Thermodynamic and kinetic aspects of agonist and antagonist binding to 1,4-dihydropyridine receptors

Molecular Basis for Ligand Modulation of a Mammalian Voltage-Gated Ca2+ Channel

The L-type voltage-gated Ca²⁺ (Ca_v) channels are modulated by various compounds exemplified by 1,4-dihydropyridines (DHP), benzothiazepines (BTZ), and phenylalkylamines (PAA), many of which have been used for characterizing channel properties and for treatment of hypertension and other disorders. Here, we report the cryoelectron microscopy (cryo-EM) structures of Ca_v1.1 in complex with archetypal antagonistic drugs, nifedipine, diltiazem, and verapamil, at resolutions of 2.9 Å, 3.0 Å, and 2.7 Å, respectively, and with a DHP agonist Bay K 8644 at 2.8 Å. Diltiazem and verapamil traverse the central cavity of the pore domain, directly blocking ion permeation. Although nifedipine and Bay K 8644 occupy the same fenestration site at the interface of repeats III and IV, the coordination details support previous functional observations that Bay K 8644 is less favored in the inactivated state. These structures elucidate the modes of action of different Ca_v ligands and establish a framework for structure-guided drug discovery.

1alpha,25(OH)2 vitamin D3 actions on ion channels in osteoblasts

Membrane-initiated cellular responses to steroids include modulation of ion channel activities via signal transduction pathways. However, the molecular mechanisms involved in nongenomic actions remain only partially understood. Our research has focused on the rapid effects of 1alpha,25(OH)(2) Vitamin D(3) [1,25D] on L-type Ca(2+) [L-Ca] and DIDS-sensitive Cl(-) channels in osteoblasts. Physiological nanomolar concentrations of hormonally active 1,25D promote rapid (1-5 min) potentiation of outward Cl(-) currents in osteosarcoma ROS 17/2.8 cells and mouse primary osteoblasts. In addition, 1,25D increases inward barium currents through L-Ca channels at low depolarizing potentials within seconds in a fashion similar to the 1,4-dihydropyridine [DHP] agonist Bay K8644. We found that second messenger cAMP is involved in 1,25D potentiation of Cl(-) and Ca(2+) channels. Nongenomic 1,25D effects on ion channel activities in osteoblasts appear to involve different mechanisms that include a possible direct interaction with the L-Ca channel molecule, on one hand, and signaling through the cAMP pathway, on the other. Rapid 1,25D actions on Cl(-) and Ca(2+) currents seem to couple to secretory activities in osteoblasts, thus contributing to bone mass formation.

Characterization of calcium channel binding

Voltage-dependent calcium channels are expressed in a variety of tissues including heart, muscles and brain. Saturation binding of a radioligand to the calcium channel is commonly used to characterize the expression level of the channel protein. Compound competition binding assay is a conventional screening method to determine the affinity of unlabeled compounds for the channel protein. This unit provides detailed experimental methods for two types of radioligand binding assays using [³H]PN200-100 and [¹²⁵I](conotoxin MVIIA. Voltage-dependent calcium channels are expressed in a variety of tissues including heart, muscles and brain.

Functional interaction between benzothiazepine- and dihydropyridine binding sites of cardiac L-type Ca2+ channels

We have previously shown, in a radioligand binding study with single ventricular myocytes, that benzothiazepine and dihydropyridine binding sites interact with each other. To further examine whether this interaction between the two binding sites is reflected in the function of L-type Ca2+ channels, the blocking action of diltiazem, nitrendipine, and the combination of these two drugs on L-type Ca2+ channel currents was investigated using baby hamster kidney cells expressing the alpha 1C, alpha 2/delta, beta and gamma subunits of the Ca2+ channel. The effects of diltiazem and nitrendipine were additive at room temperature but synergistic at 33 degrees C. The use-dependent block by 3 microM of diltiazem was significantly enhanced from 28% to 68% by addition of 30 nM of nitrendipine, which by itself did not have a blocking effect. Thus, we conclude that benzothiazepine- and dihydropyridine binding sites interact and potentiate their blocking action on L-type Ca2+ channels in a temperature-dependent manner.

Identification of L-type calcium channels associated with kappa opioid receptors in human placenta

Transduction pathways of kappa receptor activation are not fully understood. Human placenta at term expresses only this type of opioid receptors and therefore offers a unique advantage for such investigations. It has previously been postulated that kappa receptors-mediated modulation of acetylcholine and placental lactogen release from human placentas require the influx of extracellular calcium and into the cells, possibly via voltage-dependent channels. We report here that another opioid-regulated placental function, the release of human chorionic gonadotropin (hCG), depends on extracellular calcium and the modality of its influx via L-type channels. Data presented demonstrated that the stimulation of hCG secretion by the kappa-selective agonist U69,593 was abolished in presence of either EGTA or the calcium channel blocker nifedipine. Results obtained on the combined effect of opioids and dihydropyridines indicated that placental kappa opioid receptors could be directly coupled to L-type calcium channels. The identification of the latter in villus membrane preparations, reported here for the first time, further contributes to the hypothesis that, in human placenta, kappa receptors-linked transduction mechanisms involve calcium and its conductance across villus membranes.

Characterization of the binding of [3H]SR 33805 to the slow Ca2+ channel in rat heart sarcolemmal membrane

SR 33805 is a representative of a new class of compounds (indole sulfone) that inhibits L-type Ca2+ channels. [3H]SR 33805 has been shown to bind with a high affinity (Kd approximately 20 pM calculated from saturation isotherms and association/dissociation kinetics) to a single site in a purified preparation of rat cardiac sarcolemmal membranes. The binding was found to be saturable and reversible. The maximal binding capacity was in approximately 1:1 stoichiometry with that of other Ca2+ channel antagonists. Various cations (Na+, Ca2+, Cd2+, and La3+) were shown to inhibit specific [3H]SR 33805 binding, with La3+ being the most potent. Using a range of receptor or channel ligands (including omega-conotoxin and Na+ and K+ channel modulators), only the L-type Ca2+ channel antagonists were found to displace [3H]SR 33805. However, dihydropyridines, phenylalkylamines, benzothiazepines, and diphenyl-butylpiperidines were found to inhibit [3H]SR 33805 in a non-competitive manner as demonstrated by displacement experiments in addition to dissociation kinetics. In contrast, the interaction of SR 33805 with fantofarone has been found to be competitive. Binding of [3H]SR 33805 (and [3H]fantofarone) is entropy driven as opposed to that of the [3H]nitrendipine which is enthalpy driven. From these results we suggest that SR 33805 binds with a high affinity to a unique site on the L-type Ca2+ channel found in rat cardiac sarcolemmal membranes. This site is equivalent to that of fantofarone and in allosteric interaction with that of the dihydropyridines, phenylalkylamines and benzothiazepines.