Vasoactive agonists do not change the caliber of retinal capillaries of the rat

The pericytes of the retinal capillaries are considered to be contractile cells, based on indirect evidence. We attempted to detect pericyte contractility directly using a morphometric method following intravitreal injection of vasoconstricting (endothelin-1, norepinephrine) and vasodilating (forskolin, prostaglandin E1) agonists in rats. The technique involved measuring changes in the ratio of the (lumenal perimeter x mean lumenal radius) to the lumenal area (rP/A ratio, which we defined as "caliber"). Following intravitreal injection of endothelin-1 or norepinephrine there is an ophthalmoscopically visible constriction of the larger retinal arterioles. This constriction is also clearly demonstrable morphometrically (P = 0.001 for endothelin-1 and P = 0.047 for norepinephrine). However, neither for the vasoconstricting nor for the vasodilating agonists is there evidence of a significant change of the caliber of the capillaries. At least in the retina of the rat, therefore, we have no evidence that the pericytes perform a contractile function that alters flow in the capillaries.