X-ray microanalysis of elements in frozen-hydrated sections of an electrogenic K+ transport system: The posterior midgut of tobacco hornworm (Manduca sexta) in vivo andin vitro

Abstract

The lepidopteran midgut is a model for the oxygen-dependent, electrogenic K+ transport found in both alimentary and sensory tissues of many economically important insects. Structural and biochemical evidence places the K+ pump on the portasome-studded apical plasma membrane which borders the extracellular goblet cavity. However, electrochemical evidence implies that the goblet cell K+ concentration is less than 50 mM. We used electron probe X-ray microanalysis of frozen-hydrated cryosections to measure the concentration of Na, Mg, P, S, Cl, K, Ca and H2O in several subcellular sites in the larval midgut of Manduca sexta under several experimental regimes. Na is undetectable at any site. K is at least 100 mM in the cytoplasm of all cells. Typical in vivo values (mM) for K were: blood, 25; goblet and columnar cytoplasm, 120; goblet cavity, 190; and gut lumen, 180. The high K concentration in the apically located goblet cavity declined by 100 mM under anoxia. Both cavity and gut fluid are Cl deficient, but fixed negative charges may be present in the cavity. We conclude that the K+ pump is sited on the goblet cell apical membrane and that K+ follows a nonmixing pathway via only part of the goblet cell cytoplasm. The cavity appears to be electrically isolated in alimentary tissues, as it is in sensory sensilla, thereby allowing a PD exceeding 180 mV (lumen positive) to develop across the apical plasma membrane. This PD appears to couple K+ pump energy to nutrient absorption and pH regulation.

Cloning and characterization of a potassium-coupled amino acid transporter

“…The K m for sodium was five times lower than that for potassium. The high affinity of KAAT1 for Na ϩ possibly allows the midgut cells to take up this ion even from the low concentrations (Ͻ5 mM) found in the lumen of M. sexta (17).…”

Elemental changes in the brain, muscle, and gut cells of the housefly, Musca domestica, exposed to heavy metals

“…This is perhaps surprising given that the role of K þ , Cl À and Na þ ions is crucial for various essential functions in all cells, for maintenance of membrane polarization, pH stability inside and outside cells, and the transport of amino acids, carbohydrates, and HCO 3 À (Coddington and Chamberlin, 1999;Dow et al, 1984;Emery et al, 1998). Together with monovalent ions, heavy metals can reach the cell cytoplasm and be transported through epithelial cells to potential sites of deposition (Emery et al, 1998).…”

K+ absorption by locust gut and inhibition of ileal K+ and water transport by FGLamide allatostatins.

“…The fluid produced by the Malpighian tubules enters the hindgut where in terrestrial insects both ions and water can be selectively reabsorbed as required to maintain homeostasis (Coast et al, 1999). Plant-feeding insects need to cope with excess K + and Mg 2+ from their diet (Dow et al, 1984;Dow, 1987). It has been shown that adult locusts fed on grass have 1.6 times more K + in their hemolymph relative to starved locusts (Hoyle, 1954).…”