Long posterior ciliary arterial blood flow and systemic blood pressure

The purpose of the current study was to investigate the relationship between systemic blood pressure (BP) and long posterior ciliary arterial (LPCA) blood flow (BF) in response to autonomic drugs, and to determine the types of receptors involved. LPCA BF was measured in anesthetized rabbits at the retrobulbar region using laser Doppler flowmetry. The observation that most of the short posterior ciliary arteries diverge from two LPCAs in rabbits was confirmed using a vascular casting technique. Therefore, the LPCA BF at the retrobulbar region should be representative of total uveal BF. Norepinephrine (IV) increased systemic BP and decreased LPCA BF. These responses were suppressed only slightly by yohimbine, but were inhibited markedly by prazosin and by the combination of yohimbine and prazosin. B-HT 920 (IV), a selective alpha-2 adrenoceptor agonist, increased systemic BP and decreased LPCA BF. B-HT 920 (IA) also decreased LPCA BF, with all effects of B-HT 920 antagonized by yohimbine. Methacholine (IV) decreased systemic BP and increased LPCA BF whether administered IV or IA. These effects were blocked uniformly by atropine. The current results suggest that LPCA BF is controlled both by systemic BP and local ocular vascular tone. There are vasoconstrictive alpha-1 and alpha-2 adrenoceptors and vasodilative muscarinic receptors in the rabbit ocular vascular tree.