Pharmacological Characteristics of the Canine Cerebrovascular Constriction Produced by Neuropeptide Y

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In order to elucidate the role of neuropeptide Y (NPY) in cerebral circulation, we undertook the present study to examine the action of NPY itself, and the combined effects of NPY with other vasoconstrictor stimuli. NPY itself produced contractions of isolated canine basilar artery in a concentration-dependent manner, which was independent of the presence of absence of endothelium. C-terminal peptides of NPY (NPY_12-36) and (NPY_22-36) were weak agonists, while those without C-terminals were ineffective. The vasoconstriction produced by NPY was, however, strongly potentiated by increasing the K⁺ concentration in the medium up to 20 mM, or by pretreatment with tetraethylammonium, a K⁺ channel blocker and hemolysate containing oxyhemoglobin. NPY also augmented the contractile responses to prostaglandin F_2α, norepinephrine, and histamine, but not to serotonin. The contraction in response to NPY per se or in 20 mM K⁺ was effectively attenuated by Ca²⁺ antagonists such as d-cis-diltiazem, and in a Ca²⁺-free medium. These results suggest that in canine basilar artery, the activation of the Y₁ receptor modulates the availability of the L-type Ca²⁺ channel, leading to enhance Ca²⁺ influx from the extracelullar space and potentiate contractile effects of other cerebral vasoconstrictors. This might be involved in the cerebral vasospasm which occurs after subarachnoid hemorrhage.