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.