Critical issues in establishing the coupling between nNAChRs and L-type (Cav1) calcium channels while using a pharmacological approach
The phenylalkylamine (verapamil), dihydropyridine (nitrendipine) and benzothiazepine classes of Cav1 type calcium channel blockers are capable of blocking nNAChRs (Houlihan et al., 2000; Wheeler et al., 2006). The absence of the effects of nicotine (both on the calcium transient and on the QC) after the application of verapamil and nitrendipine may well be related to simple direct blockade of nNAChRs (sensitive to DHβE and permeable to Ca2+). Indeed, DHβE, verapamil and nitrendipine all lead to a decrease in the transients. At the same time, all three pharmacological agents abolish the effect of nicotine.
If even direct block of nNAChRs by verapamil and nitrendipine does take place, a number of additional facts still point to the involvement of the Cav1 calcium channels in the mechanism of modulation of calcium entry and the process of ACh release in the nerve terminal. So, after application of all three agents, the calcium entry decreases, however, the effect of verapamil and nitrendipine is almost two times more pronounced than that of DHβE. Further on, the QC does not change after the addition of DHβE, whereas in the presence of verapamil and nitrendipine it is decreased by more than 10%. In addition, activation of cholinergic receptors (by nicotine) and presumed blockade of cholinergic receptors (by verapamil and nitrendipine) have not an opposite, but a unidirectional effect - a decrease in the QC. And the last but not the least, the absence of the effect of nicotine on the calcium transient after application of cadmium, which does not affect the functioning of nNAChRs (including α4β2 nNAChRs) or even potentiates them (Wheeler et al., 2006; Garduño et al., 2012), indicates that in our case a pharmacological effect on two different targets takes place.