Tetanic failure due to decreased endogenous adenosine A2A tonus operating neuronal Cav1 (L-type) influx in Myasthenia gravis

P>In healthy motor endplates, tetanic depression is overcome by tonic adenosine A(2A)-receptor-mediated facilitation of transmitter release. The A(2A) receptor operates a coordinated shift from fast-desensitizing Ca(v)2.1 (P/Q) calcium influx to long-lasting Ca(V)1 (L) channels on motor nerve terminals. This study aimed at investigating whether A(2A) receptors-operated Ca2+ influx via Ca(V)1 (L)-type channels contribute to sustain acetylcholine release evoked by 50 Hz-bursts in toxin-induced Myasthenia gravis (TIMG) rats. In contrast to control animals, inhibition of [3H]acetylcholine (ACh) release by the Ca(V)2.1 (P/Q) channel blocker, omega-Agatoxin IVA (100 nM), in TIMG rats had a higher magnitude than that observed with the Ca(V)1 (L) channel blocker, nifedipine (1 mu M). Adenosine deaminase (0.5 U/mL) and the A(2A) receptor antagonist, ZM 241385 (50 nM), decreased [3H]ACh release by a similar amount in control rats, but their effects were smaller in magnitude in myasthenic animals. The adenosine precursor, AMP (100 mu M), increased (similar to 40%) ACh release in both control and TIMG animals. Blockade of A(2A), but not of A(1), receptors prevented AMP-induced facilitation of transmitter release; nifedipine (1 mu M) mimicked the effect of the A(2A) receptor antagonist. Video-microscopy studies designed to measure real-time transmitter exocytosis using the FM4-64 fluorescent dye fully supported radiochemical data. Thus, impairment of the adaptive shift from Ca(V)2.1 (P/Q) to Ca(V)1 (L) channels may contribute to tetanic failure in myasthenic rats. This parallels the reduction of adenosine A(2A) receptor tonus in TIMG animals, which might be restored by exogenous application of AMP.