Choroidal blood flow compensation in rats for arterial blood pressure decreases is neuronal nitric oxide-dependent but compensation for arterial blood pressure increases is not.

Choroidal blood flow (ChBF) compensates for changes in arterial blood pressure (ABP) and thereby remains relatively stable within a +/-40 mmHg range of basal ABP in rabbits, humans and pigeons. In the present study, we investigated if ChBF can compensate for increases and decreases in ABP in rats. ChBF was continuously monitored using laser Doppler flowmetry in anesthetized rats, and ABP measured via the femoral artery. At multiple intervals over a 2-4 h period during which ABP varied freely, ChBF and ABP were sampled and the results compiled across rats. We found that ChBF remained near baseline over an ABP range from 40 mmHg above basal ABP (90-100 mmHg) to 40 mmHg below basal ABP, but largely followed ABP linearly below 60 mmHg. Choroidal vascular resistance increased linearly as BP increased above 100 mmHg, and decreased linearly as BP declined from basal to 60 mmHg, but resistance declined no further below 60 mmHg. Inhibition of nitric oxide (NO) formation by either a selective inhibitor of neuronal nitric oxide synthase (NOS) (N(omega)-propyl-L-arginine) or a nonselective inhibitor of both neuronal NOS and endothelial NOS (N(omega)-nitro-l-arginine methyl ester) did not affect compensation above 100 mmHg ABP, but did cause ChBF to linearly follow declines in BP below 90 mmHg. In NOS-inhibited rats, vascular resistance increased linearly with BP above 100 mmHg, but remained at baseline below 90 mmHg. These findings reveal that ChBF in rats, as in rabbits, humans and pigeons, compensates for rises and/or declines in arterial blood pressure so as to remain relatively stable within a physiological range of ABPs. The ChBF compensation for low ABP in rats is dependent on choroidal vasodilation caused by neuronal NO formation but not the compensation for elevated BP, implicating parasympathetic nervous system vasodilation in the ChBF compensation to low ABP.