ANTI-DIURESIS IN THE BLOOD-FEEDING INSECT RHODNIUS-PROLIXUS STAL - THE PEPTIDE CAP(2B) AND CYCLIC-GMP INHIBIT MALPIGHIAN TUBULE FLUID SECRETION

Rhodnius prolixus eliminates NaCl-rich urine at high rates following i ts infrequent but massive blood meals. This diuresis involves stimulat ion of Malpighian tubule fluid secretion by diuretic hormones released in response to distention of the abdomen during feeding. The precipit ous decline in urine flow that occurs several hours after feeding has been thought until now to result from a decline in diuretic hormone re lease, We suggest here that insect cardioacceleratory peptide 2b (CAP( 2b)) and cyclic GMP are part of a novel mechanism of anti-diuresis. Se cretion rates of 5-hydroxytryptamine-stimulated Malpighian tubules are reduced by low doses of CAP(2b) or cyclic GMP. Maximal secretion rate s are restored by exposing tubules to 1 mmol l(-1) cyclic AMP. Levels of cyclic GMP in isolated tubules increase in response to CAP(2b), con sistent with a role for cyclic GMP as an intracellular second messenge r. Levels of cyclic GMP in tubules also increase as urine output rates decline in vivo, suggesting a physiological role for this nucleotide in the termination of diuresis.