Stimulation of fluid secretion of Malpighian tubules of Drosophila melanogaster Meig. by cyclic nucleotides of inosine, cytidine, thymidine and uridine

External application of the 3',5'-cyclic monophosphates of inosine, cytidin e, uridine and thymidine stimulated the fluid secretion rate (FSR) of Malpi ghian tubules isolated from Drosophila melanogaster. The evidence suggested that the cyclic nucleotides acted intracellularly in some capacity. Recept ors of the 'purinergic' type appeared not to be major contributors to fluid secretion; of three purinergic agonists tried, adenosine, adenosine 5'-mon ophosphate (AMP) and adenosine 5'-triphosphate (ATP), only adenosine had an effect, but this was not observed consistently. None of the purinergic ago nists interfered with the stimulation of the FSR by adenosine 3',5'-cyclic monophosphate (cAMP), The maximum stimulation of the fluid-secretion rate b y any cyclic nucleotide was approximately double the unstimulated (control) rate. Tubules stimulated to less than maximal FSR by one cyclic nucleotide could be stimulated maximally by an appropriate concentration of another c yclic nucleotide. Malpighian tubules bathed in solutions that contained eit her [H-3]cAMP or [H-3]cGMP accumulated radioactivity to a level many times that in the medium. Accumulation of radioactivity by tubules bathed in 430n moll(-1) [H-3]cAMP was suppressed by 1mmoll(-1) non-radioactive cyclic nucl eotides in the order cAMP much greater than cGMP>cIMP>cCMP; neither cTMP no r cUMP suppressed the accumulation of [H-3]cAMP. Approximately 35 % of the [H-3]cAMP and 80 % of the [H-3]cGMP that entered the Malpighian tubule cell s was metabolised to compounds that were not identified. It was concluded t hat cyclic nucleotides enter the Malpighian tubule cells by at least one tr ansport mechanism which is particularly sensitive to purine-based nucleotid es.