Sympathetic control of jejunal fluid and electrolyte transport. An experimental study in cats and rats.

The study was performed to test the hypothesis that the sympathetic nerves to the small intestine might be involved in the feed-back control of extra-cellular fluid volume. In a series of experiments in anaesthetized cats, we studied how efferent stimulation of the splanchnic nerves affected jejunal fluid transport rate. The mechanisms behind the observed responses were analysed and we also attempted to determine the physiological role of the sympathetic supply to the jejunal mucosa. 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, 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, a finding which indicates 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 fibres 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 below 1 Hz.