Effects of Sympathetic Nerve Stimulation on Long Posterior Ciliary Artery Blood Flow in Cats

Abstract

A new technique using ultrasonic flowmetry was developed in order to directly measure blood flow in the long posterior ciliary artery (LPCA) of anesthetized cats. Basal LPCA blood flow averaged about 0.6 ml/min and was stable over the experimental period. Electrical stimulation of the cervical preganglionic cervical sympathetic nerve produced frequency-dependent anterior segment ocular vasoconstrictor responses. Ipsilateral nictitating membrane contractions were simultaneously measured as a well-established index of neural sympathetic activation. LPCA frequency-response relationships were shifted to the right in comparison with those for the nictitating membrane. When elicited at two min intervals, submaximal evoked responses of both systems were stable for more than 90 min. Ocular vasoconstrictor and nictitating membrane responses were blocked in a dose-dependent fashion by intravenous treatment with the non-selective α-adrenoceptor antagonist, phentolamine (0.3-3.0 mg/kg), as well as with the selective α₁-adrenoceptor antagonist, prazosin (3-30 μg/kg). In contrast, neither evoked response was further antagonized by subsequent administration of the α₂-adrenoceptor antagonist, yohimbine (500 μg/kg). These results demonstrate the usefulness of ultrasonic flowmetry to study mechanisms controlling ocular anterior segment circulation and suggest that, as previously established for the nictitating membrane and anterior choroid, adrenergic neurogenic vasoconstriction in tissues perfused by the LPCA is mediated predominantly by α₁-adrenoceptors.