Myogenic reactivity of rat epineurial arterioles: potential role in local vasoregulatory events.

Local control of neural blood flow is considered to reside in innervation of epineurial and endoneurial arterioles rather than in intrinsic autoregulatory mechanisms. With the use of an isolated vessel preparation and an in vivo approach, the present studies examined intrinsic vasomotor responsiveness of epineurial arterioles. Segments of epineurial arterioles, cannulated on glass micropipettes (40 micrometers) and pressurized in the absence of intraluminal flow, showed sustained pressure-dependent (30-90 mmHg) vasoconstriction and acute myogenic reactivity. Myogenic tone was unaffected by phentolamine (10(-6) M). Removal of extracellular Ca(2+) resulted in loss of spontaneous tone and passive behavior. Concentration-response curves for norepinephrine (10(-9)-3 x 10(-6) M) and relaxation to both acetylcholine (10(-8)-10(-5) M) and adenosine (10(-8)-10(-4) M) were obtained. Acetylcholine dilator responses were inhibited by N(G)-nitro-L-arginine methyl ester. Epineurial blood flow was measured in vivo using a laser-Doppler flow probe. Blood flow declined over a 2-h period after surgery, and during this time preparations developed responsiveness to the dilator acetylcholine. Phentolamine blocked vasoconstrictor responses to exogenous norepinephrine but only partially reversed the in vivo baseline tone. The time-dependent decline in epineurial blood flow was observed despite the presence of tetrodotoxin (1 microM), further confirming that tone was predominantly caused by myogenic rather than neurogenic mechanisms. It is concluded that because epineurial arterioles exhibit intrinsic myogenic reactivity, they have the potential to participate in local regulation of neural hemodynamics independently of their own innervation.