The vasodilating effect of carbonic anhydrase inhibitors on porcine retinal arterioles depends on the perivascular retinal tissue and acidosis, but not on hypercapnia. The effect involves mechanisms other than carbonic anhydrase inhibition.

ABSTRACT.Purpose: The retinal blood flow depends on the diameter of retinal arterioles, but diameter changes in these vessels have hitherto only been assessed in vessels larger than approximately 100 lm. Therefore, a new method was developed for studying diameter changes along the vascular tree of arterioles in whole perfused segments of porcine retinas, and the effect of known vasoconstrictors on the diameter of retinal arterioles at different branching levels were studied. Methods: Thirty-four whole-mounted porcine retinas were placed in a specially designed tissue chamber. On the basis of video recordings through an inverted microscope, the diameter of retinal arterioles was measured at five different branching levels before and after addition of a high potassium concentration, or increasing concentrations of endothelin-1, the prostaglandin analogue U46619, noradrenaline or none (time controls). Results: The baseline diameter ranged from 136 lm (95% CI 132-140 lm) for 1st order arterioles to 33 lm (95% CI 21-44 lm) for 5th order arterioles. In 1st order arterioles, endothelin produced 56.6% (95% CI 47.6-64.0) and U46619 14.6% (95% CI 5.7-22.6) relative constriction compared with baseline, which for both compounds decreased significantly with increasing branching level (p < 0.0001 and p < 0.0001, respectively). The change in diameter during addition of noradrenaline did not differ significantly from the time controls (p = 0.07). Conclusions: The effect of retinal vasoconstrictors differs among larger and smaller arterioles. The study highlights the need for investigating diameter regulation in smaller retinal arterioles as a basis for understanding normal and pathological changes in retinal blood flow.