Interaction between calcium channel ligands and calcium channels

Area of Science:

• Pharmacology
• Molecular Biology
• Biochemistry

Background:

• Voltage-dependent calcium channels (VDCCs) are crucial for cellular functions and are targets for various drugs.
• Distinct drug receptors for 1,4-dihydropyridines, phenylalkylamines, and benzothiazepines are known to exist on VDCCs.
• These receptors exhibit reciprocal allosteric communication and are linked to calcium binding sites.

Purpose of the Study:

• To determine the molecular size of the 1,4-dihydropyridine receptor in different tissues.
• To compare the localization of different drug receptor sites on VDCCs.
• To clarify the molecular basis of drug interactions with calcium channels.

Main Methods:

• Radiation inactivation was used to measure the size of the 1,4-dihydropyridine receptor.
• Photoaffinity labeling with tritiated enantiomers of [3H]azidopine was employed to label receptors in skeletal muscle and cardiac membranes.
• Antibodies against the skeletal muscle channel were used to precipitate labeled polypeptides.

Main Results:

• The 1,4-dihydropyridine receptor size was consistently measured at 180 kDa by radiation inactivation across different tissues.
• Photoaffinity labeling identified a 155 kDa polypeptide as the primary site for (-)-[3H]azidopine binding in both skeletal and cardiac muscle.
• A 34 kDa band, previously misidentified as a binding site, was found to have low affinity and was not related to the channel function.

Conclusions:

• All three distinct drug receptor sites (1,4-dihydropyridines, phenylalkylamines, d-cis-diltiazem) are located on a single 155 kDa polypeptide of the voltage-dependent calcium channel.
• This 155 kDa polypeptide is the primary target for these drug classes, explaining their allosteric interactions.
• Previous studies may have yielded erroneous results due to the presence of the low-affinity 34 kDa binding site.