Procaine has opposite effects on passive Na and K permeabilities in frog skin

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Abstract
Procaine has opposite effects on the active transport of Na+ when applied on the mucosal side of the frog skin [where it produces a stimulation of the short-circuit current (I sc)] or when added on the serosal side (where it produces an inhibition ofI sc). In an attempt to reveal and localize the primary effect of procaine on either the apical or latero-basal membranes of the epithelial cells, we have tried to “chemically dissect” both membrane functions with inhibitors and ionophores. When applied on the apical side of the latero-basally depolarized epithelium, 25 mmol/l procaine increasesI sc andV oc (transepithelial open-circuit potential), while decreasing the transepithelial resistance. TheE 1−E 2 linearity domain of the I–V curves is narrowed. On the serosal side of the depolarized epithelium, the same concentration of procaine does not affectI sc andV oc (which are already inhibited) but it produces an increase in the transepithelial resistance (R t). Procaine influence on the passive K+ permeability was studied by using the ionophore nystatin, which is assumed to form channels permeable to K+, when applied on the amiloride blocked apical membrane. In nystatin-treated epithelia, 25 mmol/l procaine on the apical side decreaseI sc,V oc andR t. In parallel experiments during Cl− substitution by SO 4 2− , the procaine effects onI sc andV oc are no longer maintained, but transient. The results suggest that procaine positively influences the Na+ transient through the apical Na+-channels, and inhibits the epithelial permeability for K+, possibly by reducing K+-ions accessibility to the K+-channels.