Electrogenic proton secretion in the hindgut of the desert locust, Schistocerca gregaria

Insect acid-base physiology

Acid-base status influences many aspects of insect biology, including insect distributions in aquatic systems, insect-plant and insect-pathogen interactions, membrane transport phenomena, and the mode of action of pesticides. Acid-base status in the hemolymph and gut lumen of insects is generally well regulated but varies somewhat within individuals owing to effects of temperature, activity, discontinuous ventilation, and diet. The pH of the midgut lumen varies with the phylogeny and feeding ecology. Insect fluids have buffer values similar to those of vertebrates. The respiratory system participates in acid-base homeostasis primarily by regulating the internal carbon dioxide (partial) pressure via changes in spiracular opening and convective ventilation. The epithelia of the renal system and gut participate in hemolymph acid-base regulation by varying acid-base transport in response to organismal acid-base status. Evidence to date suggests that the dominant mechanisms for control of renal acid-base excretion involve hormonal regulation of H+-V-ATPase activity.

Nature and control of chloride transport in insect absorptive epithelia

Insect epithelia most commonly absorb from KCI-rich, low Na+ fluids. This is true of the locust hindgut, which is functionally analogous to vertebrate kidney tubules. Active absorption of Cl- at the apical membrane is the predominant transport process giving rise to a large short-circuit current (Isc) after stimulation by neuropeptides (CTSH, ITP) via cAMP as second messenger. This Cl- transport is not coupled to or driven secondarily by Na+, K+, HCO3-, Ca2+, or Mg2+ movements. An apical V-type H+ ATPase acidifies the hindgut lumen but at a rate that is 10-15% of Cl-dependent Isc. The evidence to date as to whether the resulting large apical proton gradient is used to drive Cl- transport secondarily by an apical H+/Cl- symport is mixed. Thus a primary mechanism of Cl- absorption remains an alternative possibility. The complete primary structure of a large neuropeptide stimulant (ITP: 72 amino acids) of locust ileal Cl- transport has recently been deduced from its cDNA. This is the first putative insect neuropeptide hormone shown to stimulate ion transport across absorptive epithelia for which the primary sequence has been deduced.

Intracellular pH regulation by the plasma membrane V-ATPase in Malpighian tubules of Drosophila larvae

The functional significance of the apical vacuolar-type proton pump (V-ATPase) in Drosophila Malpighian tubules was studied by measuring the intracellular pH (pHi) and luminal pH (pHlu) with double-barrelled pH-microelectrodes in proximal segments of the larval anterior tubule immersed in nominally bicarbonate-free solutions (pHo 6.9). In proximal segments both pHi (7.43 +/- 0.20) and pHlu (7.10 +/- 0.24)) were significantly lower than in distal segments (pHi 7.70 +/- 0.29, pHlu 8.09 +/- 0.15). Steady-state pHi of proximal segments was much less sensitive to changes in pHo than pH of the luminal fluid (delta pHlu/delta pHo was 0.49 while delta pHi/delta pHo was 0.18; pHo 6.50-7.20). Re-alkaliniziation from an NH4Cl-induced intracellular acid load (initial pHi recovery rate 0.55 +/- 0.34 pH.min-1) was nearly totally inhibited by 1 mmol.l-1 KCN (96% inhibition) and to a large degree (79%) by 1 mumol.l-1 bafilomycin A1. In contrast, both vanadate (1 mmol.l-1) and amiloride (1 mmol.l-1) inhibited pHi recovery by 38% and 33%, respectively. Unlike amiloride, removal of Na+ from the bathing saline had no effect on pHi recovery, indicating that a Na+/H+ exchange is not significantly involved in pHi regulation. Instead pHi regulation apparently depended largely on the availability of ATP and on the activity of the bafilomycin-sensitive proton pump.