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.