In cats anesthetized with pentobarbital, 188 direct microvascular pressure and diameter measurements were made of the tenuissimus muscle. The microvascular pressure in arterioles of 70 mum in diameter or larger was proportional to the systemic arterial pressure (PA). The arterioles with diameter-s ranging from 35 to 20 mum have been shown to be the principa-l source of arteriolar resistance regulating micropressures downstream. Across the capillary bed proper, the drop in pressures was about 15% of PA. Micropressures in the smallest venules (8-15 mum) averaged 24 mmHg and reflect closely capillary blood pressure. With a background of basic microvascular data, the vasodilatory mechanism of papaverine (P) and isoproterenol (IPR) in the skeletal muscle was analyzed. Administration of IPR decreases both arteriolar and venular pressure, while P infusion decreased the pressure in arterioles wider than 20 mum in diameter; however, in smaller arterioles there was a substantial elevation in micropressure. The data establish two basically different vasodilatory effects on the terminal vasculature: one with increased capillary pressure and fluid filtration (P), a second with a decrease in capillary pressure enhancing absorption (IPR).

In nine dogs under pentobarbital anesthesia, electromagnetic flow transducers were implanted around the ascending aorta, brachiocephalic artery, superior mesenteric artery, and iliac artery. A catheter to measure arterial pressure was implanted into the thoracic aorta. After full recovery, hemodynamic variables were measured daily in conscious dogs in five sets of recordings: control, during anticipation of food, during actual food intake, and 1 hour and 3 hours after completion of ingestion. During food intake there was a generalized sympathomimetic stimulation characterized by a significant increase in cardiac output with a peak of 142%, heart rate (176%), arterial blood pressure (147%), while the ratio of flow in the superior mesenteric artery to cardiac output decreased from 8.9% to 7.6% (P<0.05). Flow in the brachiocephalic artery increased during food intake to 196% and that in the iliac artery dropped to 75.4%. The possibility was discussed that a certain level of sympathomimetic stimulation may signal the diminishing or ending of ingestion. During digestion (at 1 and 3 hours after ingestion), there were no significant changes in cardiac output, heart rate, and mean arterial blood pressure, whereas the flow in the superior mesenteric artery increased to 133% and mesenteric regional resistance decreased to 82%, and flows in the brachiocephalic and iliac arteries decreased to 86.5% and 74.6%, respectively. The ratio of flow in the superior mesenteric artery to cardiac output increased to 8.9% and 12.5% in the third hour. These results indicate that a redistribution of blood flow occurs during digestion with a preference for the vascular bed of the superior mesenteric artery.

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