Stereoselectivity at the Calcium Channel

1,4-Dihydropyridines (DHPs) constitute a major class of ligands for L-type Ca 2؉ channels (LTCC). The DHPs have a boat-like, six-membered ring with an NH group at the stern, an aromatic moiety at the bow, and substituents at the port and starboard sides. Various DHPs exhibit antagonistic or agonistic activities, which were previously explained as stabilization or destabilization, respectively, of the closed activation gate by the portside substituents. Here we report a novel structural model in which agonist and antagonist activities are determined by different parts of the DHP molecule and have different mechanisms. In our model, which is based on Monte Carlo minimizations of DHP-LTCC complexes, the DHP moieties at the stern, bow, and starboard form H-bonds with side chains of the key DHP-sensing residues Tyr_IIIS6, Tyr_IVS6, and Gln_IIIS5, respectively. We propose that these H-bonds, which are common for agonists and antagonists, stabilize the LTCC conformation with the open activation gate. This explains why both agonists and antagonists increase probability of the long lasting channel openings and why even partial disruption of the contacts eliminates the agonistic action. In our model, the portside approaches the selectivity filter. Hydrophobic portside of antagonists may induce long lasting channel closings by destabilizing Ca 2؉ binding to the selectivity filter glutamates. Agonists have either hydrophilic substituents or a hydrogen atom at their portside, and thus lack this destabilizing effect. The predicted orientation of the DHP core allows accommodation of long substituents in the domain interface or in the inner pore. Our model may be useful for developing novel clinically relevant LTCC blockers. 1,4-Dihydropyridines (DHPs)2 form a major class of L-type Ca 2ϩ channel (LTCC) ligands. DHPs can operate as agonists or antagonists depending on their chemical structure. Importantly, activity of some DHP derivatives may shift from agonism to antagonism (and vice versa) upon site-specific mutation of the channel or modified experimental conditions (for reviews, see Refs. 1 and 2). It has been the dual nature of DHP activity (agonism and antagonism), which has made it challenging for interpretation in structural terms. The other major classes of LTCC ligands, the phenylalkylamines and benzothiazepines, are strictly antagonists.Despite a number of studies, the molecular mechanism for the activity of DHPs on LTCC remains unclear. In the present work we have addressed this problem by combining a molecular modeling approach with analyses of relevant published data. We employed a method of multiple Monte Carlo energy minimizations for docking various DHPs in our earlier reported homology model of LTCC (3), which is based on the crystal structure of the KvAP K ϩ channel (4). We constrained our analyses to potential ligand-binding modes that would be consistent with the features of the ligand-channel activity relationship described in published experiments. These include the structure-activity relationship of the m...

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“…Intriguingly, enantiomers of some DHPs, e.g. (R)-and (S)-Bay k 8644 dem-onstrate opposite, antagonistic and agonistic effects on LTCC (13,14).…”

Section: 4-dihydropyridines (Dhps)mentioning

confidence: 99%

Structural Model for Dihydropyridine Binding to L-type Calcium Channels

Tikhonov

Zhorov

2009

Journal of Biological Chemistry

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“…Wholeplant studies indicate that cytokinin-modulated Ca2+ entry takes place via dihydropyridine (DHP)-sensitive channels on the plasma membrane (13). DHPs are organic compounds that modulate Ca2+ movement through voltage-dependent Ca2+ channels in the plasma membrane of animal cells (14); DHP agonists preferentially maintain the open state and DHP antagonists maintain the closed state of the channel (15,16).…”

mentioning

confidence: 99%

Cytokinin stimulates dihydropyridine-sensitive calcium uptake in moss protoplasts.

Schumaker

Gizinski

1993

Proc. Natl. Acad. Sci. U.S.A.

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Ca2 influx through dihydropyridine (DHP)-sensitive Ca2+ channels is thought to be an early event in cytokinin-induced bud formation in moss protonema because DHP antagonists inhibit bud formation in the presence of cytokinin and DHP agonists stimulate bud formation in the absence of cytokinin [Conrad, P. A. & Hepler, P. K. (1988) Plant Physiol. 86, 684-687]. In the present study, we established the presence of a DHP-sensitive Ca2+ transport system by measuring 4-Ca2+ influx into moss protoplasts. Ca2+ influx was stimulated by external KCI (up to 5 mM), indicating that transport is voltage-dependent. K+-induced Ca2+ influx was DHP-sensitive with >50% inhibition at 500 nM nifedipine. Ca2+ influx was stimulated by increasing concentrations of the DHP Ca2+ channel agonist Bay K8644 with half-maximal effects at 25 nM; this stimulation was seen only in the absence of K+, suggesting that the agonist works preferentially on polarized membranes. Ca2+ influx was also inhibited by phenylalkylamines (verapamil) and benzothiazepines (diltiazem). The phytohormone 6-benzylaminopurine consistently stimulated Ca2+ influx with a Km value of 1 nM, whereas adenine, indoleacetic acid, and gibberellic acid had no effect on Ca2+ transport. The cytokinins kinetin and trans-zeatin caused a greater stimulation of Ca2+ influx and induced more bud formation than did 6-benzylaminopurine. These results indicate that Ca2+ is taken up into moss protoplasts through voltage-dependent DHP-sensitive Ca2+ channels on the plasma membrane and that one of the cytokinin effects in the induction of bud formation is regulation of this plasma membrane Ca2+ channel. an adenine-type cytokinin (8), and subsequently, a small initial cell is formed (6). Asymmetric division of the initial cell leads to bud formation and a change from the twodimensional filamentous protonemata to the three-dimensional "leafy" gametophore.Cytokinin applied to caulonema cells causes profuse premature bud formation (6), and localized increases in Ca2+ precede this cytokinin-induced cell division (9-13). Wholeplant studies indicate that cytokinin-modulated Ca2+ entry takes place via dihydropyridine (DHP)-sensitive channels on the plasma membrane (13). DHPs are organic compounds that modulate Ca2+ movement through voltage-dependent Ca2+ channels in the plasma membrane of animal cells (14) by fluorescent tubes. Petri dishes containing appropriately supplemented basal medium overlaid with sterile cellophane were inoculated with spore suspensions. To prepare spore suspensions, mature sporophytes were harvested, and capsules were sterilized by soaking in 3% (wt/vol) sodium hypochlorite/0.1% Triton X-100 for 10 min followed by five rinses in sterile distilled water. Capsules were opened with sterile forceps, and the spores were dispersed in sterile distilled water; plates were inoculated with 1 ml of spore suspension (104 viable spores per ml).To isolate protoplasts, protonemata were incubated with 2% (wt/vol) driselase (laminarinase, xylanase and cellulase,

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“…It is certain that mepirodipine, a single stereo isomer with (+)-(S)-(S) conformation, scarcely depressed cardiac contractility even when intraarterially administered. In this con nection, it is of interest that amongst the dihy dropyridines, there are Ca channel activators such as Bay k 8644 and Compound 202-791 whose stereoisomers showed strictly opposite actions (18,19). Hof et al (18) induced and potentiated the responses to KCI de polarization at low concentrations, but at high concentrations, they relaxed the tissue con tracted maximally at lower concentration of the same drugs (19).…”

Section: Resultsmentioning

confidence: 99%

“…In this con nection, it is of interest that amongst the dihy dropyridines, there are Ca channel activators such as Bay k 8644 and Compound 202-791 whose stereoisomers showed strictly opposite actions (18,19). Hof et al (18) induced and potentiated the responses to KCI de polarization at low concentrations, but at high concentrations, they relaxed the tissue con tracted maximally at lower concentration of the same drugs (19). These reports prompted us to elucidate whether the enantiomers of mepirodipine have a Ca channel activating property, since mepirodipine is a single stereo isomer with (+)-(S)-(S) conformation (11).…”

Section: Resultsmentioning

confidence: 99%

Reconsideration of vascular selectivity of dihydropyridine calcium antagonists: Comparison of cardiovascular profile of mepirodipine, a novel dihydropyridine consisting of a single stereoisomer with (+)-(S)-(S) conformation, with those of nifedipine and nicardipine.

Motomura¹,

Hashimoto²

1990

Jpn.J.Pharmacol

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Abstract-The coronary vasodilator effect and negative chronotropic, inotropic and dromotropic effects of mepirodipine, the single stereoisomer of a novel dihydro pyridine Ca antagonist, were compared with those of nifedipine and nicardipine using canine isolated, blood-perfused heart preparations. Drugs were injected into each nutrient artery.In the sinoatrial node preparation, the dose producing a 15% decrease in sinoatrial rate was 1.3, 6.5 and 2.5 /cg for mepirodipine, nifedipine and nicardipine, respectively.In the papillary muscle preparation, the dose causing a 50% decrease in developed tension was 44, 6.5 and 54 /cg for mepirodipine, nifedipine and nicardipine, respectively.The dose causing a 50% increase in blood flow through the anterior septal artery was 0.26, 0.18 and 2.0 /Ig for mepirodipine, nifedipine and nicardipine, respectively, while the time required for return to half maximum at the above dose was 13.1, 1.8 and 4.1 min, respectively.In the atrioventricular node preparation, the dose producing a 50% increase in AH interval was 1.6, 2.4 and 3.7 ,cg for mepirodipine, nifedipine and nicardipine, respectively. These results indicate that mepirodipine is a potent and long-lasting dihydropyridine Ca antagonist, whose vascular selectivity is highest against cardiac contractility, but less selective against sinoatrial node automaticity and atrioventricular nodal conduction, compared with nifedipine.

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Stereoselectivity at the Calcium Channel

Cited by 204 publications

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Structural Model for Dihydropyridine Binding to L-type Calcium Channels

Structural Model for Dihydropyridine Binding to L-type Calcium Channels

Cytokinin stimulates dihydropyridine-sensitive calcium uptake in moss protoplasts.

Reconsideration of vascular selectivity of dihydropyridine calcium antagonists: Comparison of cardiovascular profile of mepirodipine, a novel dihydropyridine consisting of a single stereoisomer with (+)-(S)-(S) conformation, with those of nifedipine and nicardipine.

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