Reverse Na +/Ca 2+-exchange mediated Ca 2+-entry and noradrenaline release in Na +-loaded peripheral sympathetic nerves

Abstract

[ 3H]noradrenaline ([ 3H]NA) released from sympathetic nerves in the isolated main pulmonary artery of the rabbit was measured in response to field stimulation (2 Hz, 1 ms, 60 V for 3 min) in the presence of uptake blockers (cocaine, 3 × 10 −5 M and corticosterone, 5 × 10 −5 M). The [ 3H]NA-release was fully blocked by the combined application of the selective and irreversible ‘N-type’ voltage-sensitive Ca 2+-channel (VSCC)-blocker ω-conotoxin (ω-CgTx) GVIA (10 −8 M) and the ‘non-selective’ VSCC-blocker aminoglycoside antibiotic neomycin (3 × 10 −3 M). Na +-loading (Na +-pump inhibition by K +-free perfusion) was required to elicit further NA-release after blockade of VSCCs (ω-CgTx GVIA + neomycin). In K +-free solution, in the absence of functioning VSCCs (ω-CgTx GVIA + neomycin), the fast Na +-channel activator veratridine (10 −5 M) further potentiated the nerve-evoked release of [ 3H]NA. This NA-release was significantly inhibited by KB-R7943, and fully blocked by C a o 2 + - removal . However, Li +-substitution was surprisingly ineffective. The non-selective K +-channel blocker 4-aminopyridine (4-AP, 10 −4 M) also further potentiated the nerve-evoked release of NA in K +-free solution. This potentiated release was concentration-dependently inhibited by KB-R7943, significantly inhibited by Li +-substitution and abolished by C a o 2 + - removal . It is concluded that in Na +-loaded sympathetic nerves, in which the VSCCs are blocked, the reverse Na +/Ca 2+-exchange-mediated Ca 2+-entry is responsible for transmitter release on nerve-stimulation. Theoretically we suppose that the fast Na +-channel and the exchanger proteins are close to the vesicle docking sites.