Guanine nucleotides regulate [3H]substance P binding in rat small intestine

Area of Science:

• Pharmacology
• Neuroscience
• Gastroenterology

Background:

• Substance P (SP) is a neuropeptide involved in various physiological processes, including gastrointestinal function.
• Understanding SP receptor subtypes and their signaling mechanisms is crucial for elucidating its roles.

Purpose of the Study:

• To characterize the binding properties of substance P receptors in rat small intestine membranes.
• To determine the affinity and subtype of the high-affinity SP receptor.
• To investigate the influence of guanine nucleotides and sodium on SP binding.

Main Methods:

• Radioligand binding assays using [3H]substance P.
• Non-linear regression analysis to determine binding parameters (KD, Bmax).
• Inhibition studies with unlabeled tachykinins and guanine nucleotides.

Main Results:

• Specific binding of [3H]SP to rat small intestine membranes revealed a high-affinity receptor site (KD = 0.25 nM, Bmax = 149.5 fmol/mg protein).
• The high-affinity receptor was identified as the P-subtype, showing preferential binding to SP over eledoisin and kassinin.
• Guanine nucleotides (GTP, GDP) and sodium reduced SP binding in a dose-dependent manner, with GTP and GDP being more potent.

Conclusions:

• The high-affinity SP receptor in the rat small intestine is a P-subtype receptor.
• Guanine nucleotides modulate SP binding, primarily by increasing the off-rate.
• These findings suggest the receptor is coupled to a GTP-binding regulatory protein, indicating a role in signal transduction pathways.