DHP receptor

FIGURE 17.26 The o i-subuiiit of the t-tubule Ca" chainiel/DHP receptor contains six peptide segments that may associate to form the Ca" channel. This Ca" channel polypeptide is homologous with the voltage-sensitive Na channel of neuronal tissue. 

For reasons that are not yet clear, skeletal muscle transverse (T)-tubule membranes contain 50-100-fold more high affinity DHP receptors than any other source yet identified [43,45]. Transverse tubule membranes contain 30-70 pmol/mg protein of DHP receptors that bind [ H]PN 200-100 with a of 0.1-0.2nM. The strategy utilized for the purification of L-type channels was similar to that used for the purification of other high affinity ligand binding proteins, and its success was predicted from the prior use of such an approach for the purification of other ion channels [54,55]. Thus the L-type channels were purified as high affinity DHP receptors, with the anticipation that the purified component(s) would constitute functional Ca channels. 

Reconstitution of Ca channels Since L-type Ca channels were purified as high affinity DHP receptors, it was important to demonstrate that the purified preparations could act as functional DHP-sensitive Ca channels. Purified DHP receptor preparations from skeletal... 

The recent studies with the skeletal muscle channels reconstituted in bilayers also established that an intrinsic property of the skeletal muscle Ca channels is that they possess a very low probability of opening. Even under optimal conditions of voltage, etc., these channels open only 8% of the time [95]. This previously unappreciated characteristic of these channels provides an alternative explanation to an earlier study in which it was proposed that only 5% of the DHP receptors in skeletal muscle can form functional channels [91]. In that study a probability of opening of 1.0 was assumed. However, if one adjusts this to 0.08, then the very different conclusion is that all the DHP receptors can make functional channels, but that they open only a very small percentage of the time. 

A more detailed analysis of ligand binding is possible by determination of the association rate constant ( on-kinetics ) of the dissociation rate constant ( off-kinetics ). The methodology of these estimations is illustrated for the cardiac DHP receptor. 

Jurkat-Rott K, Uetz U, Pika-Hartlaub U, Powell J, Fontaine B, Melzer W, Lehmann-Horn F (1998) Calcium currents and transients of native and heterologously expressed mutant skeletal muscle DHP receptor alphal subunits (R528H). FEBS Lett 423 198-204. 

Lerche H, Klugbauer N, Lehmann-Horn F, Hofmann F, Melzer W (1996) Expression and functional characterization of the cardiac L-type calcium channel carrying a skeletal muscle DHP-receptor mutation causing hypokalaemic periodic paralysis. Pflugers Arch 431 461-463. 

Sipos I, Jurkat-Rott K, Harasztosi C, Fontaine B, Kovacs L, Melzer W, Lehmann-Horn F(1995) Skeletal muscle DHP receptor mutations alter calcium currents in human hypokalaemic periodic paralysis myotubes. J Physiol 483 ( Pt 2) 299-306. 

The very small data set restricted generalizable conclusions, but according to the authors the results suggested that membrane solubility may constitute the critical factor that allows the specific binding to the DHP-receptor . 

Figure 7.33. Proposed mechanism for excitation-thermogenic coupling in the ocular heater organ of swordfish. Neural stimulation mediated through the T tubule voltage sensor (the dihydropyridine [DHP] receptor) may lead to opening of the Ca2+ release channel. Large amounts of Ca2+ pass from the SR into the sarcoplasm. Ca2+ entering the mitochondria may stimulate ATP synthesis. Much of the ATP produced is used by the SR Ca2+ ATPase (calsequestrin) to pump Ca2+ back into the SR. This cycling of Ca2+ between the SR and sarcoplasm represents a futile cycle. See text for additional details. (Modified after Block, 1991.)...

Conderas X, Bennasser Y, Chazal N, Moreau M, Leclerc C, Tkaczuk J, Baliiaoui E (2005) Human immunodeficiency virus type 1 Tat protein induces an intracellular calcium increase in human monocytes drat requires DHP receptors hivolvement in TNF-alpha pro-ducdon. Virology 332 316-328. 

On the other hand, DHPs can be activators (i.e.. Bay K 8644) or inhibitors (i.e., nifedipine or nitrendipine) and, therefore, are thought to act allosterically to shift the channel toward the open or closed state, rather than by occluding the ion-conducting pore. Their receptor sites consist of amino acids located in the S6 segments of domains III and IV and the S5 segment of domain III [108,110,111]. Interestingly, the DHP receptor site shares some common amino acids with the PAA receptor site. Finally, BTZs (diltiazem and related compounds) bind to a third receptor site, but the amino acids that are required for their interaction overlap also those required for PAA binding [112,113]. 

Benzothiazepines, such as diltiazem, have a specific interaction site on the ai subunit, which is supposed to be allosterically linked to the DHP receptor on the calcium... 

---