Electrophysiology of the Mantle of Anodonta Cygnea

ABSTRACT When gassed with a CO2-containing mixture, under short-circuit conditions, the isolated outer mantle epithelium (OME) of Anodonta cygnea generated a current which exhibited cyclic variations throughout the year. The intracellular potential, under short-circuit conditions, had an average value of –31 ±0·5 mV (N =65). The potential was sensitive to changes in concentration of potassium and chloride on the haemolymph side of the preparation, but not on the shell side, and was insensitive to changes in sodium concentration on either side. When the preparation was gassed with pure oxygen the current fell by 85 ± 1% (N = 8). A similar fall in current (88 ± 2%, N = 8) was observed when the solution bathing the apical side of the epithelium was prepared without bicarbonate and gassed with 95% O2 + 5% CO2. If this same bathing solution was gassed with 100% oxygen, the current fell by 67 ± 2% (N= 8) at pH7·2 and by 92±4% (N =8) at pH4·5. The short-circuit current was inhibited by DIDS (0·5 mmol 1−1) and SITS (0·5 mmol 1−1) when these drugs were applied on the haemolymph side. The current was also inhibited by DNP (1 mmol 1−1), iodoacetamide (1 mmol 1−1) and diamox (1 mmol 1−1). Amiloride (1 mmol 1−1) blocked the current but only when applied on the haemolymph side. Ouabain (0-1 mmol 1−1) did not affect the current. The net fluxes of rubidium (used as a tracer for potassium), chloride and calcium, measured with 86Rb, 36C1 and 45Ca, respectively, were very small when compared with the short-circuit current. There was a small net flux of sodium (measured with 22Na) towards the haemolymph side. The net flux of bicarbonate (measured with [14C]bicarbonate) was equal to the short-circuit current and was inhibited by DIDS. The permeability of the preparation to calcium was an order of magnitude higher than the permeability to sodium, potassium or chloride. The intracellular concentrations of potassium and chloride measured with ion-sensitive microelectrodes were 26·5 ±1·1 (N =16) and 7·9 ±0·3 (N =30) mmol 1−1, respectively. When these concentrations were measured with chemical methods the values found were 29·4 ±0·4 (N =20) and 12·9 ±0·6 (N =20) mmol 1−1, respectively. The chemically measured intracellular concentration of sodium was 14·0 ± 0·4 (N = 20) mmoll−1. When bicarbonate was removed from the haemolymph side the intracellular concentration of chloride increased. The movements of calcium across the OME are by simple diffusion and the calcification of the shell probably results from the balance between a diffusion of calcium towards the extrapallial compartment when a favourable electrochemical gradient for this ion is created and the equally passive movement of calcium which results from the uphill transport of bicarbonate ions carried out by the epithelium. The balance between these two processes might undergo annual cyclic variations.