The effects of hypercapnia on force and rate of contraction and intracellular pH of perfused ventricles from the land snail Helix lucorum (L.).

The effects of hypercapnia, together with low and high levels of extracellular Ca(2+), on heart activity and intracellular pH were examined in isolated perfused hearts from the land snail Helix lucorum. In addition, the intracellular level of Ca(2+) was determined in slices of ventricles superfused with both normal and hypercapnic salines, containing low and high concentrations of Ca(2+), to investigate whether low extracellular pH affects the entry of Ca(2+) into the heart cells. We also examined the effect of a saline that simulated the composition of the haemolymph of snails after estivating for 3 months on the heart activity and intracellular pH. The results showed that hypercapnia causes decreases in the rate and force of heart contraction, and these are more pronounced in the presence of low levels of extracellular Ca(2+). Moreover, the present results indicate that Ca(2+) maintains the contractility of the heart muscle under acidic conditions and seems to act by competing with protons for the Ca(2+ )binding sites on sarcolemma. The negative effect of hypercapnia on heart activity appears to be due to a reduction in extracellular pH rather than to changes in intracellular pH.

Excretion of thiosulphate, the main detoxification product of sulphide, by the lugworm arenicola marina L

Thiosulphate, the main sulphide detoxification product, is accumulated in the body fluids of the lugworm Arenicola marina. The aim of this study was to elucidate the fate of thiosulphate. Electrophysiological measurements revealed that the transepithelial resistance of body wall sections was 76+/-34 capomega cm2 (mean +/- s.d., N=14), indicating that the body wall of the lugworm is a leaky tissue in which mainly paracellular transport along cell junctions takes place. The body wall was equally permeable from both sides to thiosulphate, the permeability coefficient of which was 1. 31x10(-)3+/-0.37x10(-)3 cm h-1 (mean +/- s.d., N=30). No evidence was found for a significant contribution of the gills or the nephridia to thiosulphate permeation. Thiosulphate flux followed the concentration gradient, showing a linear correlation (r=0.997) between permeated and supplied (10-100 mmol l-1) thiosulphate. The permeability of thiosulphate was not sensitive to the presence of various metabolic inhibitors, implicating a permeation process independent of membrane proteins and showing that the lugworm does not need to use energy to dispose of the sulphide detoxification product. The present data suggest a passive permeation of thiosulphate across the body wall of A. marina. In live lugworms, thiosulphate levels in the coelomic fluid and body wall tissue decreased slowly and at similar rates during recovery from sulphide exposure. The decline in thiosulphate levels followed a decreasing double-exponential function. Thiosulphate was not further oxidized to sulphite or sulphate but was excreted into the sea water.