Desensitization of serotonin-stimulated adenylate cyclase in the liver fluke Fasciola hepatica.
Incubation of the intact liver fluke Fasciola hepatica with serotonin (5-HT) resulted in a time-dependent decrease in both 5-HT-stimulated adenylated cyclase activity and specific [3H]LSD binding in the subsequently prepared cell-free fluke particles. In control fluke particles, the activation of adenylate cyclase by 5-HT was biphasic, indicating a high and low affinity form of the 5-HT receptor with half-maximal activation constants (KA) of 0.35 and 2.8 microM respectively. In contrast, 5-HT activation of desensitized particles occurred through a single set of low affinity sites having a KA value of 6.3 microM. The maximal level of 5-HT activation of adenylate cyclase was also reduced in the desensitized particles. Lysergic acid diethylamide (LSD)-stimulated adenylate cyclase activity was also less in the desensitized particles. However, unlike with 5-HT, activation by LSD occurred through a single set of sites for both control and desensitized particles. [3H]LSD binding studies showed that the affinity of LSD for the 5-HT receptors in the control and desensitized particles was similar. Thus, the decrease in [3H]LSD binding and serotonin-stimulated adenylate cyclase activity in the desensitized particles appeared to be due to a preferential loss or inactivation of the high affinity form of the 5-HT receptor. A similar time-dependent loss in 5-HT-stimulated adenylate cyclase and in [3H]LSD binding occurred in cell-free fluke particles upon the addition of 5-HT or LSD. These effects were not due to protein denaturation or metabolism of the ligand during the incubation procedure. This cell-free desensitization was reversible and temperature dependent, and was not affected by ATP or other nucleotides.
D000255:Adenosine Triphosphate / Q000494:pharmacology; D000262:Adenylyl Cyclases / Q000032:analysis*; D000818:Animals; D002417:Cattle; D002474:Cell-Free System; D005210:Fasciola hepatica / Q000201:enzymology*; D008238:Lysergic Acid Diethylamide / Q000378:metabolism; D011985:Receptors, Serotonin / Q000032:analysis; D012701:Serotonin / Q000494:pharmacology*; D013696:Temperature; D014316:Tritium
D011985:Receptors, Serotonin; D014316:Tritium; D012701:Serotonin; D000255:Adenosine Triphosphate; D008238:Lysergic Acid Diethylamide; D000262:Adenylyl Cyclases