Ionic mechanisms of the cardiac pacemaker potential

The experimental evidence so far described in the literature is reviewed to explain the ionic mechanisms underlying the cardiac pacemaker potential in the sinoatrial node cell. Following gating mechanisms underlie the slow diastolic depolarization of the SA node cells. The delayed rectifier K+ channels (mainly the rapidly activated component of the delayed rectifier K+ currents, which is blocked by E-4031) activated during the preceding action potential are deactivated during diastole. The inactivation of the L-type Ca2+ channel is removed during the early diastolic period and results in an increasing inward current, provided that the amplitude of the window component is of significant amplitude. Because of its sustained nature, the removal of inactivation of the sustained inward current Ist, also generates inward current. The negative membrane potential near the maximum diastolic potential activates the hyperpolarization-activated non-selective cation current, I(f). Finally, the L-type Ca2+ channel is activated at the late phase of diastole depolarization, resulting in the maximum rate of rise of the action potential. These time- and voltage-dependent changes in membrane conductance occur in the presence of a significant background conductance. During the slow diastolic depolarization, Ist and IK may be the major component in the inward and outward currents, respectively.

Effects of lipid administration on lymphatic apolipoprotein A-IV and B output and synthesis

We examined the mesenteric lymphatic and portal venous transport of triglyceride (TG), free fatty acids (FFA), apolipoproteins (apo)A-IV and B in response to a bolus duodenal infusion of a TG-free control solution, and long-chain (18:1) and medium-chain (8:0) TG (LCT and MCT, respectively) emulsions in the rat. Additionally, intestinal and hepatic apo A-IV and apo B mRNA levels were also measured. Lymph apo A-IV, apo B (B-48), FFA, and TG output increased after LCT infusion, whereas only apo A-IV and FFA outputs increased after MCT infusion. On the other hand, portal FFA and apo A-IV transports increased at 15 min after MCT infusion but not after LCT infusion. Portal TG and apo B transports were not altered by either MCT or LCT infusion. Jejunal apo A-IV mRNA was increased after both MCT and LCT infusions. Hepatic apo A-IV mRNA levels increased only after MCT infusion. Conversely, neither LCT nor MCT had any effect on apo B mRNA levels in intestine or liver. These results indicate that apo A-IV is regulated by MCT absorption and that fatty acid reesterification and lipoprotein assembly are not prerequisite for such regulation. Conversely, it is likely that apo B-48 participates only in the formation and/or transport of chylomicrons after LCT absorption.

[Subfractionation of serum high density lipoproteins and its clinical significance]

Serum concentrations of HDL subfractions, LpAI and LpAI:AII, were simultaneously determined simply, specifically and rapidly by use of rocket immunoelectrophoresis with the home-made gel plates. The method was reproducible, and the results correlated well with those obtained with Sebia's kits. The normal levels of LpAI were 57 +/- 13 mg/dl in males and 59 +/- 15 mg/dl in females. LpAI levels were significantly lower in patients with renal failure and atherosclerosis, and tended to decrease in hepatic diseases than those in normal subjects. The levels of LpAI:AII were also decreased in these three diseased groups. Decreases in LpAI levels were more slightly in patients with hepatic or renal diseases than those in LpAI:AII levels. On the contrary, decrease in LpAI level was more markedly in patients with atherosclerosis than that in the other subfraction. The results in the present study suggest that the determination of HDL subfractions will be useful for diagnosis of various diseases.

Reduction of intestinal apo A-IV mRNA levels in the cirrhotic rat

In the present study, intestinal apo A-IV synthesis was investigated using a carbon tetrachloride (CCl4)-induced cirrhosis rat model. Triglyceride (TG) content in rat cirrhotic liver was increased markedly by 170% (P < 0.001) and apo B was increased by 20% (P < 0.05) compared with control levels. These results reflected the steatotic change in the liver. In contrast, TG levels in the small intestine of cirrhotic rats decreased significantly (P < 0.01). In addition, intestinal apo A-IV (jejunum P < 0.001; ileum P < 0.01) and its mRNA levels (jejunum P < 0.01; ileum P < 0.05) were also reduced. The decreased apo A-IV content in the jejunum was confirmed by immunohistochemical analysis. These results indicate that intestinal apo A-IV synthesis in cirrhosis is suppressed, at least under the condition of an overnight fast. Therefore, decreased intestinal apo A-IV synthesis may relate to the decreased ability to absorb fat in cirrhosis, but a fat-loading study will be necessary to confirm this hypothesis. It is unknown from the present study why serum apo A-IV level is not significantly decreased, despite a reduction in apo A-IV synthesis. The clearance of apo A-IV by the liver may be delayed or apo A-IV synthesis may be rather markedly enhanced during fat absorption in liver cirrhosis.

The effects of the beta-adrenergic-blocking agents, timolol and carteolol, on plasma lipids and lipoproteins in Japanese glaucoma patients

PURPOSE

To determine whether two topical beta-blockers, timolol and carteolol, differently affect plasma lipids and lipoproteins in normolipidemic Japanese patients with glaucoma.

PATIENTS AND METHODS

Thirty-three normolipidemic patients with primary open-angle glaucoma or ocular hypertension were randomly allocated to and completed 16 weeks of bilateral treatment with 0.5% timolol, 1.0% carteolol, or 2.0% carteolol twice daily in a three-center, prospective study. Patients using any drugs affecting plasma lipids or with a history of beta-blocker use of hyperlipoproteinemia were excluded. Fasting blood lipids and lipoproteins, including total cholesterol, high-density lipoprotein cholesterol, triglyceride, and apoproteins, were measured three times before therapy was initiated. These measurements were repeated every 4 weeks during the treatment period.

RESULTS

The level of high-density lipoprotein cholesterol significantly decreased in the timolol treatment group but did not change in the carteolol treatment groups. The ratio of total cholesterol minus high-density lipoprotein cholesterol to high-density lipoprotein cholesterol increased in the timolol treatment group.

CONCLUSIONS

Topical beta-blockers do affect plasma lipids in Japanese patients with glaucoma. The effects of timolol are greater than those of carteolol.

[Clinical significance of serum soluble interleukin-2 receptor (sIL-2R) determination in patients with rheumatoid arthritis]

It has been recently shown that soluble interleukin-2 receptor (sIL-2R) is related to the pathogenesis of autoimmune diseases. We determined serum levels of sIL-2R in patients with rheumatoid arthritis (RA). Fifty-three patients (3 males and 50 females from 35 to 70 years old) and sex- and age-matched 50 healthy subjects were included in this study. The sIL-2R concentrations positively correlated with Lansbury index of RA (r = 0.602, p < 0.01). There was a significant correlation between sIL-2R levels and the prevalence of CD3+.HLA-DR+ lymphocytes (r = 0.420, p < 0.01).

Impaired synthesis of retinol-binding protein and transthyretin in rat liver with bile duct obstruction

To gain further insight into the protein metabolism in bile duct-obstruction, we examined the synthesis of retinol-binding protein (RBP) and transthyretin (TTR) in rats with common bile duct-ligation. In these rats, liver and plasma levels of RBP and TTR decreased markedly, whereas liver retinoid contents remained unchanged. Although there appeared no decrease in the total amount of RBP or TTR mRNA expressed in the liver, the subcellular distribution of these mRNAs changed from the membrane-bound polysome fraction to the membrane-unbound polysome fraction. This abnormal distribution recovered rapidly after biliary drainage, resulting in the subsequent recovery of the plasma RBP and TTR levels. These observations suggest that cholestasis inhibits the synthesis and secretion of RBP and TTR by disrupting the binding of their mRNAs to membrane-bound polysomes. Plasma levels of RBP and TTR might be sensitive indicators of the recovery of protein synthesis after biliary drainage in patients with obstructive biliary disorders.

Cell distension-induced increase of the delayed rectifier K+ current in guinea pig ventricular myocytes

Single ventricular myocytes of guinea pig heart were distended by applying a positive pressure of 5 to 20 mm Hg in the pipette during the whole-cell voltage clamp. The amplitude of delayed rectifier K+ current (I(K)) was increased by approximately 1.5 times, whereas the inward rectifier K+ current was scarcely affected. The increase of I(K) was reversible by applying a negative pressure of -10 to -30 mm Hg accompanied by shrinkage of the inflated cell. This response of I(K) was largely attributed to the E-4031-insensitive component of I(K). The fully activated current amplitude, measured using long-lasting depolarizing pulses (> 30 seconds) to +60 mV, was increased by the cell distension. The activation time course of I(K) during the long pulse consisted of more than three exponential components, and the slowest time constant was decreased by the distension from control 20.2 +/- 7.7 seconds (n=4) to 7.6 +/- 1.6 seconds (n=5). We failed to detect an involvement of microtubules or microfilaments, protein kinase C, and Ca2+ in the inflation-mediated increase of I(K).

Non-selective cation current of guinea-pig endocardial endothelial cells

1. Endocardial endothelial (EE) cells, isolated by the enzymatic treatment of guinea-pig heart, were used for whole-cell voltage clamp experiments. 2. The inward rectifier K+ current was observed in about half of the experiments. The contribution of Ca(2+)-dependent K+ current to the resting membrane conductance was also suggested. 3. After the K+ conductances were suppressed, removal of external Na+ revealed an inward cation current (1.2 pA pF-1, at -45 mV), whose slope conductance was a saturable function of external Na+ concentration. When Na+ was totally replaced by various monovalent cations, the order of the membrane conductances was K+ > Rb+ > Cs+ > Na+ > Li+. 4. This basal non-selective cation current was blocked by either Gd3+ or La3+, and showed slight outward rectification. 5. Addition of 20 mM Ca2+ or Ba2+, but not Mg2+ or Mn2+, to the Na(+)-free solution, induced an inward current, indicating that this current possesses a significant Ca2+ permeability. 6. In approximately 15% of the experiments, ATP and histamine induced another type of non-selective cation current, which showed different ion selectivity (Na+ > K+, Cs+) and rectification (inward). 7. The basal non-selective cation current is responsible for both the low resting potential and the leak Ca2+ influx of EE cells.

Inactivation of the cardiac Na+ channels in guinea-pig ventricular cells through the open state

1. The inactivation kinetics of the Na+ current were investigated using the improved oil-gap voltage clamp method in single ventricular cells of guinea-pig hearts. 2. Activation of the Na+ current was observed on depolarization more positive than -50 mV from a holding potential of -100 mV, and inactivation was complete during these depolarizations. The time course of current decay was fitted by a double exponential at potentials between -40 and -15 mV, and virtually by a single exponential at more positive potentials. The decay time courses examined either by the double-pulse protocol or the single-pulse protocol were similar. 3. The double-pulse protocol clearly revealed a sigmoidal onset of inactivation on depolarization. The initial delay of inactivation decreased with more positive potentials. The time course of double-pulse inactivation was reconstructed by integrating the Na+ current recorded by a continuous depolarization. 4. These findings are consistent with the hypothesis that the cardiac Na+ channel inactivates exclusively through the open state.

Classification of ion channels in the luminal and abluminal membranes of guinea-pig endocardial endothelial cells

1. The ion channels on both the luminal and abluminal membranes of endocardial endothelial (EE) cells were separately recorded using the patch clamp technique in the guinea-pig heart. 2. The major population consisted of two types of non-selective cation channels, which showed open probabilities of 0.21 and 0.33 at the resting potential, and conductances of 36 and 11 pS, respectively. 3. The next major class was Cl- channels with an ohmic conductance of 409 pS. The channel was quiescent in the cell-attached mode but was activated by strong depolarization after excising the patch membrane. 4. The channels activated by intracellular Ca2+ were mainly K+ channels showing a 34 pS slope conductance and, less frequently, Ca(2+)-dependent K+ channels having a large conductance (210 pS). The inward rectifier K+ channel (32 pS) was also observed. 5. The non-selective cation channels were recorded on the luminal membrane, but scarcely on the abluminal membrane, suggesting an active transport of K+ and Na+ across the endocardium. 6. The resting membrane conductance of the EE cells may be provided mostly by non-selective cation channels and 34 pS Ca(2+)-dependent K+ channels.

Hyperpolarization induced by vasoactive substances in intact guinea-pig endocardial endothelial cells

1. The responses of guinea-pig endocardial endothelial (EE) cells to various vasoactive substances were investigated in either the small tissue preparation or freshly isolated cells using the patch clamp technique. 2. The mean resting potential of the EE cell was -44 mV in the small tissue preparation, and applications of ATP, ADP, AMP, adenosine, histamine and substance P induced transient hyperpolarizations of -22, -21, -9, -10, -23 and -15 mV, respectively. The membrane potential of EE cells failed to respond to acetylcholine, bradykinin, thrombin, atrial natriuretic peptide, vasopressin and serotonin. 3. The whole-cell voltage clamp of dissociated cells revealed a transient increase of K+ conductance underlying the ATP and histamine responses. The agonist-induced current showed no time-dependent change during voltage steps. The response was showed no time-dependent change during voltage steps. The response was prevented by adding 10 mM EGTA to the pipette solution. 4. In the cell-attached single channel recordings, ATP induced transient K+ channel activities having a slope conductance of 34 pS. In inside-out patches, similar K+ channels were activated by applying Ca2+ of more than 0.1 microM. 5. These findings are consistent with the idea that the Ca(2+)-dependent K+ channel is involved in the hyperpolarizing response of EE cells, as described in vascular endothelial cells.

A sustained inward current activated at the diastolic potential range in rabbit sino-atrial node cells

1. After blocking both the hyperpolarization-activated current and the membrane K+ conductance, depolarizations from -80 mV to between -70 and -50 mV induced a sustained current in sino-atrial node cells. We have tentatively designated this current Ist. 2. Ist was blocked by both organic and inorganic Ca2+ channel blockers, but was insensitive to tetrodotoxin (30 microM). Isoprenaline increased Ist. 3. The peak of Ist (at about -50 mV) was increased to 149 +/- 13% (n = 8, P < 0.01) by lowering the external Ca2+ concentration ([Ca2+]o) from 1.8 to 0.1 mM, in contrast to the marked depression of the L-type Ca2+ current. In 0.1 mM [Ca2+]o solution, the nicardipine-sensitive current-voltage relation showed the peak amplitude at about -50 mV and a reversal potential of +37.4 +/- 4.4 mV (n = 5). The isoprenaline-sensitive component also showed a reversal potential of about +30 mV. 4. Reducing [Na+]o from 140 to 70 mM in 0.1 mM [Ca2+]o decreased Ist to 53 +/- 5% (n = 7, P < 0.01). Increasing [Ca2+]o or [Mg2+]o decreased the amplitude of Ist. The half-maximum concentration of Mg2+ inhibition was 2.2 mM. 5. At 1.8 mM [Ca2+]o, Ist was inactivated by clamping for 5s at -10 mV, and gradually recovered after repolarization to -80 mV with a half-time of 1.36 +/- 0.4 s (n = 6). 6. The transitional sino-atrial node cell had minimal amplitude of Ist. 7. These characteristics of Ist are qualitatively comparable to those of the monovalent cation conductance of the L-type Ca2+ channel induced by depleting external divalent cations to the micromolar range. We conclude that Ist is generated by a novel subtype of L-type Ca2+ channel.

Polymorphism and Distribution of Apo(a)

Several apo(a) isoforms, controlled by a series of alleles Lp(a)F, Lp(a)B, Lp(a)S1, Lp(a)S2, Lp(a)S3, Lp(a)S4 and null, were found in 470 healthy Japanese by 4% SDS-PAGE and immunoblotting techniques. There was a strong inverse relationship between the apparent molecular weight of apo(a) isoforms and plasma concentrations of Lp(a). Lp(a) in d < 1.006 fraction increased 2-4h after oral fat load. Lp(a) exhibited a marked avidity for triglyceride-rich lipoprotein (TRL), and we suggest that the TRL-bound Lp(a) is the intact Lp(a) derived from serum. We demonstrated that the lipid-free apo(a) does not contain apo B-100 in serum, and has a molecular mass of ca 200 kDa. The free apo(a) level in normal subjects was 1.75 mg/dl (as Lp(a)) and was no different from the level in CAD patients.

[Changes in serum h-HGF levels after living-related liver transplantation]

It is well known that prognosis is very poor in patients with severe hepatic insufficiency such as congenital biliary atresia and fulminant hepatitis. The liver transplantation is only effective therapy for these patients and living-related liver transplantation is becoming popular in Japan. We observed the changes in serum human hepatocyte growth factor (h-HGF) levels of the recipients during the operation in 3 cases of congenital atresis and one of fulminant hepatitis. Serum h-HGF values in these patients reached the maximal levels (5-10-fold compared to the base line values) at the phase of portal or hepatic artery anastomosis during the operation. These observations suggest that the increase of h-HGF in the recipient is derived from the following three origins. (1) Wash outed h-HGF from the liver of the donor. (2) Induced h-HGF from other organs than the liver of the recipient. (3) The decreased catabolism of h-HGF in recipient due to total hepatectomy.