Phosopholipase C involvement in activation of the muscarinic receptor-operated cationic current in guinea-pig ileal smooth muscle cells

Abstract

In guinea-pig intestinal smooth muscle, M2 muscarinic receptors activate nonselective cationic current (I_cat). Nonetheless, M3 receptors which link to the G_q/11 protein/phospholipase C (PLC) system have also been suggested to play a crucial role in I_cat activation via Ca²⁺-independent mechanisms. Therefore, we examined using whole-cell patch clamp techniques whether PLC is involved in I_cat activation in single smooth muscle cells. 1) The PLC inhibitor U73122 (0.1-1 μM) concentration-dependently inhibited I_cat evoked by ascending application of carbachol (CCh; 1-300 μM), and completely blocked at 1 μM. It also inhibited I_cat evoked by intracellularly-applied GTPγS (400 μM). 2) The inactive analogue U73343 (0.1-1 μM) did not significantly affect CCh- or GTPγS-evoked I_cat. 3) Antibodies against the α subunit of G_q/11 proteins blocked only an oscillatory component of CCh-evoked I_cat such as that linked to G_q/11 protein-regulated PLC activity. 4) OAG (10-100 μM), an analogue of diacylgrycerol (DAG), evoked no or only small current by itself and remained CCh-evoked I_cat unchanged. These results indicate that PLC plays a crucial role in I_cat activation, and also suggest that the PLC's role is neither regulated by G_q/11 proteins nor mediated by DAG. [Jpn J Physiol 54 Suppl:S137 (2004)]