Identification and characterization of [3H]nitrendipine binding sites in rat spinal cord

The present study reports the existence of high-affinity [3H]nitrendipine ([3H]NIT) binding sites in rat spinal cord. Characterization studies revealed [3H]NIT binding to synaptosomes to be specific, rapid and saturable, occurring at a single population of sites. The Bmax was 51 fmol/mg of protein and Kd 0.22 nM with a Hill slope of 0.96. Studies with nifedipine and verapamil demonstrated that the latter binds to a site allosterically linked to the 1,4-dihydropyridine binding sites in spinal cord. The Ca++ channel agonist methyl 1,4-dihydro-2,6-dimethyl-3-nitro-4-(2-trifluoromethylphenyl)-pyridine-5 carboxylate acted competitively at the 1,4-dihydropyridine binding site and inhibited specific [3H]NIT binding completely. Organic Ca++ antagonists inhibited binding to various degrees. Treatment with EDTA reduced specific [3H]NIT binding in spinal cord by 83%. This was restored by externally added Ca++. The effect of various mono-, di- and trivalent cations on specific [3H]NIT binding as well as its restoration in EDTA-treated preparations was tested. Na+, K+, Li+, Ca++, Mg++, Mn++ and Ba++ were found to have no significant effect. Other cations inhibited binding of [3H]NIT in the sequence La greater than Cd++ greater than Cu++ greater than Co++. Regional studies in rat spinal cord demonstrated 3-fold higher specific [3H]NIT binding sites in the dorsal cord compared to the ventral cord. Moreover, further variations were also found in cervical, thoracic and lumbar regions of the spinal cord. The results demonstrate that binding of the labeled calcium antagonist in spinal cord membranes is of high affinity and completely reversible.(ABSTRACT TRUNCATED AT 250 WORDS)