Role of Phosphoinositide Hydrolysis and Protein Kinase C Activation in 5HT ₂ Receptor Function in Smooth Muscle

Abstract

Serotonin (5-hydroxytryptamine; 5HT) has long been known to play a major role in smooth muscle functioning. In general, 5HT induces contraction of several smooth muscle groups including vascular smooth muscle (both arteries and veins), iris smooth muscle, gastrointestinal smooth muscle and perhaps other smooth muscle types. Because of its pronounced activity, 5HT has been thought to play a role in the etiology of diseases which result from a dysregulation of smooth muscle activity including coronary artery disease, vascular headaches, various motility disorders of the gastrointestinal tract and retroperitoneal fibrosis. Insight into the mechanisms of 5HT action in smooth muscle could elucidate novel pharmacologic approaches to treat these diseases. Smooth muscle has also provided a convenient model system to study mechanisms of 5HT action; in fact, early studies involving the isolation of 5HT and the classification of 5HT receptor subtypes mainly employed smooth muscle preparations (Gaddum and Picarelli, 1957). It is possible, therefore, that knowledge gained about 5HT’s actions in smooth muscle could prove to be of general importance for understanding the molecular mechanisms involved in 5HT activity.

Serotonin ₂ (5-hydroxytryptamine ₂ ; 5HT ₂ ) receptors in many types of smooth muscle, similar to blowfly salivary gland (Fain and Berridge, 1979), appear to be coupled to a phospholipase C which regulates the hydrolysis of phosphoinositides like phosphatidylinositol-4,5-bisphosphate (PIP ₂ ) phosphatidylinositol-4-phosphate (PIP) and phosphatidylinositol (PI) (Roth et al, 1984). Indeed, in rat aorta (Roth et al, 1984; Nakaki et al, 1985), rabbit iris smooth muscle, rat superior vena cava (Cohen and Wittanour, 1986) and certain gastrointestinal smooth muscle types (Jafferji and Michell,1976) 5HT receptor occupation appears to induce phosphoinositide hydrolysis. A similar function for 5HT ₂ receptors has been described in many other tissues including brain(Kendall and Nahorski, 1985; Conn and Sanders-Bush, 1985), platelets (Leysen et al, 1985) and certain cell lines (see Roth and Chuang, 1987 and Chuang, 1989 for reviews).

Studies employing selective agonists and antagonists have demonstrated that it is the 5HT ₂ receptor which activates PI metabolism in these tissues (Roth et al, 1986; Conn and Sanders-Bush, 1987; Leysen et al, 1985). Because of the similar pharmacologic profile for the 5HT _1C and 5HT ₂ receptors both of which activate PI hydrolysis, it has not been completely clear which receptor is involved in the various tissues studied. Recent experiments (Roth and Ciaranello, manuscript in preparation) using cDNA probes for the 5HT ₂ and 5HT _1C receptors suggests that only the 5HT ₂ receptor is expressed in rat aorta. This would indicate that it is the 5HT ₂ receptor which is important for activating PI hydrolysis rat aorta. Also, experiments in which eukaryotic cells were transfected with a plasmid encoding the 5HT ₂ receptor provided proof that the 5HT ₂ receptor may be coupled to a PI-specific phospholipase C (Pritchett et al, 1988). Despite this abundance of evidence documenting 5HT ₂ -receptor mediated PI metabolism, the functional significance of this biochemical pathway has proved difficult to elucidate.