Binding Blockers

In addition to halopeiidol, the putative neuroleptics, limcazole (311), lemoxipiide (312), and gevotioline (313) bind to (7-ieceptois as does the dopamine uptake blocker, GBR 12909 (314) and two ligands active at the NMDA receptor, ifenprodil (315) and CNS 1102 (316). NPC 16377, (317) is a selective (7-teceptor ligand. MAO inhibitors and antidepressants also bind to (7-teceptors. Some evidence indicates that (7-teceptors in the brain are in fact a form of cytochrome which may account for the diversity of ligands interacting with (7-sites. 

Dihydropyridine receptor (DHPR) is a member of voltage-dqiendent Ca2+ channels (CaVi, L-type), which specifically binds to dihydropyridine derivatives, a group of the Ca2+ channel blockers. Cav 1.1 works as the voltage sensor for skeletal muscle contraction, and Cay 1.2, as Ca2+-influx channel for cardiac muscle contraction. 

Molecular biology studies have identified a loop containing 20-25 amino acid residues between S5 and S6 (or Ml and M2, Fig. 2) forming the pore. The G(Y/F) G motif located in the pore represents the K+-selectivity signature, which is common to all K+ channels. The external entry to the channel pore and its adjacent residues constitute binding sites for toxins and blockers. The internal vestibule of the pore and the adjacent residues in S5 and S6 contribute to binding sites for compounds such as 4-aminopyiidine and quinidine. The S4-S5 linker lies close to the permeation pathway and is required for... 

The LVA ai subunits are blocked by moderate to low (10 pM) concentrations of nickel and bind the channel blocker mibefradil and kurotoxin. Both compounds are not specific LVA channel blockers because they block also Cavl. x and Cav2.x channels at about tenfold higher concentration. Interestingly, the endogenous cannabinoid anandamide binds to LVA channels and stabilises the inactivated state. This effect decreases T-type calcium current and neuronal firing activities. 

Butyrophenones Butyrylcholinesterase C2 Domain C Kinase C-reactive Protein Ca2+-ATPase Ca2+-binding Proteins Ca2+ Channel Ca2+ Channel Antagonists Ca2+ Channel Blockers... 

Figure 8. Simultaneous measurement of intracellular Ca and oxidant production in neutrophils. Cells were labeled with Quin-2 and suspended at 2 x lo cells/mL buffer. At time zero, 1 nJf FLPEP was added (upper trace in each panel). In addition, the receptor blocker tBOC was added (3 x 10" M) after 30 s to stop further binding of the stimulus (lower trace in each panel). The excitation wavelength was 3A0 nm. Top panel Quin-2 fluorescence determined on channel B (of Figure 1) using a Corion A90-nm interference filter. The crossover from the superoxide assay has been subtracted. Middle panel Oxidant production (superoxide equivalents) determined by the para-hydroxyphenylacetate assay. Fluorescence was observed at AOO nm (on channel A of Figure 1).

Hyaluronic acid is a linear polysaccharide found in the highest concentrations in soft connective tissues where it fills an important structural role in the organization of the extracellular matrix (23,24). It has been used in ophthalmic preparations to enhance ocular absorption of timolol, a beta blocker used for the treatment of glaucoma (25), and in a viscoelastic tear formulation for conjunctivitis (26). The covalent binding of adriamycin and daunomycin to sodium hy-aluronate to produce water-soluble conjugates was recently reported (27). 

Another possible target for toxins are the receptors for neurotransmitters since such receptors are vital, especially for locomotion. In vertebrates the most strategic receptor is that for acetylcholine, the nicotinic receptor. In view of the breadth of action of the various conotoxins it is perhaps not surprising that alpha-conotoxin binds selectively to the nicotinic receptor. It is entirely possible that similar blockers exist for the receptors which are vital to locomotion in lower species. As mentioned previously, lophotoxin effects vertebrate neuromuscular junctions. It appears to act on the end plate region of skeletal muscle (79,59), to block the nicotinic receptor at a site different from the binding sites for other blockers (81). 

The NRl family is composed of one subunit with nine different alternatively spliced variants. Block by NMDA channel blockers such as ketamine, MK-801 and phencyclidine is affected by which splice variant of the NRl subunit is involved, probably because the NRl splice variant affects the kinetics of channel activation (the effectiveness of any channel blocker being dependent on how much the channel is open). The glycine binding site is on the NRl subunit and the glutamate binding site is on the NR2 and NR3 subunits. 

I have noted that NPPB is structurally related to loop diuretics of the furosemide (Fig. 2) type. These latter compounds bind to the Na 2CNK -cotransporter [16] and inhibit NaCl reabsorption in the TAL segment and NaCl secretion in epithelia such as the colonic crypt cell and rectal gland of Squalus acanthias [15]. We were able to show that only minor modification of the NPPB molecule on one side and of furosemide on the other led to compounds with altered selectivities [70,91-93]. One prototype of an intermediate blocker, i.e., a substance blocking both Na 2Cl K -cotransport and CP-channels, is torasemide (Fig. 2). Hence we have performed a systematic study in order to define the constraints defining the effectiveness of this class of substances [91]. 

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