ADRENERGIC AND NITRERGIC NEUROTRANSMITTERS ARE RELEASED BY THE AUTONOMIC SYSTEM OF THE PIG LONG POSTERIOR CILIARY ARTERY

The role played by adrenergic, muscarinic and nitric oxide putative ne urotransmitters released from autonomic nerve endings onto the pig pro ximal long posterior ciliary artery (LPCA) was determined. The proxima l LPCA in the pig usually supplies both the uveal and retinal circulat ions. In this study, in vitro ring segments of the artery, passively s tretched and with noradrenaline-induced tone, were neurogenically stim ulated (NS) using electrical field stimulation with 5-sec trains of 0. 2 msec pulses. NS produced a frequency dependent contraction in all ve ssels which was completely abolished by 10(-6) M tetrodotoxin. 40 Hz s timulation was used throughout the study as it produced a maximal NS c ontraction. 10(-5) M guanethidine abolished the NS-induced contraction and revealed a NS-induced relaxation, as did the or adrenergic blocke r, phentolamine, in vessels passively stretched. The beta adrenergic b locker, propranolol, only slightly reduced the NS-induced constriction . In vessels pre-contracted with noradrenaline, NS produced a relaxati on (D) which was proportional in magnitude to the tone (C) viz. D = (0 .30 +/- 0.04) . C + (0.24 +/- 0.06). The muscarinic blocker, atropine, had no effect on the NS-induced relaxation, implying that it is a non -adrenergic, non-cholinergic mediated system. Incubation with N-w-nitr o-L-arginine methyl ester reduced the NS-induced relaxation to 48% of its control value, a reduction which was reversed in the presence of e xcess L-arginine. Damage to endothelial cell function did not reduce t he NS-induced relaxation. It is concluded that the autonomic innervati on of the proximal LPCA releases both contraction and relaxation neuro transmitters. Contraction is mediated by an ai adrenergic neurotransmi tter. At least two neurotransmitters mediate relaxation, one of which is probably nitric oxide. There is no functional evidence for the rele ase of beta adrenergic neurotransmitter from the sympathetic system or acetylcholine from the parasympathetic system.