Vasoactivity of intraluminal and extraluminal agonists in perfused retinal arteries.

PURPOSE

To evaluate the vasoactive response of isolated perfused arteries of the pig to K+ and adrenergic agonists and to compare the effects of intraluminal (IL) and extraluminal (EL) drug delivery.

METHODS

A new microperfusion system was developed, in which short lengths of porcine retinal arteries (outer diameter 90.4 +/- 2.7 microns) were cannulated at both ends and perfused at a controlled rate (5 microliters/min) with outflow through a single side branch. The diameter of the vessel and the intraluminal pressure were monitored, and the effect of intraluminally and extraluminally applied agonists was determined. Endothelial cell function and the integrity of the blood retinal barrier was verified.

RESULTS

Consistent vasoactive responses were obtained from most vessels. 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 applied extraluminally. The largest contraction for adrenaline was 19.0% +/- 2.1% (n = 13) IL and 8.4% +/- 1.5% (n = 13) EL, and for noradrenaline, 17.8% +/- 1.9% (n = 13) IL and 6.8% +/- 1.1% (n = 13) EL. The IL 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 the existence of myogenic contractions was restricted to the special case in which vessels with no branches were pressurized under zero flow conditions.

CONCLUSIONS

Pig retinal arteries exhibited asymmetry in their responses to adrenergic agonists and K+, with contractions significantly larger when the drug was applied to the intraluminal surface rather than the extraluminal surface. This asymmetry may reflect an important property of retinal vessels. Microperfusion systems of this type may prove valuable in developing a better understanding of control mechanisms in retinal circulations.