Novel interactions of cations with dihydropyridine calcium antagonist binding sites in brain.
The effects of monovalent (Na+, Li+, K+, Rb+) and divalent (Ca2+, Mg2+, Mn2+) cations on dihydropyridine calcium antagonist binding sites in brain and cardiac membranes were investigated using a low ionic strength buffer (5 mM Tris-HCl, pH 7.4), and the dihydropyridine, [3H]-nitrendipine. At 25 degrees C, the monovalent cations Na+, Li+, and K+ (100 mM) but not Rb+ significantly decreased the apparent dissociation constant (KD) but had no effect on the maximum binding site capacity (Bmax) of [3H]-nitrendipine in brain. The divalent cations Ca2+, Mg2+, and Mn2+ (2 mM) significantly increased the Bmax, but did not affect the KD of [3H]-nitrendipine. The effects of cations were concentration-dependent (EC50 monovalent cations 10-25 mM; EC50 divalent cations 50-200 microM) and demonstrated brain region selectivity. The effect of Ca2+, but not Mg2+ or Mn2+ on [3H]-nitrendipine binding was described by a two-site model. At 25 degrees C, neither mono- nor divalent cations altered the characteristics of [3H]-nitrendipine binding to rat cardiac membranes. At 37 degrees C, Na+ (100 mM) but not K+ (100 mM) significantly increased the Bmax of [3H]-nitrendipine in rat brain membranes. Ca2+ (2 mM) significantly increased the Bmax of [3H]-nitrendipine binding to rat brain membranes to a greater extent than at 25 degrees C. Both Na+ and K+ had no effect on [3H]-nitrendipine binding to cardiac membranes, while Ca2+ (2 mM) significantly decreased the KD of [3H]-nitrendipine. It is suggested that the selective effects of mono- and divalent cations on [3H]-nitrendipine binding to rat brain and cardiac membranes may be associated with differences in the calcium current blocking activity of dihydropyridine calcium antagonists in brain and cardiac tissues.
D000818:Animals; D001665:Binding Sites; D001921:Brain / Q000187:drug effects / Q000378:metabolism*; D002118:Calcium / Q000378:metabolism; D015220:Calcium Channels; D002412:Cations / Q000378:metabolism*; D007700:Kinetics; D008094:Lithium / Q000378:metabolism; D008274:Magnesium / Q000378:metabolism; D008297:Male; D008345:Manganese / Q000378:metabolism; D009206:Myocardium / Q000378:metabolism; D009543:Nifedipine / Q000031:analogs & derivatives* / Q000378:metabolism; D009568:Nitrendipine; D009994:Osmolar Concentration; D011188:Potassium / Q000378:metabolism; D051381:Rats; D011978:Receptors, Nicotinic / Q000378:metabolism*; D012413:Rubidium / Q000378:metabolism; D012964:Sodium / Q000378:metabolism; D014325:Tromethamine / Q000378:metabolism
D015220:Calcium Channels; D002412:Cations; D011978:Receptors, Nicotinic; D014325:Tromethamine; D008345:Manganese; D009568:Nitrendipine; D008094:Lithium; D012964:Sodium; D008274:Magnesium; D009543:Nifedipine; D012413:Rubidium; D011188:Potassium; D002118:Calcium