Endothelin-1 has been suggested to stimulate vasorelaxation through endothelial NO release downstream of ET
B receptor stimulation.
7,8 In anesthetized dogs, when the NOS inhibitor N
G-monomethyl-L-arginine (L-NMMA) and ET-1 were infused together intravenously, a greater increase in systemic vascular resistance was observed compared to that observed in the presence of ET-1 alone,
36 suggesting that NO production, possibly via ET
B receptor activation,
37,38 can mitigate vasoconstrictor responses to ET-1. The potential role of NO in these previous observations is supported by a study using bioassay techniques in which administration of donor perfusate from freshly isolated endothelial cells treated with ET-1 enhances vasorelaxation of recipient arterial strips in a manner sensitive to NO scavenging.
39 Moreover, selective ET
B receptor stimulation causes retinal arteriolar vasoconstriction that is weaker than that in response to ET-1,
2 possibly a result of endothelial ET
B-induced production of NO.
39 However, using an isolated vessel approach, we found in our study that inhibition of NOS had no effect on vasoconstriction to ET-1 (
Fig. 2) and the vasoconstriction to ET
B receptor activation was not altered by endothelial removal (
Fig. 3). It appears that ET-1 does not lead to NO production to functionally counteract its vasoconstrictor activity in retinal arterioles. We previously showed that the smooth muscle-localized endothelin ET
A receptors mediate retinal arteriolar constriction to ET-1 with nominal contribution from ET
B, although there is ET
B receptor expression in endothelial and smooth muscle cells.
2 Interestingly, intravitreal injection of endothelin isoform, ET-3, has been shown to elicit an ET
B-dependent NO-mediated increase in retinal blood flow in rats.
40 However, we observed only modest concentration-dependent constriction of porcine retinal arterioles in response to ET-3 (
n = 5, data not shown). The vasoconstriction (32 ± 6%) at the highest concentration of ET-3 (10 nM) was sensitive to ET
B receptor antagonist BQ788. The species difference and/or the initiation of other indirect mechanisms by ET-3 in the above in vivo preparation in which the direct impact of this peptide on retinal arterioles was not verified might explain the observed discrepancy. Nonetheless, our data did not support a vasoactive role of endothelial ET
B receptors in retinal arterioles, in contrast with their smooth muscle counterparts, which promote weak vasoconstriction (
Fig. 3). This is in agreement with results from a study showing that ET
B receptor blockade had no effect on ET-1–induced constriction of coronary arterioles from young rats.
41 Taken together, these diverse findings may suggest heterogeneity among microvascular beds in whether ET-1 stimulation elicits endothelial ET
B-mediated dilation, or that there may be activation of other signaling pathways for NO release secondary to ET-1 stimulation.