Intravital microscopy: a new in vivo technique for visualizing and quantifying effects of regulatory peptides on choledochoduodenal junction motility

Multiple actions of secretin in the human body

The discovery of secretin initiated the field of endocrinology. Over the past century, multiple gastrointestinal functions of secretin have been extensively studied, and it was discovered that the principal function of this peptide in the gastrointestinal system is to facilitate digestion and to provide protection. In view of the late identification of secretin and the secretin receptor in various tissues, including the central nervous system, the pleiotropic functions of secretin have more recently been an area of intense focus. Secretin is a classical hormone, and recent studies clearly showed secretin's involvement in neural and neuroendocrine pathways, although the neuroactivity and neural regulation of its release are yet to be elucidated. This chapter reviews our current understanding of the pleiotropic actions of secretin with a special focus on the hormonal and neural interdependent pathways that mediate these actions.

Secretin induces variable inhibition of motility in different parts of the Australian possum sphincter of Oddi

The sphincter of Oddi (SO) may not function as a single structure. We aimed to determine the response of the proximal and distal segments of the bile duct (BD-SO) and pancreatic duct (PD-SO) components of the SO to secretin, with and without neural blockade with tetrodotoxin (TTX). In anaesthetized Australian possums, separate manometry catheters were placed in the proximal and distal BD-SO or PD-SO segments to record motility. Secretin, 50-1000 ng kg(-1), was administered, followed by TTX, and re-administration of secretin, 500 and 1000 ng kg(-1). Changes in the motility index (MI, frequency x mean amplitude) were determined. Statistical analysis utilized repeated-measures ANOVA. Secretin produced a dose-dependent decrease in MI from the proximal and distal BD-SO and PD-SO (all P < 0.001). The maximum inhibition, at 1000 ng kg(-1), was 21 +/- 4%, 33 +/- 6% and 42 +/- 5% of control (mean +/- SEM), for proximal and distal BD-SO, and distal PD-SO, respectively. The proximal PD-SO MI, however, was inhibited to 62 +/- 6% of control, at 1000 ng kg(-1). TTX enhanced the secretin-induced response to the same level at the four sites (P < 0.02). We conclude that secretin inhibits the motility of the possum SO in a nonuniform manner and is modulated by neural activity.

Proximal and distal segments of the possum sphincter of Oddi respond differently to neural and cholecystokinin octapeptide stimulation in vitro

BACKGROUND/AIMS

Previous studies have demonstrated separate pancreatic duct (PD) and bile duct (BD) components of the sphincter of Oddi (SO) and suggested distinct proximal and distal functional segments. This study was designed to determine if proximal and distal segments of the BD component of the SO (BD-SO) and PD component of the SO (PD-SO) responded equally to (1) activation of SO-duodenal neural pathways, and (2) exogenous cholecystokinin octapeptide (CCK-8).

METHODS

Intact SO-duodenum preparations from Australian brush-tailed possums (n = 6) were mounted in organ baths. SO activity was recorded from the proximal and distal segments of BD-SO and PD-SO +/- electrical activation of duodenal nerves at two separate sites. Full thickness muscle strips from the proximal and distal segments of the BD-SO and PD-SO were prepared (n = 8), mounted in organ baths, and exposed to CCK-8 (10(-9)- 10(-6) M), +/- tetrodotoxin.

RESULTS

Activation of duodenal nerves evoked different responses in some segments of the BD-SO and PD-SO, depending on the site of duodenal electrical stimulation. CCK-8 induced a concentration-dependent, tetrodotoxin-insensitive decrease in the contraction amplitude of SO muscle strips from the proximal but not the distal SO. BD-SO and PD-SO strips were not different.

CONCLUSIONS

The SO is composed of BD and PD components each of which contains proximal and distal segments that can respond independently to appropriate stimuli.

Functional innervation of the biliary sphincter of the guinea-pig revealed by anti-autonomic drugs

1. The roles of excitatory and inhibitory intrinsic motor nerves on contractions reflexly evoked by wall distension were investigated in the isolated sphincter of Oddi of the guinea-pig (SO-GP). 2. Distension of the terminal bile duct for 30-60 s time periods increased the frequency of contractions from about 2 to 12 min(-1) (n = 16). 3. Hexamethonium (HEX; 300 microM) largely prevented the distension-evoked increase in contraction frequency (4.5 min(-1), n = 8) as did atropine (ATR; 1 microM) (0.8 min(-1), n = 6), while tetrodotoxin (TTX; 1 microM) blocked the contractions triggered during distension. 4. L-nitroarginine (L-NA; 100 microM) significantly increased the frequency of contractions during and in the absence distension while apamin (APAM; 0.5 microM) significantly increased their frequency and doubled their mean amplitude during distension. 5. These results suggest that distension activates excitatory cholinergic motor nerves to increase the frequency of contractions in the SO-GP. These actions are modulated by the concomitant activation of intrinsic nitrergic and non-nitrergic inhibitory motor nerves.

Involvement of L-arginine-nitric oxide pathways in neural relaxation of the sphincter of Oddi

To evaluate if L-arginine-nitric oxide-pathways are involved in the neural relaxation of the sphincter of Oddi, we studied the effect of nitric oxide synthase inhibition on electrical field stimulation-induced relaxation of the sphincter of Oddi in the guinea pig in vitro. After incubation with atropine (1 microM), phentolamine (1 microM) and propranolol (1 microM), histamine (50 microM) and cholecystokinin-octapeptide (25 nM) produced similar increases in sphincter tone. Subsequent field stimulation induced sphincteric relaxation, that was significantly greater when the initial tone had been raised by cholecystokinin (5 Hz, 59 +/- 9%; 10 Hz, 79 +/- 9%) compared to histamine (5 Hz, 27 +/- 3%; 10 Hz, 40 +/- 7%). N-omega-Nitro-L-arginine methyl ester (L-NAME, 100 microM), which competitively inhibits nitric oxide synthase, markedly suppressed this relaxation. The subsequent addition of L-arginine (1 mM), but not D-arginine (1 mM), restored the relaxation. Hexamethonium (100 microM) did not affect the relaxation, but tetrodotoxin (1 microM) completely abolished it. Sodium nitroprusside caused a dose-dependent relaxation of the sphincter (ED50 13 nM), which was unaffected by L-NAME. In conclusion, endogenous nitric oxide synthase products represent a major transmitter of non-adrenergic non-cholinergic relaxation of the sphincter of Oddi in the guinea pig. This relaxation is partially facilitated by cholecystokinin.