NZ-105, a New 1,4-Dihydropyridine Derivative: Correlation between Dihydropyridine Receptor Binding and Inhibition of Calcium Uptake in Rabbit Aorta

The correlation between the binding of NZ-105, a newly synthesized 1,4-dihydropyridine (DHP) derivative, on DHP receptors and its inhibitory activity on transmembrane 45Ca2+ uptake was investigated. 3H-NZ-105 bound rabbit aortic microsomes more slowly than did 3H-nitrendipine (3H-NTD): the association and dissociation rate constants of 3H-NZ-105 were about 70 times and 10 times less than those of 3H-NTD. respectively. The dissociation constant (Kd) of 3H-NZ-105 (4.48 nM) was about 6 times larger than that of 3H-NTD (0.79 nM), and the maximum number of binding sites (Bmax) for 3H-NZ-105 (112.5 fmoles/mg protein) was about the same as that for 3H-NTD (106.2 fmoles/mg protein). Unlabelled NZ-105 and nicardipine fully, and in a concentration-dependent manner, displaced 3H-NZ-105 specific binding. Pre-incubation with NZ-105 also concentration-dependently (more than 0.1 μM) inhibited the transmembrane 45Ca2 + uptake increment induced by a high-K+ (50 mM) solution. The inhibitory efficacy of NZ-105 became larger as the incubation period with this compound increased (from 1 hr incubation to 3 hr incubation), and recovery was difficult even after washout for 3 hr. Based on these results, we conclude that NZ-105 causes blockade of voltage-dependent calcium channels (VDCs) by binding to DHP receptors. Moreover, the very slow onset and recovery from NZ-105-induced vasodilation may be attributable to the slow and long-lasting inhibition of transmembrane calcium uptake, which accompanies its very slow binding to and dissociation from DHP receptors.