SYMPATHETIC CONTROL OF JEJUNAL FLUID AND ELECTROLYTE TRANSPORT - AN EXPERIMENTAL-STUDY IN CATS AND RATS

  • 1 January 1984
    • journal article
    • review article
    • Vol. 535, 1-63
Abstract
Whether the sympathetic nerves to the small intestine are involved in the feedback control of extracellular fluid volume was investigated. In anesthetized cats, how efferent stimulation of the splanchnic nerves affected jejunal fluid transport rates was studied. The mechanisms behind the observed responses were analyzed and the physiological role of the sympathetic supply to the jejunal mucosa was determined. Splanchnic nerve stimulation (SNS) led to an increase in net fluid absorption provided that glucose was present in the jejunal lumen. In segments exposed to cholera toxin, SNS had a very pronounced inhibitory effect on net fluid secretion. The absorptive response was significant at a stimulation rate of 2 Hz and the maximal response occurred at 4-8 Hz. The effect was blocked by phentolamine, an .alpha.-adrenergic antagonist, and it was mimicked by a close intraarterial infusion of noradrenaline [norepinephrine], an .alpha.-adrenergic agonist. The absorptive response to SNS was still present after blockade of the intestinal vasoconstrictor nerves with hexamethonium, indicating that the sympathetic pathways to the epithelium and to vascular smooth muscle may employ different ganglionic transmitter mechanisms. The increase in net fluid absorption induced by SNS was due to the inhibition of a secretory process. The response was not secondary to changes in intestinal blood flow. The effect of noradrenaline infusion was abolished after blockade of the intestinal nerves with tetrodotoxin, indicating that the sympathetic nerves may act by modulation of the activity in intramural secretory nervous pathways. Unloading of carotid sinus baroreceptors led to an increase in jejunal fluid absorption, a response which was abolished by division of the sympathetic fibers to the segment. Unloading of cardiac mechanoreceptors enhanced jejunal fluid absorption or decreased cholera secretion, effects which were abolished by previous cervical vagotomy or local .alpha.-adrenoreceptor blockade of the jejunal segment. Afferent stimulation of the right cardiac nerve led to an inhibition of net fluid absorption if the nerve was stimulated with high-threshold parameters at frequencies < 1 Hz. The rate of jejunal fluid transport in the cat is physiologically regulated by the sympathetic nervous system. This effect is probably exerted by sympathetic modulation of intramural secretory pathways belonging to the enteric nervous system. The activity in the sympathetic fibers to the mucosa is in turn reflexly controlled by afferent activity from both carotid sinus baroreceptors and cardiopulmonary volume receptors with unmyelinated C-fiber afferents. The small intestine is involved in the feedback control of extracellular fluid volume. The potent sympathetic inhibition of intestinal secretion suggests that the sympathetic nervous system may play a key role in the physiological protection against extracellular volume depletion due to secretory diarrhea.