Voltage-sensitive nitrendipine binding in an isolated cardiac sarcolemma preparation

Dihydropyridine receptors in transverse tubules from normal and dystrophic chicken skeletal muscle

Calcium overload is a fundamental pathogenic event associated with chronic muscle degeneration in muscular dystrophies. The possibility that L-type voltage-dependent calcium channels were involved in the etiology of chicken muscular dystrophy was investigated by studying the dihydropyridine receptors in transverse tubule membranes isolated from skeletal muscle of normal (line 412) and dystrophic (line 413) chickens. The yield of T-tubular protein from dystrophic muscle was considerably increased compared with that from normal muscle (2.51 +/- 0.18 vs 1.04 +/- 0.31 mg protein x 100 g muscle-1). The binding of the calcium channel antagonist (+) [3H]PN200-110 to the dihydropyridine receptor in transverse tubule preparations was relatively slow, markedly affected by temperature and required divalent cations. (+) [3H]PN200-110 equilibrium binding assays revealed a single class of high-affinity sites and showed that maximum binding capacity (Bmax) (3.17 +/- 0.47 for normal and 3.51 +/- 0.52 pmol x mg protein-1 for dystrophic transverse tubules) and dissociation constant (Kd) (0.32 +/- 0.07 and 0.26 +/- 0.09 nM, respectively) were not significantly different in normal and dystrophic membranes. Kinetic studies indicated that normal and dystrophic transverse tubules did not differ significantly in association (2.54 x 10(6) and 2.27 x 10(6) M(-1)s(-1), respectively) and dissociation (8.5 x 10(-4) and 9.3 x 10(-4)s(-1), respectively) rate constants. Since dissociation kinetics for both preparations were monoexponential under all the experimental conditions employed, no low-affinity binding sites for (+) [3H]PN200-110 could be detected in chicken transverse tubules membranes. However, immunoblot assay, using a monoclonal antibody, revealed that dystrophic transverse tubules as compared with normal membranes were enriched twofold with the alpha 1-subunit of the dihydropyridine receptor. Therefore, although dihydropyridine-binding sites were not altered in transverse tubule membranes from dystrophic chicken skeletal muscle, both the increased yield in T-tubule vesicles and the enhanced immunodetection of the alpha 1-subunit of the dihydropyridine receptor, suggest that total content in dihydropyridine receptor is higher in dystrophic than in normal muscle.

Dihydropyridine receptors are primarily functional L-type calcium channels in rabbit ventricular myocytes

We measured [3H]PN200-110 binding and patch-clamp currents in rabbit ventricular myocytes to determine if there is a disparity between the density of dihydropyridine-specific receptors and functional L-type calcium channels, as has been reported for skeletal muscle. The dihydropyridine receptor density was 74.7 +/- 4.2 fmol/mg protein (mean +/- SEM, Kd = 1.73 +/- 0.29 nM, n = 6) in ventricular homogenates and 147 +/- 6 fmol/mg protein (Kd = 1.15 +/- 0.16 nM, n = 4) in myocytes. Ventricular homogenates contained 121 +/- 9 mg protein/g wet wt (n = 7). These values were used to calculate a dihydropyridine receptor density of 12.9 dihydropyridine sites/micron2 for ventricular homogenates and 14.8 dihydropyridine sites/micron2 for myocytes. The number of functional L-type calcium channels (N) was calculated from measurements of whole-cell current (I), single-channel current (i), and open probability (po), where N = I/(i x po). We measured sodium current through calcium channels (I(ns)) to avoid calcium-induced inactivation. Whole-cell (I(ns)) and single-channel (i(ns) and po) measurements were obtained under similar ionic conditions at a test potential of -20 mV. In six cells, the peak I(ns) was approximately 105 pA/pF. The single-channel conductance was 40.8 +/- 2.6 pS (n = 12), and i(ns) at -20 mV was 1.96 pA. The mean po at -20 mV was 0.030 +/- 0.002 in 16 patches in which only a single channel was evident. The calculated density of functional L-type calcium channels was approximately 18 channels/micron2.(ABSTRACT TRUNCATED AT 250 WORDS)

The devapamil-binding site of the purified skeletal muscle receptor for organic-calcium channel blockers is modulated by micromolar and millimolar concentrations of Ca2+

The interaction of 2,7-dimethyl-3-(3,4-dimethoxyphenyl)-3-cyan-7-aza-9-(3- methoxyphenyl) nonahydrochloride (devapamil), a stereospecific analog of (3-[2-(3,4-dimethoxyphenyl)ethyl]- methylaminopropyl-3,4-dimethoxy-(1-methylethyl)benzeneacetonitr ile (verapamil), with the purified skeletal muscle receptor for calcium channel blockers (CaCB) was studied at 4 degrees C and 30 degrees C in the absence and presence of calcium. The purified CaCB receptor bound 0.9 mol devapamil/mol calcium-channel alpha 1 subunit, with an apparent Kd of 13 +/- 2.6 nM at 4 degrees C in the presence of 0.4 microM Ca2+. The affinity, and not the density, of the devapamil-binding site was decreased by lowering the pH from 8.5-6.5, or by increasing the Ca2+ concentration from 0.4 microM to 100 mM. The same results were obtained at 30 degrees C, although the ligand-receptor complex was not stable at Ca2+ concentrations below 10 microM. These binding data were confirmed by kinetic experiments. The rate constants calculated for a pseudo-first-order and a second-order reactions were identical and yielded fourfold lower k-1/k+1 (KD) values than the equilibrium experiments performed using 1 nM and 0.4 microM Ca2+, but the same values using 1 mM Ca2+. 1 mM Ca2+ increased the k-1/k+1 (KD) by decreasing 10-fold the association rate at 4 degrees C. The dissociation rate was increased about 10-fold by 5 microM devapamil or 100 microM D-cis-diltiazem, suggesting that the high affinity site is negatively regulated allosterically by millimolar Ca2+ concentrations and by the occupation of a second low-affinity site. Incubation of the CaCB receptors in the absence of Ca2+ and devapamil at 30 degrees C, but not at 4 degrees C, resulted in an apparent loss of devapamil-binding sites. The decrease in binding sites was caused by a reduced affinity. This apparent loss of binding sites was prevented by the addition of Ca2+ with an apparent median effective concentration of 0.4 microM. The apparent half-maximal inactivation times of the devapamil-binding site were 90 s and 12 min in the presence of 1 nM and 0.4 microM Ca2+, respectively. These results show that micromolar Ca2+ concentrations stabilize the CaCB receptor in a conformation which allows high-affinity binding of phenylalkylamines. Millimolar Ca2+ concentrations induce a low-affinity state of the devapamil-binding site on a stable CaCB receptor.

Binding Ca2+ to intracellular or to extracellular sites of dihydropyridine receptor of rabbit skeletal muscle discriminates between in vitro binding of Ca2+-channel agonist and antagonist

Transverse tubule membrane vesicles contain dihydropyridine receptor of rabbit skeletal muscle in an insideout orientation. Digitonin-solubilized, purified dihydropyridine receptor is embedded in digitonin vesicles in an outside-out orientation. Ca2+ selectively stimulates binding of the Ca2+-channel antagonist [3H]PN200-110 to dihydropyridine receptor in the outside-out but not the inside-out orientation. The dissociation constant for binding Ca2+ to the extracellular Ca2+-specific binding site of dihydropyridine receptor is 2-3 microM. The data demonstrate that binding Ca2+ to the extracellular high-affinity Ca2+-binding site is required for binding dihydropyridines to dihydropyridine receptor. This binding is inhibited, however, by 1-10 mM concentrations of any divalent cation tested (Ba2+, Mn2+, Mg2+). Also, Ca2+ selectively stimulates binding of the Ca2+-channel agonist [3H]BayK8644 to dihydropyridine receptor in the inside-out orientation. The titration of this Ca2+ dependence indicates that the dissociation constant for binding Ca2+ to the intracellular Ca2+-specific binding site of dihydropyridine receptor is in the millimolar range. Thus, binding Ca2+-channel agonist or antagonist to dihydropyridine receptor is modulated by binding Ca2+ to different sites of the receptor. Measurements of dissociation rate constants for binding [3H]PN200-110 to dihydropyridine receptor in the presence of diltiazem, verapamil and/or Ca2+ indicate that Ca2+, like diltiazem or verapamil, is an allosteric effector of this receptor.

Role of the sodium pump and the background K+ channel in passive K+(Rb+) uptake by isolated cardiac sarcolemmal vesicles

A simple procedure was developed for the isolation of a sarcolemma-enriched membrane preparation from homogenates of bullfrog (Rana catesbeiana) heart. Crude microsomes obtained by differential centrifugation were fractionated in Hypaque density gradients. The fraction enriched in surface membrane markers consisted of 87% tightly sealed vesicles. The uptake of 86Rb+ by the preparation was measured in the presence of an opposing K+ gradient using a rapid ion exchange technique. At low extravesicular Rb+ concentrations, at least 50% of the uptake was blocked by addition of 1 mM ouabain to the assay medium. Orthovanadate (50 microM), ADP (2.5 mM) or Mg (1 mM) were also partial inhibitors of Rb+ uptake under these conditions, and produced a complete block of Rb+ influx in the presence of 1 mM ouabain. When 86Rb+ was used as a tracer of extravesicular K+ (Rb+0 less than or equal to 40 microM, K+0 = 0.1-5 mM) a distinct uptake pathway emerged, as detected by its inhibition by 1 mM Ba2+ (K0.5 = 20 microM). At a constant internal K+ concentration (K+in = 50 mM), the magnitude of the Ba2+-sensitive K+ uptake was found to depend on K+0 in a manner that closely resembles the K+ concentration dependence of the background K+ conductance (IK1) observed electrophysiologically in intact cardiac cells. We conclude that K+ permeates passively this preparation through two distinct pathways, the sodium pump and a system identifiable as the background potassium channel.

Effect of divalent cation chelation on dihydropyridine binding in isolated cardiac sarcolemma vesicles

The effect of divalent cation chelation on specific nitrendipine and ouabain binding has been determined in a highly enriched sarcolemma preparation isolated from canine ventricle. Maximal high-affinity nitrendipine binding measured in the absence of added calcium or magnesium was 997 +/- 103 fmol/mg protein. Nitrendipine binding in the presence of EDTA significantly decreased to 419 +/- 42 fmol/mg protein (P less than 0.001) which equates to 42.0% of control. The simultaneous presence of EDTA and A23187 in the binding buffer resulted in a decrease in nitrendipine binding to below detectable levels. These results suggest that divalent cations trapped within vesicles can support high affinity nitrendipine binding. Evaluation of dihydropyridine binding at various pH values suggested that the loss of binding below pH 7.0 and above pH 8.0 may result indirectly from a change in divalent cation binding rather than a direct effect on dihydropyridine binding per se. The maximal binding of ouabain determined in the presence of magnesium and inorganic phosphate averaged 340 +/- 7.4 pmol/mg protein. Pre-treatment of the preparation with sodium dodecyl sulfate (SDS) in order to express binding in sealed inside-out (IO) vesicles, increased ouabain binding to 471 +/- 27 pmol/mg protein. Thus, these preparations averaged 27.8% sealed IO vesicles. Addition of EDTA in the absence of magnesium in the binding buffer reduced ouabain binding to 204 +/- 7.7 and 11.7 +/- 3.5 pmol/mg protein in control and SDS-treated preparations, respectively. These findings suggest that this sarcolemma preparation consists of 43.6% sealed right-side-out (RO) vesicles which contain sufficient endogenous divalent cation trapped in the intravesicular space, to support ouabain binding. The correspondence between the percentage of ouabain binding that remains in the presence of EDTA and the percentage of nitrendipine binding observed under the same conditions is consistent with the hypothesis that divalent cations support nitrendipine binding by interaction with a site or sites accessible only from the cytoplasmic membrane surface and that nitrendipine and ouabain binding sites occur in the same vesicles (i.e., the nitrendipine binding site is of sarcolemma origin).

Bradykinin-induced increases in cytosolic calcium and ionic currents in cultured bovine aortic endothelial cells

The goal of the present study was to determine if voltage-sensitive calcium channels are present in bovine aortic endothelial cell plasmalemma and if they contribute to the rise in cytosolic calcium produced by bradykinin. After bradykinin (100 nM) exposure, endothelial cell associated fura-2 fluorescence peaked within 10-20 seconds and then declined to a steady level 2- to 3-fold above resting values. Pretreatment with lanthanum (20 microM) abolished the steady level produced by bradykinin but had little effect on the initial, transient rise in cytosolic calcium. Chelation of extracellular calcium with EGTA before addition of bradykinin resulted in a substantial decrease in the fura-2 transient and elimination of the long-lasting component. Nimodipine (3 microM) and nitrendipine (1 microM) were without effect on either phase of the bradykinin-induced response. Moreover, elevation of extracellular potassium failed to produce a rise in intracellular calcium. With the use of the tight seal technique to voltage clamp the cells, inwardly rectifying and calcium-activated potassium currents were found to exist in the endothelial cells. Addition of bradykinin (100 nM) elicited a calcium-activated potassium current that was eliminated in the absence of intracellular potassium. No voltage-sensitive calcium currents were activated when the cells were exposed to 10 mM or 110 mM calcium chloride in the presence or absence of bradykinin. The binding of [3H](+)PN200-110 to endothelial cell membrane preparations was 1-3 orders of magnitude lower than that observed in PC-12, GH3, or BC3H1 cell membranes.(ABSTRACT TRUNCATED AT 250 WORDS)

Inhibition of myocardial Ca2+ channels by three dihydropyridines with different structural features: potential-dependent blockade by Ro 18-3981

Inhibition of myocardial Ca2+ channels was investigated for three dihydropyridines with different structural features: Ro 18-3981, darodipine (PY 108-068) and nifedipine. Ro 18-3981 contains a sulphamoyl acetyl side-chain. In voltage-clamps experiments with isolated cardiac myocytes of guinea-pig, Ro 18-3981 caused a concentration-dependent inhibition of the Ca2+ current, which was influenced by the membrane holding potential. A markedly greater inhibition by Ro 18-3981 was observed when myocytes were depolarized (to +10 mV) from a holding potential (Vh) of -20 mV (IC50 = 2.3 nm) than at -50 mV (IC50 = 100 nM). The three dihydropyridines caused a concentration-dependent reduction in contractile force of isolated, electrically-stimulated left atria of the guinea-pig. Elevation of the extracellular K+ concentration from 5.9 to 24 mM resulted in a significant reduction in negative inotropic IC50 values for Ro 18-3981 (137 fold), darodipine (8 fold) and nifedipine (20 fold). The affinity of these drugs for the high-affinity (+)-[3H]-PN 200-110 binding site was determined in guinea-pig cardiac membranes. The KD value of Ro 18-3981 (1.0 nM) was similar to the IC50 value for blockade of ICa at a Vh of -20 mV (2.3 nM), i.e. at a level of near-maximal depolarization. Thus, structurally-different dihydropyridines exert potential-dependent inhibition of myocardial Ca2+ channel activity consistent with the modulated receptor hypothesis. These results demonstrate that blockade of myocardial excitation-contraction coupling by Ca2+ entry blockers is also potential-dependent.

Multiple calcium channels and neuronal function

Recent investigations have demonstrated that neurons have a number of different types of calcium channels, each with their own unique properties and pharmacology. These calcium channels may be important in the control of different aspects of nerve activity. Some of the possibilities can now be discussed.