Epithelial Apical Membrane Research Articles

Effects of Vasopressin on Epithelial Transport

Vasopressin increases the permeability of responsive epithelia for a variety of chemical species, such as sodium, urea, water, and lipophilic molecules. The analysis of the mechanisms leading to the stimulation of water transport (hydrosmotic action) is the main topic of this review. A model implying the quantal insertion of water channels into the apical epithelial membrane by an exocytosislike process is discussed with reference to biophysical, biochemical, and ultrastructural results obtained in the past few years. It is also shown that, in nonexcitable cells, insertion of transport units packed in intracytoplasmic structures may be a rather general mechanism for controlling membrane transport rates.

Effects of theophylline and cAMP on intestinal allosteric chloride transport in freshwater prawns

Active transmural Cl- transport across the intestine of the freshwater prawn Macrobrachium rosenbergii and the transmural potential difference resulting from the net movements of this ion and Na+ are significantly reduced or abolished in the presence of 10 mM theophylline or 1 mM dibutyryl adenosine 3',5'-cyclic monophosphate (DB-cAMP). The locus of theophylline or DB-cAMP action appears to be at the epithelial apical membrane where increased intracellular cAMP or other cyclic nucleotide such as guanosine 3',5'-cyclic monophosphate (cGMP) may interact directly or through another agent, such as the divalent cation Ca2+, to reduce or eliminate coupled Na+-Cl- influx into the cell from the gut lumen. Under control conditions, cotransport of Na+ and Cl- across the epithelial apical membrane of prawn intestine occurs by an allosteric carrier protein exhibiting sigmoidal influx kinetics for the two monovalent ions. Exogenous Ca2+ is a third binding ligand for this multisubunit protein, with the divalent cation serving as an allosteric activator of the transport system. Bilateral intestinal incubation with 10 mM theophylline abolished Cl- influx (control, 0.71 +/- 0.23; theophylline treated, 0.03 +/- 0.03 mumol X cm-2 X h-1). Addition of 1 mM DB-cAMP to intestinal incubation saline significantly (P less than 0.01) reduced the maximal Cl- influx velocity from 0.76 +/- 0.08 to 0.33 +/- 0.05 mumol X cm-2 X h-1 without altering the anion affinity constant (control, 102 +/- 5; DB-cAMP, 96 +/- 19 mM). In addition, the cyclic nucleotide had no effect on the Hill interaction coefficient of the transfer mechanism (control, 3.22 +/- 0.12; DB-cAMP, 3.42 +/- 0.68).(ABSTRACT TRUNCATED AT 250 WORDS)