RESUMO
Electrical depolarisation-(2 Hz, 1 ms)-induced [3H]noradrenaline ([3H]NA) release was measured from the isolated main pulmonary artery of the rabbit in the presence of uptake blockers (cocaine, 3 x 10(-5)M; corticosterone, 5 x 10(-5)M) and after blocking the MAO-enzyme by pargyline (1,2 x 10(-4)M). Substitution of most of the external Na+ by Li+ (113 mM; [Na+]0: 25 mM) slightly potentiated the stimulation-induced release of [3H]NA in a tetrodotoxin (TTX, 10(-7)M) sensitive manner. The reverse Na+/Ca2+-exchange inhibitor KB-R7943 (3 x 10(-5)M) failed to inhibit the stimulation-evoked release of [3H]NA, but increased the resting outflow of neurotransmitter. The 'N-type' voltage-sensitive Ca2+-channel (VSCC) blocker omega-conotoxin (omega-CgTx) GVIA (10(-8)M) significantly and irreversibly inhibited the release of [3H]NA on stimulation (approximately 60-70%). The 'residual release' of NA was abolished either by TTX or by reducing external Ca2+ from 2,5 to 0,25 mM. The 'residual release' of NA was also blocked by the non-selective VSCC-blocker neomycin (3 x 10(-3)M). Direct correlation was obtained between the extent of VSCC-inhibition and the transmitter release enhancing-effect of presynaptic alpha2-receptor blocker yohimbine (3 x 10(-7)M). When the release of [3H]NA was blocked by omega-CgTx GVIA plus neomycin, yohimbine was ineffective. Inhibition of the Na+-pump by removal of K+ from the external medium increased both the resting and the stimulation-evoked release of [3H]NA in the absence of functioning VSCCs (i.e. in the presence of neomycin and after omega-CgTx treatment). Under these conditions the stimulation-evoked release of NA was abolished either by TTX or by external Ca2+-removal (+1 mM EGTA). Similarly, external Li+ (113 mM) or the reverse Na+/Ca2+ exchange blocker KB-R7943 (3 x 10(-5)M) significantly inhibited the nerve-evoked release of NA in 'K+-free' solution. KB-R7943 decreased the resting outflow of NA as well. Under conditions, in which the Na+-pump was inhibited in the absence of functioning VSCCs, yohimbine (3 x 10(-7)M) further enhanced the release of neurotransmitter, while l-noradrenaline (l-NA, 10(-6)M), an agonist of presynaptic alpha2-receptors, inhibited it. The yohimbine-induced enhancement of NA-release was abolished by Li+-substitution and significantly inhibited by KB-R7943 application. It is concluded that after blockade of VSCCs, brief depolarising pulses may reverse Na+/Ca2+-exchange and release neurotransmitter in Na+-loaded sympathetic nerves. Further, similar to that of VSCCs, the reverse Na+/Ca2+-exchange may also be inhibited by presynaptic alpha2-receptor activation.
