Masahiro Aomine

Nature of “residual fast channel” dependent action potentials and slow conduction in guinea pig ventricular muscle and its modification by isoproterenol☆

The nature of residual fast channel dependent action potentials and conduction was studied in guinea pig ventricular papillary muscle in which the resting potential was reduced to 58 +/- 1 mV in high K+ (16.7 mM) Tyrodes solution, with or without isoproterenol (0.1 microM). In the absence of isoproterenol, the action potential had a slur on the upstroke and the maximum rate of rise (Vmax) was composed of 2 separable peaks: the early large (32 +/- 7 V/s, Vmax,fast) and the late small (10 +/- 2 V/s, Vmax,slow) ones. The conduction velocity ranged from 30 to 35 cm/s. The Vmax,fast was selectively depressed by l-verapamil (1 microgram/ml), indicating that the impulses were transmitted through incompletely inactivated (residual) fast channels. Lidocaine (2 micrograms/ml) depressed Vmax,fast with reduction of conduction velocity to about 20 cm/s just before the conduction block. Isoproterenol quadrupled the Vmax,slow but decreased Vmax,fast. As a result, the Vmax,slow overrode the Vmax,fast without change in the conduction velocity. In the absence of isoproterenol, there was no rate-dependent change in the action potential duration and the effective refractory period among the rates of 0.1, 0.5, and 0.9 Hz. Isoproterenol produced rate-dependent shortening in action potential duration with rate-independent shortening of postexcitation refractoriness, thereby resulting in a significant shortening in the effective refractory period at faster rates (0.5 and 0.9 Hz). Results suggest that the residual fast channel could produce slow conduction and that the ionic channels (fast or slow) responsible for the slow conduction may alternate according to local concentrations of tissue catecholamines.

Further studies on the mechanism of uptake of D-glucose by Tetrahymena pyriformis GL.

Abstract 1. 1. The glucose uptake system indicated the stereospecificity. Of the compounds tested as inhibitors of glucose uptake, only 2-deoxy- d -glucose, 3-O-methyl- d -glucose, α,β-methyl- d -glucoside, maltose, d -galactose, d -fructose and d -xylose were effective. 2. 2. Sulfhydryl reagent, p-chloromercuribenzoate (pCMB) markedly inhibited the glucose uptake. 3. 3. The order of ionic requirements for glucose uptake in Tetrahymena pyriformis is: Na+ ≥ Li+ > Rb+ ≥ choline+ Anions (Cl−, CH3COO−3 and SO2−4) and divalent cation (Mg2 +) did not play any definite roles in the glucose uptake. 4. 4. The rate of glucose uptake was found to be depressed by the extracellular K+ but was a function of the Na+ concentration of the medium. 5. 5. The glucose uptake by cells in the inorganic medium can be divided into Na+-sensitive and Na+-insensitive components. 6. 6. It is suggested that the Na+-sensitive component is the same component as the part influenced by K+, and Na+-insensitive component is the same as the part which is not influenced by K+.

The mechanism of sugar uptake in Tetrahymena pyriformis—I. Sugar specificity

Abstract 1. 1. In Tetrahymena pyriformis , d -glucose was metabolized, but 2-deoxy- d -glucose (2DG) was not metabolized. 2. 2. The curve of uptake velocity vs concentration of the sugars obeyed Langmuirs isotherm, showing the carrier-mediated process. 3. 3. The 2DG uptake system in T. pyriformis showed a high stereo-specificity, i.e. its uptake was inhibited only by d,l -glucose, d -mannose,α,β,-methyl- d -glucoside and 2DG. 4. 4. Counter-flow phenomena of 2DG were observed by the addition of various inhibitory sugars.

The mechanism of sugar uptake in Tetrahymena pyriformis—III general characterization

1. n1. 2-Deoxy-d-glucose was taken up by Tetrahymena pyriformis against the concentration gradient. n n2. n2. The 2DG uptake system was mediated by the carrier of specificity and activated by external Na+ and inhibited by external K+, suggesting the co-transport of sugars with Na+. n n3. n3. The external K+ concentration resulted in an inhibition of 2DG uptake in the presence of external Na+ ions. In the absence of external Na+ ions, however, such an inhibition by K+ ions was not observed. n n4. n4. The order of ionic requirements for 2DG uptake was: Na+ ≥ Li+ ≥ Rb+ > choline+ ≥ Cs+. n n5. n5. Anoxia did not inhibit the 2DG uptake. n n6. n6. The uptake of 2DG in T. pyriformis may be nearly independent of the formation of food vacuoles.

The mechanism of sugar uptake in Tetrahymena pyriformis—II. Effects of various inhibitors and chemical modificators

Abstract 1. 1. Cycloheximide significantly inhibited the 2-deoxy- d -glucose (2DG) uptake by Tetrahymena pyriformis . This indicates that protein component is at least partly responsible for the glucose transport. 2. 2. Effects of metabolic inhibitors on glucose and 2 DG uptakes were studied. Some glycolytic inhibitors significantly inhibited them. 3. 3. Ouabain, a specific inhibitor of Na + ,K + -ATPase, showed no inhibition at the concentration of 10 −3 M. 4. 4. The hydroxyl groups, sulfhydryl groups, carboxyl groups, amino groups, guanide groups, arginine residues and lysine residues are considered as the functional amino acid residues concerned with 2 DG uptake.

The relationship between contractile tension and intracellular cyclic AMP level in frog atrium: An analysis by using NAF, verapamil and ouabain

Abstract 1. The relationship between contractile tension and intracellular cyclic AMP level was studied in bullfrog atrium, modifying the external Ca ion concentration and using NaF, verapamil and ouabain. 2. Withdrawal of Ca led to a decrease of contractile tension and a significant increase of cyclic AMP level. 3. Likewise, application of verapamil (8 μg/ml) reduced contractile tension and enhanced the level of cyclic AMP. 4. NaF showed a marked positive inotropic effect on the muscles at a concentration of 10−5 M, but a negative inotropic effect at 10−2 M. 10−5 M of NaF produced a transient increase and afterwards a later decrease of the intracellular cyclic AMP. Conversely, on the addition of 10−2 M, the level of cyclic AMP remained elevated. The level of cyclic AMP was decreased with time of incubation with ouabain, while an augmentation in the contractile tension was observed. 5. These results are interpreted as indicating that the intracellular free Ca ions may regulate the intracellular level of myocardial cyclic AMP.

Effects of 5-methoxyindole-2-carboxylic acid on growth, glucose uptake and glycogen content in Tetrahymena pyriformis GL

1. n1. Low concentrations of 5-methoxyindole-2-carboxylic acid (MICA) considerably prolonged the log phase in cultures of Tetrahymena pyriformis GL. n n2. n2. Glucose uptake by cells in the stationary phase was inhibited by approx 50% after 45 min exposure to 27.25 μg/ml of MICA. n n3. n3. MICA (0.54 μg/ml) reduced the cellular content of glycogen and increased the glycogen phosphorylase activity in vivo. n n4. n4. MICA, however, showed neither activating nor inhibitory effects on this enzyme activity in vitro. n n5. n5. It may be thought that MICA does not interfere directly with the mechanism of glucose uptake. but MICA has influence on the metabolic control systems.

Diabetes Prolongs the Action Potential Duration in Rat Ventricular Muscle Probably Via Enhanced Calcium Current

Diabetic cardiomyopathy arises in patients suffering from diabetes mellitus for relatively long periods, and it is not secondary to hypertension, coronary artery disease, or valvular disease (1–4). A number of studies using diabetic rats as an experimental model, disclosed mechanical, biochemical and morphological changes in the heart (5,6). Fein et al. (7) found a marked slowing of relaxation and a substantial decrease of shortening velocity in the ventricular papillary muscles obtained from streptozotocin-induced diabetic rats, in which prolongation of action potential duration (APD) was also shown (8.)

Comparison of voltage-tension relationship between peak and terminal tonic tensions induced by longer depolarization in the bullfrog myocardium

Abstract 1. 1. The comparison of voltage-tension relationship between peak and terminal tonic tensions elicited by longer depolarizing pulses was performed in bullfrog atrium under voltage-clamp conditions using the double sucrose gap technique. 2. 2. The experiments were done in the absence of Mn ions which are used to block slow inward current responsible for the generation of phasic tension. 3. 3. The differences of the sensitivity between the peak and terminal tonic tensions for various conditions (1. changes in temperature, 2. exposure to Ca-rich or Na-poor solution, 3. effects of conditioning pulses, 4. effects of ouabain and verapamil) were observed, suggesting that the nature of the terminal tonic tention is considerably different from that of peak tonic tension.