FIG. 4. Schematic representation of two proposed 5-HT4 transduction pathways. The positive coupling to adenylyl cyclase is presumably through Gs, leading to a reduction in certain voltage-dependent K+ currents (IK(V)) through the protein kinase A phosphorylation pathway. The coupling of 5-HT4 receptors to the calcium-activated potassium current (IK(Ca)) does not appear to involve cAMP.