EFFECTS OF OMEGA-TOXINS ON NORADRENERGIC NEUROTRANSMISSION IN BEATINGGUINEA-PIG ATRIA

The effects of four omega-toxins, known to block various subtypes of n euronal voltage-activated Ca2+ channels, on the beating guinea pig lef t atrium have been analyzed. Atria were suspended in oxygenated Krebs- bicarbonate solution at 32 degrees C and driven with electrical pulses delivered by a stimulator at 1 Hz, 1 ms, 4 V. A 10-fold increase of v oltage caused a potent and rapid enhancement of the size of contractio ns (about 3- to 4-fold above basal), which reflects the release of end ogenous noradrenaline from sympathetic nerve terminals. omega-Conotoxi n MVIIC, omega-conotoxin MVIIA and omega-conotoxin GVIA inhibited the inotropic responses to 10 X V stimulation with IC50 values of 191, 44 and 20.4 nM, respectively. omega-Agatoxin IVA did not affect the contr actile responses. The inotropic responses to exogenous noradrenaline w ere unaffected by the toxins. The potent blocking effects of omega-con otoxin GVIA were present even in conditions in which the release of no radrenaline was strongly facilitated by presynaptic alpha(2)-adrenocep tor blockade by phenoxybenzamine. These effects were not reversed upon repeated washing of the tissue with toxin-free medium. In contrast, t he blockade induced by omega-conotoxin MVIIC and omega-conotoxin MVIIA were fully reversed, with t(1/2) of 13.5 and 31.2 min, respectively. omega-Conotoxin MVIIC (1 mu M) protected against the irreversibility o f the blockade induced by omega-conotoxin GVIA (100 nM). The results a re compatible with the following conclusions: (i) at cardiac sympathet ic neuromuscular junctions, the release of noradrenaline and the norad renergic transmission is maintained and regulated by N-type Ca2+ chann els; (ii) the modulation by presynaptic alpha(2)-adrenoceptors of nora drenaline release is exerted via N-type Ca2+ channels; (iii) omega-con otoxin MVIIC and omega-conotoxin MVIIA reversibly block N channels and the noradrenergic neurotransmission; (iv) omega-conotoxin GVIA and om ega-conotoxin MVIIC seem to recognize the same binding site on the N-t ype Ca2+ channels; however, their binding kinetics might considerably differ; (v) P-type Ca2+ channels are unlikely involved in the regulati on of transmitter release at cardiac sympathetic nerve terminals.