Characteristics of Cerebrovascular Response to Intrinsic Vasoactive Substances in Sika Deer (Cervus nippon yesoensis) and the Possible Effects of Gravity on Adrenergic Responses

Gravity may exert species-specific effects on quadrupedal vasoreactivity, reflecting variations in the vertical displacement of the cardiocranial axis from the dorsal plane. Deer show markedly displaced cardiocranial axes compared to their closest phylogenetic relatives, but their relative cerebrovascular responses remain unelucidated. Accordingly, we investigated the responses to noradrenaline (NA), acetylcholine (ACh), 5-hydroxytryptamine (5-HT), histamine, angiotensin (Ang) II, and bradykinin (BK) in cervine basilar arterial rings. NA and 5-HT induced slight contraction, and ACh induced relaxation, which contrasts with the findings reported in pigs and cattle. The cumulative response to ACh was abolished by endothelial denudation and inhibited by <i>N^ω</i>-nitro-L-arginine (a nitric oxide synthase inhibitor), atropine (a nonselective muscarinic antagonist), and p-fluoro-hexahydro-sila-difenidol (an M₃ antagonist). Pirenzepine (an M₁ antagonist) and methoctramine (an M₂ antagonist) showed no significant effects. Histamine induced contractions, with its concentration-response curve shifted to the right in parallel by diphenhydramine (an H₁ antagonist). However, cimetidine (an H₂ antagonist) showed no significant effects. Ang II and BK had no vasomotive effects. NA and ACh induced different cerebrovascular responses in sika deer versus cattle, but histamine and BK did not. Our findings suggest that cerebrovascular responses are influenced by the similarity of animal species and the head and heart positions relative to gravity.