Effects of polyamines on the muscarinic receptor‐operated cation current in guinea‐pig ileal smooth muscle myocytes

The effects of extracellular and intracellular polyamines (PAs), spermine and putrescine, on the cation current (mICAT) evoked either by activating muscarinic receptors with carbachol or by intracellularly applied GTPγS (in the absence of carbachol) were studied using patch‐clamp recording techniques in single guinea‐pig ileal myocytes. Extracellular spermine and putrescine rapidly and reversibly inhibited mICAT in a concentration‐ and voltage‐dependent manner with the IC50 values at −40 mV of about 1 and 5 mM, respectively. Membrane depolarization relieved the blocking action of PAs although cation conductance activation curve remained N‐shaped. The inhibition was similar for both carbachol‐ and GTPγS‐evoked currents, suggesting that the cation channel rather than the muscarinic receptor was the primary site of the PA action. In outside‐out membrane patches, both cation channel unitary conductance and open probability were reduced. In perforated‐patch experiments used to retain cytoplasmic PAs sustained 100 μM carbachol‐induced mICAT was significantly smaller (478±76 pA, n=7) compared to that recorded using conventional whole‐cell configuration with nominally PA‐free pipette solution (1314±76 pA, n=12), but comparable in size to mICAT with 0.3 mM spermine in the pipette solution (509±41 pA, n=19). Intracellular putrescine inhibited mICAT less potently compared to spermine. In conclusion, these results show a novel role of intestinal PAs in mICAT inhibition, which can contribute to their well‐known suppressing effect on the gastrointestinal smooth muscle excitability and contractility. British Journal of Pharmacology (2004) 143, 968–975. doi:10.1038/sj.bjp.0706010