The key role of the mitochondria-rich cell in Na+ and H+ transport across the frog skin epithelium

We have investigated the possibility that the mitochondria-rich (MR) cells participate in sodium and proton transport, when the frog skin epithelium is bathed on its apical side with solutions of low Na⁺ concentration, by comparing transport rates with morphological observations (MR cell number and MR cell pit surface area). Frogs were adapted to various salinities or the isolated skins were treated with the following hormones, deoxycorticosterone acetate (DOCA), arginine vasotocin (AVT) and oxytocin in order to modify the transport of sodium and hydrogen ions. Adaptation of the frogs (either 3–4 days or 7–10 days) to distilled water, NaCl (50 mmol/l), KCl (50 mmol/l) or Na₂SO₄ (25 mmol/l) solutions modified the Na⁺ transport rate and the morphology of the epithelium. The highest Na⁺ transport rates were found for the animals adapted to the Na⁺ free solutions and were correlated with an increase in the total MR cell pit-surface area (number of MR cells x individual cell pit-surface area). The KCl adaptated group showed the largest increase in sodium and proton transport and also presented a metabolic acidosis as reflected by plasma acidification (pCO₂ increase and HCO₃^– decrease). Proton secretion and sodium absorption were also found to be stimulated by either serosal DOCA addition (10^–6 M) or during acidification of the epithelium by serosally applied CO₂. Na⁺ transport was enhanced by AVT (10^–6 M) or oxytocin (100mU/ml) when the skin was bathed on its apical side with a high Na⁺ containing solution (115 mmol/l), whereas these hormones did not exert any effect on Na⁺ transport when the apical solution was low in Na⁺ (0.5mmol/l). It is concluded that MR cells play a key role in Na⁺ and H⁺ transport through the frog skin epithelium when bathed on its apical side with a low Na⁺ containing solution. Distinct pathways for sodium transport through two cell types (MR cells and granular cells) are proposed depending on the Na⁺ concentration of the solution bathing the apical side of the epithelium.