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)