Retinal Autoregulation: The Myogenic Response in Retinal Arterioles

Abstract

Abstract: : Purpose: Retinal vessels exhibit no apparent autonomic innervation, thus highlighting the importance of local vascular control mechanisms in the regulation of retinal blood flow. Indirect measurements have suggested that autoregulation occurs in the retinal vasculature; however, the autoregulatory mechanism manifested by the myogenic response in the retinal microvessels has not been systematically studied. To address this issue, we characterized the myogenic response in isolated retinal arterioles without confounding influences from systemic or local neural, hormonal, hemodynamic, and metabolic changes. Methods: Two different sizes of retinal arterioles (120–200 µm and 80–120 µm in diameter corresponding to first– and second–order arterioles, respectively) were isolated from porcine eyes. The isolated vessels were cannulated and pressurized in a dual–reservoir system. After an equilibration procedure, the pressure–diameter relation was determined by measuring internal diameter by a videomicroscopic technique at various lumenal pressures between 35 and 125 cmH₂O in a 10 cmH₂O stepwise change. The retinal arterioles were then maximally relaxed with sodium nitroprusside (10^–4 M) in calcium free PSS, and the same series of pressure changes were performed to obtain the passive pressure–diameter relation. Results: All vessels developed basal tone at 36–37°C, but second–order arterioles exhibited a 20% higher basal tone than the upstream first–order arterioles. At pressures between 35 and 65 cmH₂O, first–order arterioles demonstrated myogenic activity with maintenance of a relatively constant lumenal diameter. Increasing intraluminal pressure above 65 cmH₂O caused a passive distention of these vessels. In contrast, the second–order arterioles exhibited a stronger myogenic constriction in response to a stepwise increase in intraluminal pressure from 35 to 95 cmH₂O. The vasodilation was observed at higher pressures (> 95 cmH₂O) in these vessels. In the presence of sodium nitroprusside (10^–4 M), all arterioles responded to pressure changes passively with no significant differences between first– and second–order arterioles. Conclusions: This is the first in vitro study to demonstrate myogenic activity in retinal arterioles and differences in the active myogenic response between first– and second–order retinal arterioles. Myogenic mechanisms, particularly in second–order arterioles, appear to actively contribute to the autoregulation of retinal blood flow.

Keywords: retina • neuroprotection • blood supply