Importance of the Microcirculation to Intestinal Secretion
This chapter discusses some normal and pathologic influences on the secretion of salt and water by the intestine. It details the secretion, the general characteristics of the microcirculation, the physical forces that can affect secretion, and finally specific secretory stimuli. Neural fibers are present in close proximity to the villus and crypt epithelium and to the microcirculatory vessels. Adrenergic, cholinergic, purinergic, peptidergic, and other classes of neurotransmitters are effectors, either directly or through effects on the serial release of other neurotransmitters. Sympathetic nerve stimulation caused in all layers of the intestine a transient reduction in blood flow that then recovered or escaped toward control values within a few minutes. Extrinsic sympathetic stimulation in the rat, at 4 Hz, decreased serosal but not mucosal blood flow, as determined by direct visualization. Cholinergic stimuli cause secretion both in vivo and in vitro. The secretion is most likely active, but interactions with other stimuli and microcirculatory effects may modulate the magnitude of secretion.