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Department of Physiology, Medical School, Birmingham.

1. In anaesthetized rats, supramaximal baroreceptor stimulation by carotid sinus inflation evoked a reflex fall in systemic arterial pressure and an increase in gross mesenteric vascular conductance, but no significant change in gross mesenteric blood flow. 2. Principal arteries (80-350 microns internal diameter (i.d.)) which supply the intestine and mesentery, small arteries (30-40 microns) and some terminal arterioles (18-30 microns) of the mesentery showed a diameter increase beginning with the fall in arterial pressure, but remaining terminal arterioles and precapillary arterioles (10-18 microns) showed a diameter increase beginning when arterial pressure neared its lowest level. 3. Application of phentolamine to the mesentery abolished diameter increases that began with the reflex fall in arterial pressure. Thereafter, 50% of small arteries and terminal and precapillary arterioles showed diameter increases that began when the fall in arterial pressure neared its zenith. 4. It is proposed that the proximal arterial vessels, which are known to be sympathetically innervated, predominantly showed dilatation mediated by reflex inhibition of sympathetic tone, while more distal arterioles showed myogenic dilatation secondary to the fall in systemic arterial pressure. 5. Small veins (30-50 microns) of the mesentery and principal veins (100-560 microns) that drain mesentery and intestine also showed a diameter increase beginning with the reflex fall in systemic arterial pressure. Since they are known to have a sympathetic noradrenergic supply, and in the absence of changes in mesenteric blood flow likely to cause passive changes in venous diameter, they apparently showed dilatation due to inhibition of sympathetic tone. 6. The above responses are all compatible with, and so suggest underlying mechanisms for, changes in blood flow, regional blood volume, and capillary filtration evoked by baroreceptor stimulation in studies of whole mesenteric circulation, i.e. intestine as well as mesentery.

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