The relationship of mucoid substances and ion and water transport, with new data on intestinal goblet cells and a model for gastric secretion.
It is known from the physical chemistry that mucoid substances (glycosaminoglycans, proteoglycans, glycoproteins) constitute polyanionic gels with anomalous colligative behaviour and differential preference for binding some cations such as H+, K+, Ca2+. The mucus secreting membranes in animals also constitute ion and water transporting epithelia with morphologically characterised pericellular compartments. These pericellular compartments are not free-fluid spaces as is assumed in conventional physiological thought, but contain polyanionic mucoid matrices with a prevalence of sulphated varieties such as heparan sulphates. Measurements with electron probe X-ray microanalysis of cryosections (EPXMA) show that at the various pericellular sites the local dry mass of these matrices can vary from less than 5% to greater than 30% of the wet mass. EPXMA measurements in several epithelia from insects and from vertebrates (gastro-intestinal tract) also show that the total concentrations of the major chemical elements (P, S, Na, K, Mg, Cl) at these pericellular sites is distinct both from their concentrations in the cytoplasm and in the bulk mucosal and serosal compartments. In all the examples a particular role of pericellular polyanions in sequestering K+ and Ca2+, and excluding Cl- is indicated. It is proposed that the general K-sequestering ability of these pericellular mucoids conserves the K+ ions leaking out of the cells and thus reduces the energy load for active transport in the cells through a K-recycling mechanism. In the midgut caeca of the desert locust, Schistocerca gregaria a dense mucoid matrix (30% dry mass) in the luminal crypts (cristae) helps to maintain a high K+ activity at the mucosal surface of the absorptive cells, apparently necessary for a K-linked absorption of nutrients and water. In the electrically isolated apical cavities of the midgut 'goblet' cells of Manduca sexta larvae, a similar matrix provides fixed counter anions for the electrogenically secreted K+, thus enabling the gut to sustain, in vivo, a transepithelial potential (TEPD) of some 150 mV (lumen + ve). This TEPD is used for maintaining a high pH of 11-12 in the midgut lumen. A similar mechanism of maintaining a high TEPD for sensory perception is suggested in insect sensilla and in vertebrate cochlea. A much lower concentration (10% dry mass) of the matrix in the apical invaginations (canaliculi) of Calliphora salivary glands facilitates a fluid secretion of isotonic KCl at high rates.(ABSTRACT TRUNCATED AT 400 WORDS)
Gupta
B L
BL
Department of Zoology, Cambridge University, UK.
D000818:Animals; D001692:Biological Transport; D004848:Epithelium / Q000378:metabolism; D005753:Gastric Mucosa / Q000378:metabolism*; D007313:Insecta / Q000378:metabolism; D007413:Intestinal Mucosa / Q000378:metabolism*; D007477:Ions; D008954:Models, Biological; D009093:Mucus / Q000378:metabolism*; D014867:Water / Q000378:metabolism*
D007477:Ions; D014867:Water