Serotonin Stimulates Both Cytosolic and Membrane‐Bound Guanylate Cyclase in NG108–15 Cells

Abstract: The cyclic GMP (cGMP) content was rapidly (>30 s) increased by serotonin [5‐hydroxytryptamine (5‐HT)] (EC₅₀= 10 μM), and the increase lasted for > 10 min in NG108–15 cells. The 5‐HT‐induced elevation of cGMP level (EC₅₀= 10 μM) at 20 s (“fast” elevation) was inhibited by ICS 205–930 or MDL 72,222 and by Ca²⁺ deficiency in the reaction medium but not by organic Ca²⁺ antagonists. The 5‐HT effect at 10 min (“slow” elevation) was not inhibited by several antagonists for 5‐HT receptors of the IA, IB, IC., ID, 2, and 3 subtypes and was independent from external Ca²⁺ concentration. The fast and slow effects of 5‐HT were similar to the effects of bradykinin and atrial natriuretic peptide (ANP), respectively, in aspects of both Ca²⁺ dependency and time course of the effects. Bradykinin transiently stimulated formation of inositol phosphates as well as accumulation of cGMP, a finding suggesting that intracellular Ca²⁺ is involved in bradykinin‐induced cGMP accumulation as shown in the fast response to 5‐HT. ANP. an activator of membrane‐associated guanylate cyclase (mGC), slowly (∼60 s) increased the cGMP content (EC₅₀= 10 nAf), a result lasting for >10 min, and the effects were independent from external Ca²⁺, as shown in the slow response to 5‐HT. 5‐HT and ANP did not induce formation of inositol phosphates. These results suggest that (a) the fast effects of 5‐HT on cGMP level elevation are mediated by 5‐HT₃ receptors, which activate cytosolic guanylate cyclase through Ca²⁺ entry via ion channels other than voltage‐sensitive Ca²⁴ channels, and (b) the slow effects seem to be due to an unidentified subtype of 5‐HT receptor that activates ANP‐sensitive mGC.