Effects of environmental conditions on mitochondrial-rich cell density and chloride transport in toad skin

Chloride flux across amphibian skin is usually passive, yet largely conductive; previous reports have suggested that aldosterone influences this pathway. The conductive Cl^– pathway and its regulation were examined further, across the abdominal skin of toads (Bufo marinus) adapted to various environments. Short-circuit current (I_sc), total conductance (G_t) and Cl^– influx (J_Cl) were measured in conditions such that there was net Cl^– movement in absence of Na⁺ transport. In salt-deprived animals compared to salt-adapted ones, there was a significant increase in J_Cl (563 vs 200 pmol cm^–2 s^–1), aldosteronaemia (4.2 vs 1.1 nmol/l), as well as MRC density (1458 vs 851 mm^–2). After adaptation to dilute Na₂SO₄ compared to MgCl₂, J_Cl (631 vs 313 pmol cm^–2 s^–1) as well as the density of mitochondria-rich cells (MRC) (1306 vs 710 mm^–2) practically doubled, while the toads' aldosteronaemia was lower (2.4 vs 10.8 nmol/1). In all groups of toads, J_Cl was matched by I_sc, and there was a close correlation between G_t and J_Cl (r=0.96), which confirms the conductive nature of transepithelial Cl^– movement. Furthermore, the relationship between J_Cl and MRC density (r=0.75) argues in favour of a role played by MRC on Cl^– conductance of epithelia such as amphibian skin. As aldosterone injected for 1 week into NaCl-adapted toads did not influence MRC density and as aldosteronaemia was not correlated with Cl^– conductance, this hormone does not emerge as the determinant of these parameters.