Carbonic Anhydrase Inhibitors of Different Structures Dilate Pre-Contracted Porcine Retinal Arteries

Abstract

Carbonic anhydrase inhibitors (CAIs), such as dorzolamide (DZA), are used as anti-glaucoma drugs to lower intraocular pressure, but it has been found that some of these drugs act as vasodilators of retinal arteries. The exact mechanism behind the vasodilatory effect is not yet clear. Here we have addressed the issue by using small vessel myography to examine the effect of CAIs of the sulfonamide and coumarin type on the wall tension in isolated segments of porcine retinal arteries. Vessels were pre-contracted by the prostaglandin analog U-46619, and CAIs with varying affinity for five different carbonic anhydrase (CA) isoenzymes found in human tissue tested. We found that all compounds tested cause a vasodilation of pre-contracted retinal arteries, but with varying efficacy, as indicated by the calculated mean EC50 of each compound, ranging from 4.12 µM to 0.86 mM. All compounds had a lower mean EC50 compared to DZA. The dilation induced by benzolamide (BZA) and DZA was additive, suggesting that they may act on separate mechanisms. No clear pattern in efficacy and affinity for CA isoenzymes could be discerned from the results, although Compound 5, with a low affinity for all isoenzymes except the human (h) CA isoform IV, had the greatest potency, with the lowest EC50 and inducing the most rapid and profound dilation of the vessels. The results suggest that more than one isozyme of CA is involved in mediating its role in controlling vascular tone in retinal arteries, with a probable crucial role played by the membrane-bound isoform CA IV.

Keywords: carbonic anhydrase; vascular tone; vasodilation.

Conflict of interest statement

The authors declare no conflict of interest.

Figures

Figure 1

Chemical structures of compounds 1–6 used in this study.

Figure 2

The effects of U-46619, BZA and DZA on wall tension in porcine retinal arteries. A: Continuous myography recording of wall tension in one artery. The vertical arrow on the left indicates the time point when 10−6 M U-46619 was added to the bath. The wall tension then increases and reaches a peak, and 10−3 M BZA was added at the time indicated by the right vertical arrow. Note the rapid vasodilation induced by BZA to the baseline level. B: Myography recording from another artery. After vasoconstriction induced by U-46619 reached a peak, 10−3 M DZA was added at the time indicated by the middle arrow. The left vertical arrow indicates an addition of 10−3 M BZA. Note the additional dilation induced by BZA. C: Mean concentration-response curves for the dilatory effects of BZA (filled circles) and a (open triangles), expressed as a percentage of the maximum contraction induced by U-46619. Symbols represent the mean ± standard error of the mean (SEM) dilation at each concentration. D: The mean ± SEM dilation induced by BZA and DZA in response to a single dose of 10−3 M added to the bathing solution, as a percentage of the maximum contraction induced by U-46619. An asterix (*) above bars indicates statistically significant vasodilation (p < 0.002)

Figure 3

The effects of U-46619 and compounds 3 and 4 on retinal arterial wall tension. A: Continuous recording of wall tension. First, 10−6 M U-46619 was added to the bath, and when the vasoconstriction reached a peak in the experiment shown, a single dose of 10−3 M of compound 3 was added at the point indicated by the right vertical arrow. B: Continuous recording of wall tension from another vessel. At the peak of vasoconstriction induced by U-46619, a dose of 10−3 M of compound 4 was added at the point indicated by the right arrow. C: Mean concentration-response curves for the dilatory effects of compounds 3 (filled circles) and 4 (open triangles), expressed as a percentage of U-46619 induced maximum contraction. D: The mean dilation induced by compounds 3 and 4 in response to a single dose of 10−3 M added to the bathing solution, as a percentage of the maximum contraction induced by U-46619. An asterix (*) above a bar indicates statistically significant vasodilation (p < 0.01 and p < 0.002).

Figure 4

The effects of U-46619, and compounds 5 and 6 on retinal arterial wall tension. A: Continuous recording of wall tension. After adding 10−6 M U-46619 to the bath, and when constriction had reached a peak, a single dose of 10−3 M of compound 5 was added, as indicated by the right vertical arrow. B: Recording of wall tension from a second vessel. At the peak of vasoconstriction induced by U-46619, a dose of 10−3 M of compound 6 was added at the point indicated by the right arrow. C: Mean concentration-response curves for the dilatory effects of compounds 5 (filled circles) and 6 (open triangles), expressed as a percentage of U-46619 induced maximum contraction. D: The mean dilation induced by compounds 5 and 6 in response to a single dose of 10−3 M added to the bathing solution, as a percentage of the maximum contraction induced by U-46619. Asterix (*) above the bars indicate statistically significant vasodilation (p < 0.001 and p < 0.002).