VASOACTIVITY OF INTRALUMINAL AND EXTRALUMINAL AGONISTS IN PERFUSED RETINAL ARTERIES

Purpose. To evaluate the vasoactive response of isolated perfused arte ries of the pig to K+ and adrenergic agonists and to compare the effec ts of intrauminal (IL) and extraluminal (EL) drug delivery. Methods. A new microperfusion system was developed, in which short lengths of po rcine retinal arteries (outer diameter 90.4 +/- 2.7 mu m) were cannula ted at both ends and perfused at a controlled rate (5 mu l/min) with o utflow through a single side branch. The diameter of the vessel and th e intraluminal pressure were monitored, and the effect of intraluminal ly and extraluminally applied agonists was determined. Endothelial cel l function and the integrity of the blood retinal barrier was verified . Results. Consistent vasoactive responses were obtained from most ves sels. The resting diameter of the vessel was not greatly influenced by changes in flow rate or intraluminal pressure over the physiological range. Adrenaline and noradrenaline caused dose-dependent contractions , which were larger when applied intraluminally than they were when ap plied extraluminally. The largest contraction for adrenaline was 19.0% +/- 2.1% (n = 13) IL and 8.4% +/- 1.5% (n = 1.3) EL, and for noradren aline, 17.8% +/- 1.9% (n = 13) IL and 6.8% +/- 1.1% (n = 13) EL. The I L contraction to 124-mM K+, 19.0% +/- 1.6% (n = 21), was also greater than that for El application, 5.0% +/- 1.0% (n = 13). We found that th e existence of myogenic contractions was restricted to the special cas e in which vessels with no branches were pressurized under zero flow c onditions. Conclusions. Pig retinal arteries exhibited asymmetry in th eir responses to adrenergic agonists and k(+), with contractions signi ficantly larger when the drug was applied to the intraluminal surface rather than the extraluminal surface. This assymmetry may reflect an i mportant property of retinal vessels. Microperfusion systems of this t ype may prove valuable in developing a better understanding of control mechanisms in retinal circulations.