The nature of transepithelial potential of the liver

The method of continuous monitoring of rat liver transepithelial electrical potential in situ (the potential between electrodes inserted into the common bile duct and blood flow) with simultaneous measurement of liver blood filling by photoelectroplethysmography is described. It was suggested that spontaneous oscillations of potential value with the period 20-30 sec found in the experiment were associated with synchronous oscillations of blood vessels and bile ducts cross section square caused by synchronization of pre- and postcapillary sphincters activity.     

Effect of modeccin on rat liver ribosomes in vivo.

1. Rat liver microsomes isolated at 6 and 12 h of poisoning with 3 x LD50 (0.3 microgram/100 g body wt.) of modeccin, the toxin of Adenia digitata, have a decreased capacity of protein synthesis in vitro. 2. A similar decrease of protein synthesis is observed with polysomes at 6 h of poisoning. Experiments with recombined ribosomal subunits demonstrate that this is due to inactivation of the 60 S ribosomal subunit. 3. At 6 h of poisoning there is a marked vesiculation and degranulation of the hepatocyte rough endoplasmic reticulum, which is completely fragmented at 24 h of poisoning. Hepatocyte mitochondria are swollen at 6 h and shrunk at 24 h of poisoning. 4. It is concluded that modeccin penetrates inside hepatocytes in vivo, and damages ribosomes in the same manner as it does in vitro. However, mitochondrial damage indicates that ribosomes may not be the only target of modeccin in vivo.     

Effect of thyroxine in vitro on the transmembrane potential of rat liver mitochondria

It has been shown in in vitro experiments that a certain latent period after the addition of thyroxine (T4) and triiodothyronine (T3) was necessary for the manifestation of their effects on transmembrane potential (TMP) of the rat liver mitochondria. The duration of the lag-period decreased upon an increase in the concentrations of these hormones, and T4 at a dose of 2.10(-4) M produced a fall in TMP immediately after its addition. The rate of TMP fall was in proportion with the concentrations of thyroid hormones introduced into the cell, with T3 30-40% more effective than T4. It was established that the action of I2 resembled that of thyroid hormones, namely, a fall in TMP, mitochondrial swelling, activation of transhydrogenase Kl was ineffective. It is suggested that the appearance of the lag-period upon the action of thyroid hormones might be explained by the period of time necessary for the formation of the active iodine forms, as well as by the formation of fatty acids (donators of H+) by mitochondrial phospholipases. All these factors lead to TMP fall resulting in decreased formation of sufficient ATP quantities in mitochondria.     

Alveolar transepithelial potential difference and ion transport in adult rat lung

The complex morphology of the mammalian lung complicates characterization of solute transport across the intact alveolar epithelium. We impaled the subpleural alveolar epithelium with microelectrodes and measured the transepithelial potential difference (PD) of the liquid-filled vascular-perfused left lobe of the rat lung. When the air space was filled entirely with Krebs-Ringer-bicarbonate, the PD was 4.7 mV (lumen negative). The PD was not affected significantly by agents that modify either Na+ or Cl- transport, but replacement of luminal Cl- with gluconate resulted in a fourfold hyperpolarization, a response also noted for large airways. When the airways were blocked by an immiscible nonconducting fluorocarbon, basal PD was not different from unblocked lobes (4.0 mV) but was inhibited 73% by luminal amiloride. Cl(-)-free Krebs-Ringer-bicarbonate blocked in the alveoli with fluorocarbon did not induce hyperpolarization. This result suggests that 1) Cl- permselectivity of the alveolar epithelium is less than that of large airway epithelium and 2) airway PD dominates the voltage across the liquid-filled lung, even when measurements are made from alveoli. When airways are blocked by fluorocarbon, the PD across the alveolar epithelium is largely dependent on Na+ flow through a path with amiloride-sensitive channels.     

Aging is associated with increased clonogenic potential in rat liver in vivo

Cancer increases with age and often arises from the selective clonal growth of altered cells. Thus, any environment favoring clonal growth per se poses a higher risk for cancer development. Using a genetically tagged animal model, we investigated whether aging is associated with increased clonogenic potential. Groups of 4-, 12-, 18-, and 24-month-old Fischer 344 rats were infused (via the portal vein) with 2x10(6) hepatocytes isolated from a normal syngenic 2-month-old donor. Animals deficient in dipeptidyl-peptidase type IV (DPP-IV-) enzyme were used as recipients, allowing for the histochemical detection of injected DPP-IV+ cells. Groups of animals were sacrificed at various times thereafter. No growth of DPP-IV+ transplanted hepatocytes was present after either 2 or 6 months in the liver of rats transplanted at young age, as expected. In striking contrast, significant expansion of donor-derived cells was seen in animals transplanted at the age of 18 months: clusters comprising 7-10 DPP-IV+ hepatocytes/cross-section were present after 2 months and were markedly enlarged after 6 months (mean of 88+/-35 cells/cluster/cross-section). These results indicate that the microenvironment of the aged liver supports the clonal expansion of transplanted normal hepatocytes. Such clonogenic environments can foster the selective growth of pre-existing altered cells, thereby increasing the overall risk for cancer development associated with aging.     

The influence of thyroxine administered in vivo on the transmembrane protonic electrochemical potential difference in rat liver mitochondria.

When mitochondria from normal and thyroxine-treated rats were energized by incubation with succinate, phosphate and MgCl2, it was found that the hormone treatment increased the transmembrane protonic electrochemical potential difference by 16mV and the respiration rate by 46%. Other experiments show these changes to be associated with increases in the intramitochondrial K+ and phosphate concentrations.     

Effect of irradiation on the in vivo and in vitro subcellular distribution of radiostrontium in rat liver

Hepatic mitochondrial strontium uptake is enhanced in whole body irradiated rats. Effect is observed on animals but not on irradiated organ slices. It is likely that a hypophyso - adrenal axis hormone plays a role in changes observed in vivo.     

Effect of angiotensin II on transepithelial potential in the isolated perfused flounder (Platichthys flesus) gill

Angiotensin II (Asp¹, Val⁵) perfused through isolated flounder gills inhibited the transepithelial potential by up to 25 per cent at a concentration of 10⁻⁹M. There was no effect on gill haemodynamics and the subsequent response to 10⁻⁵ M adrenaline was normal.     

Glucagon effect on rat liver protein synthesis in vivo

The in vivo effect of glucagon administration on hepatic polyribosomal profiles has been studied. Glucagon did not change significantly total, free or bound polyribosomal fractions 30–45 minutes after its administration. The combined administration of glucagon plus antiinsulin serum failed to show any significant effect of glucagon over the antiinsulin serum treated control. Glucagon increased valine production in the perfused isolated liver. These results suggest that the well known amino acid catabolic action of glucagon may be preferentially mediated through an increased proteolysis. Since it is known that glucagon increases considerably in vivo the liver cyclic AMP levels then its lack of effect on polyribosomal profiles might indicate that the postulated role for the cyclic nucleotide on liver protein synthesis must be taken cautiously.     

Lithium's effects on rat liver glucose metabolism in vivo

Oral administration of lithium carbonate to fed-healthy rats strongly decreased liver glycogen content, despite the simultaneous activation of glycogen synthase and the inactivation of glycogen phosphorylase. The effect seemed to be related to a decrease in glucose 6-phosphate concentration and to a decrease in glucokinase activity. Moreover, in these animals lithium markedly decreased liver fructose 2,6-bisphosphate, which could be a consequence of the fall in glucose 6-phosphate and of the inactivation of 6-phosphofructo-2-kinase. Liver pyruvate kinase activity and blood insulin also decreased after lithium administration. Lower doses of lithium carbonate had less intense effects. Lithium administration to starved-healthy and fed-streptozotocin-diabetic rats caused a slight increase in blood insulin, which was simultaneous with increases in liver glycogen, glucose 6-phosphate, and fructose 2, 6-phosphate. Glucokinase, 6-phosphofructo-2-kinase, and pyruvate kinase activities also increased after lithium administration in starved-healthy and fed-diabetic rats. Lithium treatment activated glycogen synthase and inactivated glycogen phosphorylase in a manner similar to that observed in fed-healthy rats. Glycemia was not modified in any group of animals. These results indicate that lithium acts on liver glycogen metabolism in vivo in at least two different ways: one related to changes in insulinemia, and the other related to the direct action of lithium on the activity of some key enzymes of liver glucose metabolism.     

Kinetics of sulfation in the rat in vivo and in the perfused rat liver

Sulfation of phenols and similar low-molecular-weight substrates in the rat in vivo is a rather complex process. Besides enzyme kinetic parameters, cosubstrate availability (indirectly measured by serum sulfate concentration) and competition with glucuronidation also play a role. For some substrates extensive extrahepatic sulfation occurs, accounting for more than 50% of the total-body sulfation capacity. However, the hepatic contribution may be under-estimated when drugs are administered into the hepatic portal vein, because saturation of hepatic metabolism may occur under those conditions. Inside the liver, sulfation is located primarily in zone 1, the periportal area. This can be shown in the single-pass perfused rat liver by perfusion in either the normal or retrograde flow direction. In the rat sulfate conjugates are eliminated preferentially in urine, whereas glucuronides are excreted to a high extent in bile. Therefore, it is important to collect both bile and urine in the characterization of pharmacokinetics of conjugation in vivo. Selective inhibition of sulfation by pentachlorophenol and 2,6-dichloro-4-nitrophenol facilitates studies of the role of sulfation in elimination of its substrates, and the competition between sulfation and glucuronidation for the same substrate.     

Effect of membrane potential on equilibrium poise between cytochromea and cytochromec in rat liver mitochondria

The object of this work was to test the suggestion that the equilibrium poise between cytochromea and cytochromec in mitochondria might be influenced by the membrane potential.

1. The midpoint potentials of cytochromes (c+c ₁) and cytochromea (CO present) were found to be 250 mV and 245 mV, respectively, by equilibrating rat liver mitochondria with mixtures of ferrocyanide and ferricyanide anaerobically in presence of antimycin A and measuring the redox state of the cytochromes spectrophotometrically. In absence of CO, cytochrome oxidase gave an anomalous redox titration curve with a “midpoint” at about 275 mV.
2. When the mitochondria were equilibrated with ferricyanide/ferrocyanide, the redox poise of cytochromea (CO present) and of cytochromes (a+a ₃) but not of cytochromes (c+c ₁) was dependent on the sign and magnitude of the membrane potential developed by treating the mitochondria as follows: by adding ATP, by chaging the composition of the suspension medium so as to vary the Donnan or Nernst potential, by adding valinomycin in a medium of low K⁺ ion content, or by adding a pulse of acid or alkali when the membrane was made permeable to protons with FCCP.
3. The findings agree with the suggestion that the respiratory chain is arranged across the cristae membrane with cytochromesc ₁ andc in contact with the outer phase and cytochromesa anda ₃ plugged through, so that the equilibrium distribution of electrons between thec anda cytochromes is influenced by the electric field across the membrane.