5-Hydroxytryptamine stimulates fluid secretion in locust malpighian tubules independently of cAMP

1. 5-Hydroxytryptamine (5-HT) stimulates fluid secretion by semi-isolated Malpighian tubules of Locusta in a dose-dependent manner. 2. The threshold of stimulation is between 10^-8 and 10^-7 M 5-HT; maximal activation occurs at doses greater than 10^-6 M. 3. Relative to the activation induced by diuretic hormone (storage lobe extracts), 5-HT increases the rate of fluid secretion by only 65%. 4. Phentolamine, the α-adrenergic blocker, failed to inhibit either DH or 5-HT stimulated secretion. 5. Diuretic hormone raises the levels of intracellular of cAMP, and activates adenylate cyclase in plasma membrane preparations of Locusta Malpighian tubules. 6. 5-HT (10^-4 M) has no effect in either assay system. 7. Thus 5-HT can stimulate fluid secretion independently of cAMP. 8. A hypothetical model for hormone stimulated fluid secretion by Locusta Malpighian tubules, involving dual-receptor activation, is proposed. 9. Other biogenic amines, including octopamine, adrenalin, dopamine, synephrine and the formamidine chlordimeform were tested for their ability to stimulate fluid secretion. 10. Only dopamine showed a weakly stimulatory effect.