Transient responses of rabbit cerebral blood vessels to norepinephrine: correlation with intrinsic myogenic tone.

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Abstract
Transient contractile responses to norepinephrine (NE) of vascular segments from the rabbit vertebral, internal carotid, and basilar arteries rise to a peak within several seconds, and in the presence of the agonist, reverse rapidly, relaxing with a half-time of 15 +/- 3.2 seconds. In the basilar artery, peak contraction is approximately 25% of the maximum response mediated via the "alpha-like" adrenoreceptors and is elicited by NE 10(-7) M. Steady state contractions are seen with higher concentrations. Transient contractile responses are absent in segments from the brachiocephalic and external carotid arteries, and their incidence increases the more rostral along the length of the vertebral and internal carotid artery the origin of the segment studied. They were seen in all preparations of the intracranial vertebral and basilar arteries. There is a good correlation between the occurrence in any particular vascular segment of the transient contractile response and intrinsic tone as assessed by relaxation to papaverine (10(-6) M). The response was blocked by alpha-adrenergic receptor blocking agents and was not elicited by d-NE nor tetrahydrazoline or oxymetazoline. This response may be analogous to the first phase of the biphasic contraction found in many other blood vessels. Since in cerebral vessels the agonist concentration to elicit the first phase is several orders of magnitude lower than the second, it can appear in the absence of the latter.