[Model experiments of physiologic and pharmacologic interactions with epithelial ionic currents induced by...

The present experiments were conducted to reveal regulatory mechanisms that control the stable electric potential (PD) and reversible changes in this potential during mechanical stimulation (dPD). The aim of the present work was to determine the role of C fibre endings and adrenergic and cholinergic receptors in the regulation of ion transport and to assess the applicability of frog skin as a model for physiological and pharmacological analysis of sodium and chloride currents. The experimental material consisted of 413 fragments of frog skin from 139 animals. Cross-epithelial electric potential (PD in mV) and electric resistance (R in ohms) of isolated frog skin placed in Ussing apparatus were measured. Mechanical stimulation of sensory receptors resulted in reversible hyperpolarisation (Tab. 1, Fig. 1). Preincubation of frog skin in the presence of amiloride (AMI) completely inhibited this reaction (Tab. 1, Fig. 2a), whereas incubation in bumetanide (BUME) lowered PD and did not induce hypersensitivity to mechanical stimuli (Tab. 1). Addition of capsaicin (KAPSA) to the preparations incubated in Ringer's solution (RH) and in Ringer's solution reinforced with BUME resulted in depolarisation (Tab. 1). On the other hand, incubation with KAPSA caused hyperpolarisation in the AMI group. KAPSA supplementation influenced the sensitivity to mechanical stimulation in all experimental groups. It was found that ambroxol (ABX) added to the preparations incubated in RH solution and in BUME, inhibited the hyperpolarisation and did not affect the magnitude of the reaction in the AMI group (Tab. 2, Fig. 3). ABX administration caused a decrease in the reactivity to mechanical stimuli in the RH group. Blocking of both adrenergic and cholinergic receptors with a solution containing thymolol, benextramine, hexamethonium, and atropine (ABHT) inhibited the hyperpolarisation in all groups studied and was without effect on the magnitude of mechanical stimulation in AMI and BUME groups (Tab. 2). In the RH group, ABHT weakened the reaction to mechanical stimulation. Changes in the pH of Ringer's solution (pH 6.4) caused a decrease in the electric potential and in the reactivity to mechanical stimuli, without altering resistance (Tab. 3). Addition of toluene diisocyanate (TDI) to preparations incubated in RH solution and in AMI increased the hyperpolarisation (Tab. 4). TDI also led to changes in the reaction to mechanical stimulation, i.e. induced depolarization in both groups. On the other hand, TDI administration to frog skin incubated with BUME did not cause any reaction change. The following conclusions were drawn from the present findings: 1. Hyperpolarisation caused by physiological and pharmacological stimuli is effected principally through increased sodium flux. 2. Stimulation of C fibre endings, as well as of adrenergic and cholinergic receptors, inhibits ion transport. 3. Isolated frog skin is a useful model for studying sodium and chloride transport through mammalian epithelium.