TEMPERATURE-DEPENDENT MODULATION OF [NITRENDIPINE-H-3 BINDING BY THE CALCIUM-CHANNEL ANTAGONISTS VERAPAMIL AND DILTIAZEM IN RAT-BRAIN SYNAPTOSOMES

  • 1 January 1984
    • journal article
    • research article
    • Vol. 229  (2) , 333-339
Abstract
Binding of the dihydropyridine Ca2+ channel antagonist [3H] nitrendipine to an intact rat brain mitochondrial-synaptosomal fraction (P2) was specific, saturable, temperature-dependent and of high affinity (Kd = 115-467 pM). The effects of the Ca2+ channel antagonists verapamil and diltiazem on [3H]nitrendipine binding and their temperature dependence were investigated. At 0 and 25.degree. C, verapamil inhibited [3H]nitrendipine binding incompletely in a manner consistent with an allosteric modulation and nearly independent of the incubation temperature. The effects of diltiazem were highly temperature-dependent. At 25 and 37.degree. C, 10 .mu.M diltiazem enhanced [3H]nitrendipine binding to values of 140 and 200% of control, respectively. At 0.degree. C, 10 .mu.M diltiazem inhibited [3H]nitrendipine binding to a value of 68% of control. Analysis of saturation isotherms at steady state demonstrated that at all temperatures studied the effects of verapamil and diltiazem on [3H]nitrendipine binding were due to alterations in the ligand Kd. At 25.degree. C, these alterations were mediated by changes in the rate of ligand-receptor complex dissociation. Competition studies of verapamil and diltiazem at 25 and 0.degree. C indicate that the effects of these 2 drugs on [3H]nitrendipine binding are mutually exclusive. Evidently the binding of [3H]nitrendipine is allosterically modulated by spacially related binding sites for verapamil and diltiazem.