Abstract

Cell-free particles from the liver fluke Fasciola hepatica contain a highly active serotonin stimulated adenylate cyclase. Serotonin (5-HT) stimulates this enzyme 25 to 30-fold over basal activity to activities of 0.80 ± 0.10 nmoles/min·mg protein. Histamine, dopamine, octopamine, epinephrine, and carbachol failed to activate this adenylate cyclase. The kinetics of activation by 5-HT showed apparent negative cooperativity with a Hill coefficient of 0.7. The lower apparent affinity half-maximal activation by 5-HT occurred at 2.1 ± 0.3 µM. Any substitutions on the 5-HT molecule reduced apparent affinity and degree of activation. Apparent affinity and intrinsic activity of indoleamines decreased with decreasing structural similarity to 5-HT. All indoleamines tested compete for the serotonin receptor. Derivatives of lysergic acid also activate this adenylate cyclase, with very high apparent affinity. D-lysergic acid diethylamide (LSD) was the most potent derivative activating maximally about 25% of 5-HT stimulated activity. Half-maximal activation by D-LSD occurred at 40 nM. Activation by LSD was totally stereospecific, with the L-isomer inactive even at 1 mM. Both D- and L-LSD antagonized 5-HT stimulation, but the L-isomer had a 500-fold decreased affinity. 2-bromo LSD (BOL) competitively antagonized 5-HT activation as shown by Schild analysis. BOL also directly inhibited basal (nonactivated) adenylate cyclase. The direct inhibition and antagonism of 5-HT by BOL both involved a single population of receptor sites with the same inhibition constant. This result suggests that BOL inhibits adenylate cyclase by interacting with the 5-HT site and that BOL has a "negative" efficacy for this receptor. The evidence supports a single class of adenylate cyclase regulated only by serotonin receptors.