Fig. 4. Mechanisms of action of mosquito natriuretic peptide (MNP). MNP probably belongs to the CRF family of insect diuretic peptides. In Malpighian tubules of the yellow fever mosquito Aedes aegypti, MNP selectively increases the rates of transepithelial Na⁺ secretion by increasing (+) the Na⁺ conductance of the basolateral membrane and by activating Na⁺/K⁺/2Cl^- cotransport. The second messenger of MNP and CRF-like diuretic peptides is cAMP. Whether cAMP also stimulates proton extrusion across the apical membrane is unknown. Table 2 documents the cAMP stimulation of NaCl secretion and not KCl secretion.