The enterins inhibit the contractile activity of the anterior aorta of Aplysia kurodai.

The anterior aorta is one of the largest blood vessels in the marine mollusc Aplysia kurodai. We examined the actions of recently identified neuropeptides, the enterins, on this blood vessel. Immunohistochemistry revealed that the enterin-immunopositive nerve fibers and varicosity-like structures are abundant in the aorta. When the enterins were applied to the aorta, the basal tonus of the arterial muscles was diminished. The enterins also decreased the contraction amplitude of the anterior aorta evoked either by the application of an Aplysia cardioactive peptide, NdWFamide, or by the stimulation of a nerve innervating the aorta (the vulvar nerve). We found that the enterins activate the 4-aminopyridine (4-AP)-sensitive K(+) channels, and thereby hyperpolarize the membrane potential of the aortic muscles. In the presence of 4-AP, the enterins failed to inhibit the muscle contraction evoked by the vulvar nerve stimulation, suggesting that the inhibition is mainly due to the activation of the 4-AP-sensitive K(+) channels. The inhibition of the NdWFamide-evoked contraction by the enterin was not, however, affected by 4-AP. These results suggest that the enterins are involved in inhibitory regulation of the contractile activity of the anterior aorta, and that the inhibition could be due to multiple mechanisms.