Alpha 2-adrenoceptor antisecretory responses in the rat jejunum

Area of Science:

• Pharmacology
• Gastroenterology
• Physiology

Background:

• Adrenergic receptors play a role in regulating gastrointestinal function.
• Alpha-adrenoceptors are implicated in modulating ion transport in the jejunum.

Purpose of the Study:

• To investigate the specific alpha-adrenoceptor subtypes involved in regulating short-circuit current (SSC) in the rat jejunum.
• To determine the mechanism by which noradrenaline affects jejunal ion transport.

Main Methods:

• Ussing chamber experiments were performed on rat jejunum sheets.
• Transepithelial short-circuit current (SSC) was measured.
• The effects of various adrenergic agonists (noradrenaline, UK-14,304, phenylephrine) and antagonists (idazoxan, corynanthine, prazosin) were assessed.
• Experiments were conducted in normal, chloride-free, and bicarbonate-free physiological salt solutions (PSS).

Main Results:

• Noradrenaline, UK-14,304, and phenylephrine significantly decreased resting and theophylline-stimulated SSC.
• The selective alpha 2-adrenoceptor antagonist idazoxan inhibited these responses, while the alpha 1-antagonist corynanthine did not.
• UK-14,304 was the most potent agonist, followed by noradrenaline, then phenylephrine.
• Idazoxan caused parallel rightward shifts in agonist concentration-response curves, indicating competitive antagonism.
• Noradrenaline-induced reduction of SSC was not affected by tetrodotoxin, suggesting a direct mucosal mechanism.
• The effect was dependent on chloride but not bicarbonate ions.

Conclusions:

• The decrease in SSC in rat jejunum induced by noradrenaline, UK-14,304, and phenylephrine is mediated by the stimulation of postjunctional alpha 2-adrenoceptors.
• These receptors are located on the mucosal side and directly influence ion transport, independent of neural pathways.
• Chloride ions are essential for this alpha 2-adrenoceptor-mediated response.