L-type calcium channels unmasked by cell-permeant Ca2+ buffer at mouse motor nerve terminals

The involvement of the different types of voltage-dependent calcium channel s (VDCC) in both DM-BAPTA-AM-incubated and EGTA-AM-incubated mature mice le vator auris neuromuscular junctions (NMJ) was studied. We evaluated the eff ects of omega-agatoxin IVA (omega-Aga IVA), nitrendipine and omega-conotoxi n GVIA (omega-CgTX) (P/Q-, L- and N-type VDCC blockers, respectively) on pe rineurial calcium currents (I-Ca) and nerve-evoked transmitter release. The application of omega-Aga IVA (100 nM) drastically reduced perineurial I-Ca (>90%) and nerve-evoked transmitter release (>90% of reduction in quantal content, m) at both DM-BAPTA-AM-incubated and EGTA-AM-incubated NMJ. The L- type VDCC antagonist nitrendipine (10 mu M) caused a significant reduction (23+/-9%, n=5) of perineurial I-Ca at DM-BAPTA-AM-incubated NMJ. In additio n, after the block of P/Q-type VDCC with omega-Aga IVA (100 nM), nitrendipi ne reduced (>90%, n=2) the remaining perineurial I-Ca. Such reduction was n ot observed at EGTA-AM-incubated NMJ, before or after the total block of P/ Q-type VDCC. Moreover, nitrendipine did not significantly reduce the quanta l content of DM-BAPTA-AM-incubated NMJ. Finally, the application of omega-C gTX (5 mu M) did not significantly affect perineurial I-Ca or nerve-evoked transmitter release at either DM-BAPTA-AM-incubated or EGTA-AM-incubated NM J. These results show the existence of a nitrendipine-sensitive, L-type com ponent of perineurial I-Ca in DM-BAPTA-AM-incubated NMJ of mature mice.