Comparison of binding affinities of ω-conotoxin and amlodipine to N-type Ca2+ channels in rat brain.

AIM: To compare the binding affinities of omega-conotoxin (CTX) and amlodipine to N-type Ca2+ channels in rat brains. METHODS: Whole rat brains were homogenized in HEPES buffer 50 mmol . L-1 (pH 7.4) and centrifuged at 40 000 x g to obtain the membrane-entriched fraction. I-125-omega-conotoxin (I-125-omega-CTX) was used as a radioligand. Using radioligand binding assay K-d and B-max values of the radioligand were determined by Scatchard analysis. The IC50 value for each drug was obtained from displacement experiments. RESULTS: No differences in B-max values of I-125-omega-CTX binding sites between frozen and fresh tissues were observed. Values of K-d and B-max of N-type Ca2+ channels were 0.02 +/- 0.01 nmol . L-1 and 1029 +/- 108 pmol/g protein, respectively. The pK(i) values of omega-CTX and amlodipine were 9.51 and less than 4, respectively. The pK(i) values of propranolol, prazosin, atropine, and histamine were very low. CONCLUSION: The binding affinity of the L-type Ca2+ -antagonist amlodipine to N-type Ca2+ channels in the rat brain was very low.