Effect of mucosal H+ and chemical modification on transcellular K+ current in frog skin

Plin5 Bidirectionally Regulates Lipid Metabolism in Oxidative Tissues

Cytoplasmic lipid droplets (LDs) can store neutral lipids as an energy source when needed and also regulate the key metabolic processes of intracellular lipid accumulation, which is associated with several metabolic diseases. The perilipins (Plins) are a family of proteins that associate with the surface of LDs. As a member of Plins superfamily, perilipin 5 (Plin5) coats LDs in cardiomyocytes, which is significantly related to reactive oxygen species (ROS) production originated from mitochondria in the heart, consequently determining the progression of diabetic cardiomyopathy. Plin5 may play a bidirectional function in lipid metabolism which is in a state of dynamic balance. In the basic state, Plin5 inhibited the binding of comparative gene identification-58 (CGI-58) to adipose triglyceride lipase (ATGL) by binding CGI-58, thus inhibiting lipolysis. However, when the body is under stress (such as cold, fasting, exercise, and other stimuli), protein kinase A (PKA) phosphorylates and activates Plin5, which then causes Plin5 to release the binding site of CGI-58 and ATGL, prompting CGI-58 to bind to ATGL and activate ATGL activity, thus accelerating the lipolysis process, revealing the indispensable role of Plin5 in lipid turnover. Here, the purpose of this review is to summarize the present understanding of the bidirectional regulation role of Plin5 in oxidative tissues and to reveal its potential role in diabetic cardiomyopathy protection.

The dependence of electrical transport pathways in Malpighian tubules on ATP

The relationship between the intracellular ATP concentration [ATP](i) and the electrical properties of principal cells was investigated in Malpighian tubules of the yellow fever mosquito, Aedes aegypti. Under control conditions, [ATP](i) was 0.91 mmol l(-1), the input resistance of the principal cell (R(pc)) was 334.1 k Omega, and the basolateral membrane was marked by a large K(+)-conductance and a membrane voltage (V(bl)) of -75.8 mV. Peritubular cyanide (CN, 0.3 mmol l(-1)) reduced [ATP](i) to 0.08 mmol l(-1) in less than 2 min; however, V(bl) dropped to -8 mV and R(pc) increased to 3150.8 k Omega in 8 min, while the K(+)-conductance of the basolateral membrane disappeared. Upon washout of CN, V(bl) and R(pc) returned to control values within 2 min, and the basolateral membrane recovered its K(+)-conductance. The recovery of normal [ATP](i) took 15 min. Dose-dependence and EC(50) values for the CN-inhibition of V(bl) and the increase in R(pc) were strikingly similar (184.0 micromol l(-1) and 164.4 micromol l(-1)). Similar effects of metabolic inhibition were observed with dinitrophenol (DNP), but the EC(50) values were 50.3 micromol l(-1) and 71.7 micromol l(-1) for the effects on V(bl) and R(pc), respectively. Barium, a blocker of K(+)-channels, significantly hyperpolarized V(bl) to -89.1 mV and increased R(pc) to 769.4 k Omega under control conditions, but had no effects during metabolic inhibition. These results illustrate a temporal relationship between [ATP](i) and electrogenic and conductive transport pathways in principal cells that is consistent with the role of ATP as an integrator of transport steps at apical and basolateral membranes of the cell. When [ATP](i) drops to levels that are 10% of control, the V-type H(+)-ATPase is inhibited, preventing the extrusion of K(+) to the tubule lumen. At the same time, basolateral membrane K(+)-channels close, preventing the entry of K(+) from the hemolymph. Intracellular K(+) homeostasis is thus protected during metabolic inhibition, allowing the cell to re-establish K(+) transport when ATP is synthesized again.

Insect midgut K+ secretion: concerted run-down of apical/basolateral transporters with extra-/intracellular acidity

In lepidopteran larvae, three transport mechanisms are involved in the active and electrogenic K+ secretion that occurs in the epithelial goblet cells of the midgut. These consist of (i) basolateral K+ channels, allowing K+ entry from the haemolymph into the cytosol, (ii) apical electrogenic K+/2H+ antiporters, which are responsible for secondary active extrusion of K+ from the cell into the gut lumen via the goblet cavity and (iii) apical V-ATPase-type proton pumps. The latter energize apical K+ exit by building up a large, cavity-positive electrical potential that drives the antiporters. Net K+ secretion (IK) can be measured as short-circuit current (Isc) across the in vitro midgut mounted in an Ussing chamber. We investigated the influence of protons on the transepithelial IK and the partial reactions of the basolateral K+ permeability (PK) and the apical, lumped ‘K+ pump’ current (IP) at various extra- and intracellular pH values. In particular, we wanted to know whether increased cellular acidity could counteract the reversible dissociation of the V-ATPase into its V1 and Vo parts, as occurs in yeast after glucose deprivation and in the midgut of Manduca sexta during starvation or moulting, thus possibly enhancing K+ transport.When intact epithelia were perfused with high-[K+] (32 mmol l–1) salines with different pH values, IK was reversibly reduced when pH values fell below 6 on either side of the epithelium. Attempts to modify the intracellular pH by pulsing with NH4+ or propionate showed that intracellular acidification caused a reduction in IK similar to that obtained in response to application of external protons. Treatment with azide, a well-known inhibitor of the mitochondrial ATP synthase, had the same effect as pulsing with ammonium or propionate with, however, much faster kinetics and higher reversibility. Breakdown of the basolateral or apical barrier using the antibiotic nystatin allowed the intracellular pH to be clamped to that of the saline facing the nystatin-treated epithelial border. Cell acidification achieved by this manipulation led to a reduction in both apical IP and basolateral PK. The transepithelial IK showed an approximately half-maximal reduction at external pH values close to 5 in intact tissues, and a similar reduction in IP and PK values was seen at an intracellular pH of 5 in nystatin-permeabilised epithelia. Thus, the hypothesized V1Vo stabilization by cell acidity is not reflected in the pH-sensitivity of IP. Moreover, all components that transport K+ are synchronously inhibited below pH 6. The significance of our findings for the midgut in vivo is discussed.

Na+ and Cl- net absorption by the isolated skin of Rana esculenta

In the last five years, several measurements of 22Na+ influx (Ji) and outflux (Jo) across symmetrical parts of the isolated skin of Rana esculenta, under permanent short circuitation, were performed in our Institute. The mean value of the 22Na+ net fluxes (Ji-Jo) exceeded the mean value of the short circuit current measurements (1.14 +/- 0.04 versus 0.98 +/- 0.02 microE.cm-2.h-1, 253 experiments). Since this discrepancy could be due to a concomitant Cl- net absorption, 36Cl- unidirectional fluxes were detected under similar experimental conditions. The Cl- net flux mean value was 0.11 +/- 0.02 microE.cm-2.h-1 (316 experiments) which accounts for 70% of the discrepancy between the Na+ net flux and short circuit current. This Cl- net absorption occurred in the absence of electrochemical gradients and was very likely maintained by a Na+/K+/2Cl- cotransport located at the outermost membrane of the epithelium. In fact bumetanide challenge (10(-5) M in the external fluid) strongly inhibited 36Cl- influx and 22Na+ influx across this tissue and cleared off the discrepancy between short circuit current and sodium net flux.

Common channels for water and protons at apical and basolateral cell membranes of frog skin and urinary bladder epithelia. Effects of oxytocin, heavy metals, and inhibitors of H(+)-adenosine triphosphatase

We have compared the response of proton and water transport to oxytocin treatment in isolated frog skin and urinary bladder epithelia to provide further insights into the nature of water flow and H+ flux across individual apical and basolateral cell membranes. In isolated spontaneous sodium-transporting frog skin epithelia, lowering the pH of the apical solution from 7.4 to 6.4, 5.5, or 4.5 produced a fall in pHi in principal cells which was completely blocked by amiloride (50 microM), indicating that apical Na+ channels are permeable to protons. When sodium transport was blocked by amiloride, the H+ permeability of the apical membranes of principal cells was negligible but increased dramatically after treatment with antidiuretic hormone (ADH). In the latter condition, lowering the pH of the apical solution caused a voltage-dependent intracellular acidification, accompanied by membrane depolarization, and an increase in membrane conductance and transepithelial current. These effects were inhibited by adding Hg2+ (100 microM) or dicyclohexylcarbodiimide (DCCD, 10(-5) M) to the apical bath. Net titratable H+ flux across frog skin was increased from 30 +/- 8 to 115 +/- 18 neq.h-1.cm-2 (n = 8) after oxytocin treatment (at apical pH 5.5 and serosal pH 7.4) and was completely inhibited by DCCD (10(-5) M). The basolateral membranes of the principal cells in frog skin epithelium were found to be spontaneously permeable to H+ and passive electrogenic H+ transport across this membrane was not affected by oxytocin. Lowering the pH of the basolateral bathing solution (pHb) produced an intracellular acidification and membrane depolarization (and an increase in conductance when the normal dominant K+ conductance of this membrane was abolished by Ba2+ 1 mM). These effects of low pHb were blocked by micromolar concentrations of heavy metals (Zn2+, Ni2+, Co2+, Cd2+, and Hg2+). Lowering pHb in the presence of oxytocin (50 mU/ml) produced a transepithelial current (3 microA.cm-2 at pHb 5.5) which was blocked by 100 microM of Hg2+, Zn2+, or Ni2+ at the basolateral side, and by DCCD (10(-5) M) or Hg2+ (100 microM) from the apical side. The net hydroosmotic water flux (JH2O) induced by oxytocin in frog bladder sacs was blocked by inhibitors of H(+)-adenosine triphosphatase (ATPase). Diethylstilbestrol (DES 10(-5) M), oligomycin (10(-8) M), and DCCD (10(-5) M) prevented JH2O when present in the lumen. These effects cannot be attributed to inhibition of metabolism since cyanide (10(-4) M), or 2-deoxyglucose (10(-3) M) had no effect on JH2O.(ABSTRACT TRUNCATED AT 400 WORDS)