Desensitization of serotonin-stimulated adenylate cyclase in the liver fluke Fasciola hepatica

Abstract

Incubation of the intact liver fluke Fasciola hepatica with serotonin (5-HT) resulted in a time-dependent decrease in both 5-HT-stimulated adenylate 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 ( K A ) of 0.35 and 2.8 μM respectively. In contrast, 5-HT activation of desensitized particles occurred through a single set of low affinity sites having a K A value of 6.3 μM. 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.