. Difference in functional properties and sequential appearance during rat skeletal myogenesis

High and low affinity binding sites for tetrodotoxin have been found in rat skeletal muscle cells in vitro using a radiolabeled tetrodotoxin derivative and 22Na+ flux studies. High affinity binding sites for tetrodotoxin (KD(tetrodotoxin) = 1.6 nM) cannot be detected at the myoblast stage. They appear and increase in density as myoblasts fuse into myotubes to reach a maximum binding capacity of 50 fmol/mg of proteins. Na+ channel structures with a high affinity for tetrodotoxin cannot be activated by neurotoxins specific for the Na+ channel such as veratridine and sea anemone toxinII. They are not expressed in the action potential. Na+ channels with a low affinity for tetrodotoxin (IC50(tetrodotoxin) = 1 microM) are functional since they can be activated by veratridine and sea anemone toxinII. They are already expressed in myoblasts and their density is not modified during the fusion of myoblasts into myotubes; they remain functional throughout the differentiation process. It is suggested that neuronal factors are not required for the synthesis of structures with high affinity binding sites for tetrodotoxin in the rat muscle and that they are only involved for the maturation of these structures from a nonfunctional to a functional form.

Affinity labeling and localization of the ATP binding site in the (Na+,K+)-ATPase

The (Na+,K+)-ATPase has been covalently labeled with oxidized ATP. The extent of oxidized ATP incorporation which inactivates the enzyme corresponds to one nucleotide site per equivalent of ouabain binding site. ATP and ADP protect against the covalent labeling by oxidized ATP; the dissociation constants for the protective effects of ATP and ADP are 0.6 mM and 2.5 mM, respectively. The covalent incorporation is accelerated by K+ and by ouabain in the presence of Mg2+. The site of incorporation of oxidized ATP is located on a Mr = 58,000 fragment situated at the COOH-terminal end of the Mr = 94,000 alpha chain, while the phosphorylation site and the ouabain site are in the other (NH2-terminal) part of the chain.

Receptor binding characterization in kidney membrane of [3H]U-37883, a novel ATP-sensitive K+ channel blocker with diuretic/natriuretic properties

U-37883 (4-morpholinecarboximidine-N-1-adamantyl-N-cyclohexyl), a known blocker of ATP-sensitive K+ (KATP) channels, produces natriuresis/diuresis in vivo by a direct effect on the kidney. In the present study, the binding characteristics of the U-37883 receptor were investigated using pig kidney cortex microsomes. [3H]U-37883 (0.5-5 nM, 50 Ci/mmol) exhibited specific binding, which was reversible, increased linearly with protein concentration (50-500 micrograms/ml), and was destroyed after treatment with proteases. Scatchard plots derived from the competition experiments suggested the presence of a single class of low affinity binding sites, with a Kd of 225 nM and a Bmax of 7.8 pmol/mg of protein. A similar Kd value was derived from complementary studies dealing with association and dissociation kinetics. The binding of [3H]U-37883 was tissue specific, because very little specific binding could be detected in microsomes from rat insulinoma cells (RINm5F) and brain. In contrast, these membranes displayed high affinity specific binding of [3H]glyburide, another KATP channel blocker. Finally, analogs of U-37883 that were found to be active KATP channel blockers in isolated rabbit mesenteric artery and active in vivo as diuretics/natriuretics were also found to be active in displacing specific binding of [3H]U-37883, whereas the inactive analogs (no vascular KATP channel-blocking activity and no in vivo diuresis/natriuresis) were inactive in this binding assay. We suggest that the U-37883 binding site represents a functional receptor that mediates the KATP channel antagonism and natriuresis observed with this class of compounds.

Disruption of mitochondrial respiration inhibits volume-regulated anion channels and provokes neuronal cell swelling

Hypoxia and inhibitors of mitochondrial respiration impair the regulatory volume decrease (RVD) of cerebellar granule neurons after hypotonic swelling. RVD is linked to the opening of volume-regulated anion channels (VRACs). VRACs are outwardly rectifying, inactivate slowly during maintained depolarization, and are permeable to the cellular organic osmolyte taurine. Channel activation requires nonhydrolytic ATP binding and is not modulated by intracellular ADP. VRAC opening is reversibly depressed by hypoxia and by mitochondrial inhibitors such as oligomycin, rotenone, and antimycin A. These results demonstrate that neuronal VRAC activation and swelling are both tightly linked to cellular energy. Moreover, the findings reported in this work may have a particular significance for inherited mitochondrial human diseases, such as mitochondrial myopathy, encephalopathy, lactic acidosis, and stroke-like episodes (MELAS), which cause brain swelling and edema.