TRICYCLIC ANTIDEPRESSANTS INHIBIT VOLTAGE-DEPENDENT CALCIUM CHANNELS AND NA+-CA2+ EXCHANGE IN RAT-BRAIN CORTEX SYNAPTOSOMES

We have used a resting (5 mM K+) or depolarizing (60 mM K+) choline-based medium, and a nondepolarizing sodium-based or choline-based medium, to characterize the inhibitory potential of tricyclic antidepressants against the voltage-dependent calcium channels or the Na+-Ca2+ exchange process, respectively, in synaptosomes from rat brain cortex. Imipramine, desipramine, amitriptyline, and clomipramine inhibited net K+-induced 45Ca uptake with similar IC50 values (26-31 μM), and this uptake was also inhibited by diltiazem with an IC50 of 36 μM; these results indicate an inhibition of voltage-dependent calcium channels by tricyclic antidepressants. The net uptake of 45Ca induced by Na+-Ca2+ exchange was also inhibited by the four tricyclic antidepressants tested, but not by diltiazem; imipramine (IC50 = 94 μM) was a more potent inhibitor of this process than desipramine (IC50 = 151 μM), and the IC50 values of amitriptyline (107 μM) and clomipramine (97 μM) were similar to that of imipramine. Some degree (~25%) of brain calcium channel blockade could be present at the steady-state concentrations of tricyclic antidepressants expected to occur during therapeutic use of these compounds to treat depression or panic disorder.