Presynaptic α 2-receptors regulate reverse Na +/Ca 2+-exchange and transmitter release in Na +-loaded peripheral sympathetic nerves

Abstract

Electrical depolarisation-(2 Hz, 1 ms)-induced [ 3 H ]noradrenaline ([ 3 H ]NA) release has been measured from the isolated main pulmonary artery of the rabbit in the presence of uptake blockers (cocaine, 3×10 −5 M; corticosterone, 5×10 −5 M). Substitution of most of the external Na + by Li + (113 mM; [Na +] 0: 25 mM) slightly potentiated the axonal stimulation-evoked release of [ 3 H ]NA in a tetrodotoxin (TTX, 10 −7 M) sensitive manner. The reverse Na +/Ca 2+-exchange inhibitor KB-R7943 (3×10 −5 M) failed to inhibit the stimulation-evoked release of [ 3 H ]NA, but increased the resting outflow of neurotransmitter. The ‘N-type’ voltage-sensitive Ca 2+-channel (VSCC) blocker ω-conotoxin (ω-CgTx) GVIA (10 −8 M) significantly and irreversibly inhibited the release of [ 3 H ]NA on stimulation (∼60–70%). The ‘residual release’ of NA was abolished either by TTX or by reducing external Ca 2+ from 2.5 to 0.25 mM. The ‘residual release’ of NA was also blocked by the non-selective VSCC-blocker neomycin (3×10 −3 M). Correlation was obtained between the extent of VSCC-inhibition and the transmitter release-enhancing effect of presynaptic α 2-receptor blocker yohimbine (3×10 −7 M). When the release of [ 3 H ]NA was blocked by ω-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 axonal stimulation-evoked release of [ 3 H ]NA in the absence of functioning VSCCs (i.e., in the presence of neomycin and after ω-CgTx treatment). Under these conditions the stimulation-evoked release of NA was abolished either by TTX or by external Ca 2+-removal (+1 mM EGTA). Similarly, external Li + (113 mM) or the reverse Na +/Ca 2+ exchange blocker KB-R7943 (3×10 −5 M) significantly inhibited the stimulation-induced transmitter release 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 (3×10 −7 M) further enhanced the release of neurotransmitter, while l-noradrenaline (l-NA, 10 −6 M), an agonist of presynaptic α 2-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 +/Ca 2+-exchange and release neurotransmitter in Na +-loaded sympathetic nerves. Further, similar to that of VSCCs, the reverse Na +/Ca 2+-exchange may also be regulated by presynaptic α 2-receptors.