Precapillary control of the splanchnic vascular bed.

The accessibility of the mesenteric vasculature of the rat permits it to be used for intravital microscopy. Venous-arteriolar response and autoregulation are localized in the precapillary and terminal arterioles. Responses to intravasal application of noradrenaline are not influenced by arteriolar tone, induced by a venous-arteriolar response. This indicates a strong regulatory influence on the precapillary vessels originating from the venous side. Arteriolar and venous-arteriolar autoregulations are differentially effective in the individual organ circulations. Noradrenaline dose-response curves and frequency-response curves in the saline-perfused mesenteric vascular bed were analyzed with respect to the reactivity of the different groups of precapillary vessels. The sensitivity to noradrenaline, quantified by the ED50, was significantly smaller in the arterioles than in small precapillary arterioles or in larger arteries. Autoregulatory escape was recognized as a typical mechanism of flow redistribution consisting partially of vascular adjustment. Escape from vasoconstrictor influences was more pronounced during electrical stimulation than during responses to noradrenaline. The data suggest that distribution of splanchnic blood flow to individual organs is governed by venous-arteriolar and arteriolar autoregulation, whereas the sympathetic nerves and autoregulatory escape are major factors in controlling the redistribution of blood flow within the tissue or tissue layers.