Calcium channel L-type

Protein kinase A (PKA) is a cyclic AMP-dependent protein kinase, a member of a family of protein kinases that are activated by binding of cAMP to their two regulatory subunits, which results in the release of two active catalytic subunits. Targets of PKA include L-type calcium channels (the relevant subunit and site of phosphorylation is still uncertain), phospholam-ban (the regulator of the sarcoplasmic calcium ATPase, SERCA) and key enzymes of glucose and lipid metabolism. 

A large number of diugs interfere with the smooth muscle contraction. These compounds lower blood pressure and are referred to as antihypertensive. In this section, only those coumpounds will be mentioned that have a direct effect on smooth muscle tone. Phenylephrine is an agonist on most smooth muscles and activates ax adrenoceptors. Carbachol is an agonist on some smooth muscles and activates contraction through muscarinic receptors. Blockers of the ax-adrenoceptors such as prazosin and urapidil are competitive inhibitors of the ax-receptor in vascular and bladder smooth muscle. Phenoxybenzamine is an ineversible blocker of ax receptors and phentol-amine blocks ax and a2 receptors. Ca2+ channel blockers such as the dihydropyiidines, phenylalkyla-mines and benzothiazepines lower smooth muscle tone by blocking the L-type calcium channel. 

Voltage-dependent Ca2+ channels are a family of multi-subunit complexes of five proteins responding to membrane depolarisation with channel opening allowing the influx of calcium into a cell. Voltage-dependent calcium channels are subdivided into three subfamilies the HVA DHP-sensitive L-type calcium channels, the HVA DHP-insensitive calcium channels and the LVA T-type calcium channels [2]. 

Coetzee and Opie (1988, 1992) have suggested that the cellular calcium overload induced by oxidant stress is mediated by an increase in calcium influx through the L-type calcium channel. However, a number of other studies, using different radical-generating systems, experimental conditions and species, have described no significant effects on the calcium channel over the period of exposure necessary to induce cellular calcium overload (Bhatnagar etal., 1990 Shattock etal., 1990 Beresewicz... 

L-type calcium channels (voltage-gated calcium channels L-subtype) Similarity to Diltiazem and a second ligand. ZINC db ( 50 K commercially available subset screened but most filtered to achieve desired PK profile using VolSurf). SHOP similarity, and feature-presence filtering down to 36 compounds 7 hits 18 tested, active in a vasorelaxant assay and some had novel structures. [67]... 

Veng LM, Mesches MH and Browning MD (2003). Age-related working memory impairment is correlated with increases in the L-type calcium channel protein alD (Cavl.3) in area CA1 of the hippocampus and both are ameliorated by chronic nimodipine treatment. Molecular Brain Research, 110, 193-202. 

The two most frequently studied compounds with T-type calcium channel antagonist properties are ethosuximide 1 and mibefradil 3. However, the modest potency of ethosuximide ( 200 pM) [48] and the poor selectivity of mibefradil [49] make these compounds suboptimal tools for the investigation of these channels. Guided by a pharmacophore model [50], several analogs of 3 were prepared. Compound 4 represents the most potent compound identified (IC50 8 nM, patch-clamp assay) with good selectivity over the L-type calcium channel [51], Compound 4 showed a modest brain-to-plasma ratio (0.25) after oral dosing to rats at 50 mg/kg. However, no in vivo efficacy assay results have been reported with this compound. 

Table 7 Modulation of hERG and L-type calcium channel activity following fluorine substitution...

L-type Ca2+ channels—L-type calcium channels (in muscles and neurons ... 

Obviously, the effects of tamoxifen and derivatives and of raloxifene on L-type calcium channels from aortic and other blood vessels would reduce vascular smooth muscle contractility. This action, in synergy with the aforementioned effect on BK channels, would reduce blood peripheral resistance and blood pressure, which may partially account for the reduction in cardiovascular risk (Da Costa et al. 2004 Trump et al. 1992) (Fig. 4.1). 

Kitazawa T, Kobayashi S, Horiuti K, Somlyo AV, Somlyo AP 1989 Receptor coupled, permeabilized smooth muscle role of the phosphatidylinositol cascade, G proteins and modulation of the contractile response to Ca2+. J Biol Chem 264 5339-5342 Lopez-Lopez JR, Shacklock PS, Balke CW, Wier WG 1995 Local calcium transients triggered by single L-type calcium channel currents in cardiac cells. Science 268 1042-1045 Marks AR, Fleischer S, Tempst P 1990 Surface topography analysis of the ryanodine receptor/ junctional channel complex based on proteolysis sensitivity mapping. J Biol Chem 265 13143-13149... 

FIGURE 2 Examples of the variety of structures obtained in natural product screens. I, zaragozic acid A, is an inhibitor of mammalian and fungal sterol synthesis, obtained from fungi (48) II, L-696,474, is an inhibitor of the HIV protease, obtained from fungi (51) III, dehydrosoyasaponin I, is an agonist of the calcium-activated potassium channel, obtained from a medicinal plant (58) IV, tetrandrine, is an inhibitor of L-type calcium channels, obtained from a plant (78). 

However, the reverse is not necessarily true all compounds that block the hERG channels do not prolong action potentials. Part of the reason lies in the fact that many compounds have a mixed effect on ion channels, particularly due to the blocking effect on both hERG and the L-type calcium channel [21], which is responsible for phase 2 of the cardiac action potential (Figure 16.1). Examples for such dual-blockers include bepridil, verapamil and mibefradil [22], all blocking hERG and L-type calcium channels at the therapeutic concentrations. However, only verapamil has nearly no cardiac liabilities. 

A next-level assay is usually an isolated heart/cardiac tissue preparation. The canine Purkinje fiber assay (GLP) measures several action potential parameters, like resting membrane potential, upstroke velocity, action potential duration and shape, but also if a drug acts reverse-use dependently [72]. Based on changes of the action potential shape it is possible to conclude which ion channels are modulated (e.g., L-type calcium channel block would abolish the plateau phase). The papillary muscle assay (e.g., guinea pigs) determines similar parameters [73]. 

Splawski, 1., Timothy, KW., Decher, N., Kumar, P., Sachse, F.B., Beggs, A.H., Sanguinetti, M.C. and Keating, M.T (2005) Severe arrhythmia disorder caused by cardiac L-type calcium channel mutations. Proceedings of the National Academy of Sciences of the United States of America, 102, 8089-8096. 

Surface membranes of excitable cells contain a voltage-dependent L-type calcium channel, a membrane-spanning complex of several proteins. This complex is a valuable marker for this membrane type. It binds very specific calcium channel antagonists, such as, for instance, drugs of the dihydropyridine (DHP) type as (-i-)-PN 200-110 or nitrendipine. 

Dolmetsch R, Pajvani U, Fife K, Spotts JM, Greenberg ME (2001) Signaling to the nucleus by an L-type calcium channel-calmodulin complex through the MAP kinase pathway. Science 294 333-339... 

Autonomic receptors further regulate calcium influx through the sarcolemma (Fig. 15.1). (3-Adrenergic stimulation results in the association of a catalytic subunit of a G protein coupled to the (3-receptor. This stimulates the enzyme adenylyl cyclase to convert ATP to cyclic adenosine monophosphate (cAMP). Increasing cAMP production results in a cAMP-dependent phosphorylation of the L-type calcium channel and a subsequent increase in the probability of the open state of the channel. This translates to an increase in transsarcolemmal calcium influx during phase 2 (the plateau phase) of the cardiac muscle action potential. The effects of transient increases in intracellular levels of cAMP are tightly con-... 

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