The neurosecretory effect of ouabain on isolated rabbit ileal mucosa.

Ouabain, when added to fluid bathing rabbit ileal mucosa mounted in a flux chamber, transiently increases short circuit current, implying a paradoxical secretory response. To determine the cause of this change, we studied unidirectional fluxes of 36Cl and 23Na and the effects of ion substitution, of reduced Ca concentration, verapamil, tetrodotoxin and atropine. Ouabain 0.1 mM, transiently increased the serosal to mucosal flux of Cl and Na, increased Isc and PD and reduced ion conductance. The Isc response to ouabain was diminished by reducing the bath fluid concentration of Cl, of Ca, and by adding verapamil. Tetrodotoxin both delayed and reduced the maximal Isc response; atropine had no effect. We conclude that ouabain acts by releasing a neurotransmitter of unknown identity and by increasing the serosal to mucosal flux of Cl.

Effect of neuropeptide Y on ion transport by the rabbit ileum.

Neuropeptide Y (NPY) is present in fibers extending from the submucous plexus to the epithelium of the small intestine where the liberation of NPY might affect ion transport. We sought the effects of NPY on rabbit ileal mucosa stripped of muscularis propria and mounted in a flux chamber. NPY reduced the transmural electrical potential difference and short circuit current (Isc) and increased total ionic conductance. Threshold and maximal effects were evoked at concentrations of 1 nM and 1 microM, respectively. NPY increased chloride absorption, JCl(net), by increasing the flux of Cl from mucosa to serosa, JCl(ms), and by decreasing JCl(sm). JNa(net) actually diminished because JNa(sm) rose more than JNa(ms). In the presence of NPY theophylline 5 mM caused Cl secretion, increased potential difference and Isc and reduced total ionic conductance, indicating that the tissue could respond to a secretagogue. Tetrodotoxin 0.1 microM did not diminish the Isc reduction caused by NPY, and desensitization did not alter the response of the tissue to electrical field stimulation. Like somatostatin and norepinephrine, which are also present in the submucous plexus, NPY increases Cl absorption, but unlike them, it reduces rather than augments Na absorption. The lack of effect of tetrodotoxin on the Isc response to NPY implies that NPY does not act by liberating a second neurotransmitter; the lack of effect of NPY desensitization indicates that the liberation of NPY plays no significant role in the response of the tissue to electrical field stimulation.

Ion transport in normal and inflamed human jejunum in vitro. Changes with electric field stimulation and theophylline.

We studied ion transport in human jejunal mucosa under basal conditions and when tissues were stimulated electrically (EFS) and with theophylline 5 mM (N = 12 pairs). Tissues were mounted in a flux chamber to measure unidirectional fluxes of 22Na, 36Cl, short-circuit current, Isc (mueq/cm2/hr), electrical potential difference, PD (mV), and total ionic conductance, G (mS/cm2). Six pairs of tissues that were normal or less inflamed responded to theophylline by increasing PD and Isc and by reducing JCl(ms), the mucosal to serosal flux of Cl. In six pairs that did not respond to theophylline or to EFS, inflammation was more intense, and in the basal state, G was lower, PD was higher, and unidirectional fluxes of Na and Cl were halved. We conclude: (1) normal jejunum secretes Cl and responds to theophylline or EFS by reducing JCl(ms) thereby increasing Cl secretion; (2) unlike human ileum, neither theophylline nor EFS affect JCl(sm); (3) inflammation eliminates the response to theophylline and to EFS.

Secretory reflexes in ileum and jejunum: absence of remote effects.

We have tested the hypothesis that luminal secretagogues initiate neural reflexes that alter ion transport in small intestinal segments proximal or distal to the site of the secretory stimulus. Effects of secretagogues that act by different mechanisms were studied in vitro by measuring short circuit current (ISC) of ileum or jejunum mounted in a unique flux chamber while proximal mucosa in neural continuity with the tissue was perfused with secretagogues (Na deoxycholate. Escherichia coli STa, 5-hydroxytryptamine, theophylline) or was stimulated electrically (EFS). No proximal stimulus affected distal ISC. We also studied in vivo adjacent segments of ileum or jejunum in neural continuity but with unconnected lumens. In anesthetized rabbits, we measured transmural electrical potential difference and fluid movement (Phenol red marker). Stimulation of proximal segments of ileum or jejunum with STa, or of ileum with 5-HT or Na deoxycholate did not affect distal transport. Stimulation of distal segments of ileum and jejunum with STa or 5-HT, or of jejunum with Ha deoxycholate did not affect proximal transport. We conclude that the secretion caused by luminal secretagogues in the rabbit small intestine is limited to the area of stimulation.

Noradrenergic influence on epithelial responses of rabbit ileum to secretagogues.

Norepinephrine is one of three neurotransmitters that may act directly on enterocytes to enhance absorption; its interaction with secretagogues is of physiological importance. We have studied the influence of norepinephrine on the short-circuit current (Isc) responses to acetylcholine (ACh; 10 microM), vasoactive intestinal polypeptide (VIP; 100 pM-10 nM), peptide histidine isoleucine (PHI; 100 pM-10 nM), histamine (0.1 mM), and to electrical field stimulation (EFS) of rabbit ileum mounted in flux chambers. Tetrodotoxin reduced the response to norepinephrine (10 microM) by 40% and to histamine by 32% but did not affect responses to VIP or PHI. Norepinephrine decreased the ACh response (EC50, 70 nM) and reduced the responses to PHI (less than or equal to 87%), to EFS (less than or equal to 75%), and to histamine (less than or equal to 42%). Norepinephrine decreased the response to VIP (500 pM) but not to higher or lower VIP concentrations. It enhanced the response to VIP (10 nM) and to theophylline (5 mM). We conclude that 1) norepinephrine increases absorption by acting on nerves and enterocytes; 2) the failure of norepinephrine to reduce the Isc response to VIP when the VIP-induced increment in Isc is comparable to that caused by EFS is evidence that VIP does not mediate the EFS response; 3) PHI might mediate the EFS response; and 4) VIP, PHI, and histamine affect enterocytes directly; histamine also affects intrinsic nerves.