The fluid secretion rate (FSR) of isolated Malpighian tubules

of Drosophila melanogaster Meig. is stimulated by the 3′,5′monophosphates of adenosine (cAMP; Dow et al., 1994), guanosine (cGMP; Davies et al., 1995), inosine (cIMP), cytidine (cCMP), thymidine (cTMP) and uridine (cUMP) (Riegel et al., 1998). The way in which these compounds influence fluid secretion is not known, but stimulation may follow from the accumulation of cyclic nucleotides within the cells of the Malpighian tubules: both [3H]cAMP and [3H]cGMP are accumulated, and it is assumed that the same is true of cIMP, cCMP, cTMP and cUMP. Two characteristics of the accumulation of [3H]cAMP suggested that carriermediated transport might be involved: the uptake was saturable, and it could be inhibited by large excesses (2000fold) of non-radioactive cAMP, cGMP, cIMP or cCMP. Although radioactive cAMP and cGMP were metabolised, it was estimated that sufficient of these compounds remained intact to be effective in stimulating the FSR (Riegel et al., 1998). Thus, cyclic nucleotide uptake by Malpighian tubules of Drosophila melanogaster exhibits two characteristics typical of transport mechanisms for organic anions and cations (Pritchard and Miller, 1993, 1996): first, the mechanism has an affinity for more than one molecular species and, second, uptake of one species is influenced by the presence of other transported species. A logical first step in characterising the putative transport mechanism for cyclic nucleotides in the Malpighian tubules was to discover whether non-nucleotide anionic species would 2339 The Journal of Experimental Biology 202, 2339–2348 (1999) Printed in Great Britain © The Company of Biologists Limited 1999 JEB1996