Modes of control of insect Malpighian tubules: synergism, antagonism, cooperation and autonomous regulation

Rates of fluid and ion secretion by insect Malpighian tubules are controlle d by peptides, including CRF-related peptides and kinins, and in some speci es by serotonin. It now appears to be a general rule that tubule secretion rate is controlled through the interaction of two or more haemolymph-borne factors. In this review we suggest that these interactions may be classifie d as synergistic, cooperative, or antagonistic. When presented together, tw o diuretic factors may act in synergism, so that fluid secretion is stimula ted to a greater extent than the sum of their individual effects. Synergism may involve one or more second messenger systems. Alternatively, diuretic factors may act in cooperation, so that although their overall effects are additive, cation and anion transport pathways are controlled separately by distinct second messenger systems. There is also one example of antagonism between factors controlling tubule secretion and between their respective s econd messengers; one factor is stimulatory, the other is inhibitory. In ad dition to the complex control of fluid and ion transport by haemolymph-born e factors, sophisticated autonomous regulatory mechanisms have been identif ied in Malpighian tubules. When triggered by appropriate stimuli, these mec hanisms play homeostatic roles, preserving haemolymph osmolality or ionic c omposition. (C) 2000 Elsevier Science Ltd. All rights reserved.