Molecular pharmacology of the calcium channel: evidence for subtypes, multiple drug-receptor sites, channel subunits, and the development of a radioiodinated 1,4-dihydropyridine calcium channel label, [125I]iodipine

J Cardiovasc Pharmacol. 1984;6 Suppl 4:S608-21.

Abstract

Radiolabeled Ca2+ antagonists (1,4-dihydropyridines, verapamil, and D-cis-diltiazem) were used to study voltage-operated Ca2+ channels in different excitable tissues. The concept of three subtypes of Ca2+ channels, represented by brain, heart, and skeletal-muscle isoreceptors for 1,4-dihydropyridines, is developed. The three subtypes are characterized by a variety of criteria. Despite the biochemical differences between the subtypes, they have the same Mr in situ by target-size analysis (Mr approximately equal to 180,000, when evaluated by [3H]nimodipine). The concept of the metalloprotein nature of the channel and the interaction of channel drugs with the Me2+ binding sites of the ionic pore is demonstrated. Distinct but interacting drug-receptor sites of the Ca2+ channel are found by direct labeling as well as indirectly by drug competition studies. We distinguish between the 1,4-dihydropyridine site, the verapamil site, and the D-cis-diltiazem site. Each receptor site can exist in high and low-affinity state; the distribution of receptor sites in these states is regulated by temperature, ions, and drugs. The concept of intrinsic activity of drugs to stabilize the high-affinity state is exemplified for the 1,4-dihydropyridines. A change in the channel architecture is induced by binding of D-cis-diltiazem to its drug receptor site. This is proven by target-size analysis of the channel in situ. Partially purified t-tubule membranes from skeletal muscle are an extremely rich source of Ca2+ channel drug-receptor sites. The stoichiometry was determined in this preparation and found to be four verapamil:two 1,4-dihydropyridine:one D-cis-diltiazem site. A novel Ca2+ channel probe, [125I]iodipine (2,200 Ci/mmol), was synthetized, and the properties of this ligand are presented.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Animals
  • Anura
  • Autoradiography
  • Calcium Channel Blockers / pharmacology*
  • Calcium Channels
  • Cattle
  • Diltiazem / pharmacology
  • Guinea Pigs
  • Humans
  • Hydrogen-Ion Concentration
  • In Vitro Techniques
  • Iodine Radioisotopes
  • Iodobenzenes / metabolism*
  • Ion Channels / drug effects*
  • Ligands
  • Membranes / metabolism
  • Metalloproteins / metabolism
  • Microsomes / metabolism
  • Rats
  • Receptors, Nicotinic / metabolism*
  • Species Specificity
  • Verapamil / pharmacology

Substances

  • Calcium Channel Blockers
  • Calcium Channels
  • Iodine Radioisotopes
  • Iodobenzenes
  • Ion Channels
  • Ligands
  • Metalloproteins
  • Receptors, Nicotinic
  • iodipine
  • Verapamil
  • Diltiazem