The electrogenic sodium pump of the frog retinal pigment epithelium.
Miller S S,Steinberg R H,Oakley B
The Journal of membrane biology
It was previously shown that ouabain decreases the potential difference across an in vitro preparation of bullfrog retinal pigment epithelium (RPE) when applied to the apical, but not the basal, membrane and that the net basal-to-apical Na+ transport is also inhibited by apical ouabain. The suggested the presence of a Na+ - K+ pump on the apical membrane of the RPE. In the present experiments, intracellular recordings from RPE cells show that this pump is electrogenic and contributes approximately - 10 mV to the apical membrane potential (VAP). Apical ouabain depolarized VAP in two phases. The initial, fast phase was due to the removal of the direct, electrogenic component. In the first one minute of th response to ouabain, VAP depolarized at an average rate of 4.4 +/- 0.42 mV/min (n = 10, mean +/- SEM) and VAP depolarized an average of 9.6 +/- 0.5 mV during the entire fast phase. A slow phase of membrane depolarization, due to ionic gradients running down across both membranes, continued for hours at a much slower rate, 0.4 mV/min. Using a simple diffusion model and K+-specific microelectrodes, it was possible to infer that the onset of the ouabain-induced depolarization coincided with the arrival of ouabain molecules at the apical membrane. This result must occur if ouabain affects an electrogenic pump. Other metabolic inhibitors, such as DNP and cold, also produced a fast depolarization of the apical membrane. For a decrease in temperature of congruent to 10 degrees C, the average depolarization of the apical membrane was 7.1 +/- 3.4 mV (n = 5) and the average decrease in transepithelial potential was 3.9 +/- 0.3 mV (n = 10). These changes in potential were much larger than could be explained by the effect of temperature on an RT/F electrodiffusion factor. Cooling the tissue inhibited the same mechanism as ouabain, since prior exposure to ouabain greatly reduced the magnitude of the cold effect. Bathing the tissue in 0 mM [K+] solution for 2 hr inhibited the electrogenic pump, and subsequent re-introduction of 2 mM [K+] solution produced a rapid membrane hyperpolarization. We conclude that the electrogenic nature of this pump is important to retinal function, since its contribution to the apical membrane potential is likely to affect the transport of ions, metabolites, and fluid across the RPE.
Permeability of the arachnoid and pia mater. The role of ion channels in the leptomeningeal physiology.
Filippidis Aristotelis S,Zarogiannis Sotirios G,Ioannou Maria,Gourgoulianis Konstantinos,Molyvdas Paschalis-Adam,Hatzoglou Chrissi
Child's nervous system : ChNS : official journal of the International Society for Pediatric Neurosurgery
PURPOSE:The purpose of this paper is to study the ionic permeability of the leptomeninges related to the effect of ouabain (sodium-potassium-ATPase inhibitor) and amiloride (epithelial sodium channel (ENaC) inhibitor) on the tissue, as well as identify the presence of ion channels. METHODS:Cranial leptomeningeal samples from 26 adult sheep were isolated. Electrophysiological measurements were performed with Ussing system and transmembrane resistance values (R(TM) in Ω*cm(2)) obtained over time. Experiments were conducted with the application of ouabain 10(-3) M or amiloride 10(-5) M at the arachnoidal and pial sides. Immunohistochemical studies of leptomeningeal tissue were prepared with alpha-1 sodium-potassium-ATPase (ATP1A1), beta-ENaC, and delta-ENaC subunit antibodies. RESULTS:The application of ouabain at the arachnoidal side raised the transmembrane resistance statistically significantly and thus decreased its ionic permeability. The addition of ouabain at the pial side led also to a significant but less profound increment in transmembrane resistance. The addition of amiloride at the arachnoidal or pial side did not produce any statistical significant change in the R(TM) from controls (p > 0.05). Immunohistochemistry confirmed the presence of the ATP1A1 and beta- and delta-ENaC subunits at the leptomeninges. CONCLUSIONS:In summary, leptomeningeal tissue possesses sodium-potassium-ATPase and ENaC ion channels. The application of ouabain alters the ionic permeability of the leptomeninges thus reflecting the role of sodium-potassium-ATPase. Amiloride application did not alter the ionic permeability of leptomeninges possibly due to localization of ENaC channels towards the subarachnoid space, away from the experimental application sites. The above properties of the tissue could potentially be related to cerebrospinal fluid turnover at this interface.
10.1007/s00381-012-1688-x