Transport pathways for Na+ and Br- (Cl-) in noradrenaline-stimulated frog skin (Rana temporaria)

In vitro relation between preganglionic sympathetic stimulation and activity of cutaneous glands in the bullfrog

1. Activation of cutaneous glands was studied by measuring changes in transepithelial potentiation (TEP) after pre- and postganglionic sympathetic stimulation in the bullfrog, Rana catesbeiana. 2. In normal Ringer solution, TEP was 20-90 mV with the basolateral (inside) surface positive. Single shocks to the preganglionic B pathway decreased TEP by up to 3 mV. Cutaneous depolarizations had a latency of 1.2 s, a rise time of 2.5 s, and decayed with an exponential time constant of 15 s. Similar depolarizations were evoked by postganglionic stimulation. 3. Cutaneous depolarizations summed during repetitive stimulation and > 0.05 Hz. For trains of three stimuli, peak amplitude increased with frequency and saturated at 2 Hz. In some preparations, longer trains evoked polyphasic changes in TEP. Preganglionically evoked cutaneous responses were abolished by (+)-tubocurarine. Postganglionically evoked cutaneous depolarizations were antagonized by phentolamine, but not propranolol. 4. Repetitive preganglionic stimulation of the C pathway (> 100 at 20 Hz) evoked little change in TEP and did not modulate depolarizations evoked through the B pathway. In nicotine, peptidergic cotransmission was enhanced in the ganglia, and repetitive C pathway stimulation evoked cutaneous depolarizations whose time course mirrored that of the postganglionic peptidergic after-discharge. The after-discharge and associated cutaneous depolarization were blocked by a luteinizing hormone-releasing hormone antagonist. 5. The results show cutaneous glands are selectively innervated by B neurones and respond to low levels of neural activity. Asynchronous postganglionic firing mediated by peptidergic cotransmission can provide a basis for heterosynaptic interactions between the B and C pathways.

Relative expression of the human epithelial Na+ channel subunits in normal and cystic fibrosis airways

The availability of the newly cloned subunits (alpha, beta, gamma) of the epithelial Na+ channel (ENaC) permits molecular studies of the pathogenesis of the abnormal Na+ transport rates of cystic fibrosis (CF) airway epithelia. Northern analyses of airway epithelia showed that both normal and CF airway epithelia express ENaC subunit mRNAs in a ratio of alpha > beta > gamma. In situ hybridization studies revealed expression of all three ENaC subunits in the superficial epithelium and the alpha- and beta-subunits in the gland ductular and acinar epithelium of both normal and CF airways. Ribonuclease protection assays revealed that the steady-state levels of alpha-, beta-, and gamma-ENaC mRNAs were similar in CF and normal airway superficial epithelia. These findings indicate that 1) Na+ transport defects in CF airways disease may be expressed in glandular acinar and ductal epithelium as well as superficial epithelium, and 2) the molecular pathogenesis of Na+ hyperabsorption in CF airways does not reflect increased levels of Na+ channel mRNAs, and probably number, but reflects an absence of the normal inhibitory regulation of Na+ channels by CF transmembrane conductance regulator proteins.

Roles of external and cellular Cl- ions on the activation of an apical electrodiffusional Cl- pathway in toad skin

This study is concerned with the short-circuit current, Isc, responses of the Cl(-)-transporting cells of toad skin submitted to sudden changes of the external Cl- concentration, [Cl]o. Sudden changes of [Cl]o, carried out under apical membrane depolarization, allowed comparison of the roles of [Cl]o and [Cl]cell on the activation of the apical Cl- pathways. Equilibration of short-circuited skins symmetrically in K-Ringer's solutions of different Cl- concentrations permitted adjustment of [Cl]cell to different levels. For a given Cl- concentration (in the range of 11.7 to 117 mM) on both sides of a depolarized apical membrane, this structure exhibits a high Cl- permeability, P(Cl)apical. On the other hand, for the same range of [Cl]cell but with [Cl]o = 0, P(Cl)apical is reduced to negligible values. These observations indicate that when the apical membrane is depolarized P(Cl)apical is modulated by [Cl]o; in the absence of external Cl- ions, intracellular Cl- is not sufficient to activate P(Cl)apical. Computer simulation shows that the fast Cl- currents induced across the apical membrane by sudden shifts of [Cl]o from a control equilibrium value strictly follow the laws of electrodiffusion. For each experimental group, the computer-generated Isc versus [( Cl]cell - [Cl]o) curve which best fits the experimental data can only be obtained by a unique pair of P(Cl)apical and Rb (resistance of the basolateral membrane), thus allowing the calculation of these parameters. The electrodiffusional behavior of the net Cl- flux across the apical membrane supports the channel nature of the apical Cl- pathways in the Cl(-)-transporting cells. Cl- ions contribute significantly to the overall conductance of the basolateral membrane even in the presence of a high K concentration in the internal solution.

The effect of amiloride and benzimidazoleguanidine added to the inside medium on electrolyte pathways in the frog skin glands

Serosal amiloride inhibits Na+, K+ and Cl- efflux and reduces short-circuit current and transepithelial conductance in noradrenaline-stimulated frog skin in which the sodium channels in the apical membrane are blocked by amiloride. BIG (benzimidazoleguanidine) inhibits Na+ and K+ efflux and reduces the short-circuit current. In some skins BIG decreased and in others it increased Cl- efflux and conductance. The variable response appears to be due not only to inhibition of salt extrusion from the gland cells but also to activation of Cl- transport through the mitochondria-rich cells, since it was shown that BIG could increase Cl- efflux in gland-free preparations. Different target mechanisms in the gland cells where the two substances may exert their effect are discussed.