The mosquito is a vector responsible for transmitting various pathogens to humans, and their prominence as chief vectors of human disease is largely due to their anthropophilic blood feeding behaviour. Larval stage mosquitoes must deal with the potential dilution of their haemolymph in freshwater, whereas the haematophagus female faces the challenge of excess ion and water intake after a blood meal. The excretory system, composed of the Malpighian tubules (MTs) and hindgut, is strictly controlled by neuroendocrine factors, responsible for the regulation of diuresis across all developmental stages. The highly studied insect MTs are influenced by a variety of diuretic hormones and, in some insects, anti-diuretic factors. In the present study, we investigated the effects of CAPA-1 neuropeptide on larval and adult female MTs stimulated with various diuretic factors including serotonin (5-HT), a corticotropin-related factor (CRF) diuretic peptide, a calcitonin-related diuretic hormone (DH) and a kinin-related diuretic peptide. Overall, our findings establish that CAPA-1 specifically inhibits secretion of larval and adult MTs stimulated by 5-HT and DH, whilst having no activity on MTs stimulated by other diuretic factors. Furthermore, although CAPA-1 acts as an anti-diuretic, it does not influence the relative proportions of cations transported by adult MTs, thus maintaining the kaliuretic activity of 5-HT and natriuretic activity of DH In addition, we tested the effects of the second messenger cGMP in adult MTs. We established that cGMP has similar effects to CAPA-1, strongly inhibiting 5-HT- and DH-stimulated fluid secretion, but with only minor effects on CRF-stimulated diuresis. Interestingly, although CAPA-1 has no inhibitory activity on kinin-stimulated fluid secretion, cGMP strongly inhibited fluid secretion by this diuretic hormone, which targets stellate cells specifically. Collectively, these results support thatCAPA-1 inhibits select diuretic factors acting on the principal cells and this probably involves cGMP as a second messenger. Kinin-stimulated diuresis, which targets stellate cells, is also inhibited by cGMP, suggesting that another anti-diuretic factor in addition to CAPA-1 exists and may utilize cGMP as a second messenger.
