Inhibition of Transmitter Release from Sympathetic Nerve Endings by Omega-conotoxin

1. The effects of the calcium channel blocker, omega-conotoxin, on sympathetic neuroeffector function in the guinea-pig vas deferens have been investigated using a combination of mechanical and electrophysiological recording techniques. 2. Biphasic contractions evoked by electrical field stimulation were irreversibly abolished by omega-conotoxin (10-100 nmol/L). 3. Electrically evoked excitatory junction potentials and currents were irreversibly blocked by omega-conotoxin (10-100 nmol/L). Spontaneous excitatory junction potentials and currents were unaffected by this treatment. 4. omega-Conotoxin did not block impulse propagation in the nerve terminals. However, in three of four experiments omega-conotoxin caused a decrease in the size of the nerve terminal impulse. 5. These findings support the suggestion the omega-conotoxin acts prejunctionally to inhibit sympathetic neuroeffector by interfering with depolarization-secretion coupling.