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Direct observations of responses of mesenteric microcirculation of the rat to circulating noradrenaline.

The responses of mesenteric microcirculation of the rat to circulating noradrenaline were studied by in vivo microscopy, using a photo‐electric device placed on a television monitor to measure changes in diameter of individual vessels. Blood flow was measured in the anterior mesenteric artery with an electromagnetic flow transducer, mesenteric vascular conductance was computed on‐line from mesenteric artery flow and arterial pressure. I.V. injection of noradrenaline induced a decrease in diameter of arterioles (less than 30 microns) which began simultaneously with the rise in arterial pressure and averaged 15% at 15‐20 s. By contrast small and principal arteries (30‐40 microns and 80‐350 microns respectively) generally showed a diameter increase which averaged 10% and began at the peak of the pressor response. Venules and veins (12‐560 microns) showed a diameter decrease which averaged 12‐15% at 25‐30 s. Meanwhile mesenteric vascular conductance fell by 65‐75% in 10 s and rose to 30‐50% above control in 25‐30 s. The diameter increases of small and principal arteries were not reflex dilator responses initiated by the rise in systemic arterial pressure, nor due to beta‐adrenoreceptor stimulation. However, local application of phentolamine abolished responses of all sections of the vascular tree indicating that they all depended on activation of alpha‐adrenoreceptors. During I.V. infusion of noradrenaline small arteries showed a maintained increase in diameter which began at the peak of the pressor response, while arterioles initially decreased in diameter but then relaxed, often attaining resting diameter before infusion ceased. Meanwhile mesenteric flow and conductance decreased transiently, but then returned to near control levels, i.e. 'autoregulatory escape' occurred. It is argued that noradrenaline induced alpha‐mediated contraction of all sections of the vascular tree; the tendency of arteries to constrict was counteracted by the rise in intravascular pressure caused by arteriolar constriction, active constriction of venous vessels may have been augmented by passive collapse secondary to arteriolar constriction. The secondary relaxation of arterial vessels reflects an inherent property of their smooth muscle to relax from the constrictor influence of noradrenaline and is more marked in proximal than distal vessels. It is proposed that the initial decrease in mesenteric vascular conductance in response to circulating noradrenaline may be attributed to active constriction of distal arterioles and the secondary increase in conductance ('escape') to secondary relaxation of more proximal arterial vessels.