Adrenergic inhibition of neurally evoked secretion in ground squirrel intestine

The role of norepinephrine in the mucosal response to neural stimulation was examined in the ground squirrel jejunum. Electrical field stimulation of jejunal sheets in flux chambers evoked a tetrodotoxin-sensitive increase in short-circuit current (42 +/- 6 microA/cm2) that declined during the stimulus period, followed by a second rise in current after the stimulus was terminated. The initial increase in current was inhibited by bumetanide and significantly reduced in chloride-free solutions, suggesting neurotransmitter stimulation of electrogenic chloride secretion during electrical stimulation. Atropine reduced the neurally evoked secretory response to 35% of control values. Blockade of alpha 2-adrenoceptors with phentolamine or yohimbine, or prevention of norepinephrine release with bretylium enhanced the atropine-resistant portion of the response and abolished the post-stimulus rise in current. The neurally evoked response was not affected by prazosin or propranolol. Catecholamine immunoreactivity was detected in nerve fibers of submucosal and myenteric plexuses but none was found in cell bodies of either plexus. The results suggest that electrical stimulation of the ground squirrel jejunum evokes active chloride secretion that is mediated by acetylcholine and other secretory transmitter(s), and simultaneously inhibited by norepinephrine released from adrenergic fibers in the mucosa.