Effects of physical training on pulmonary arterial pressure during exercise under hypobaric hypoxia in rats

In this investigation, we assessed the effects of physical training on exercise-induced systemic and pulmonary hemodynamic changes under hypobaric hypoxia in catheter-implanted rats. We made continuous measurements of pulmonary and systemic arterial pressures during progressive treadmill exercises under hypobaric hypoxia (equivalent to altitudes of 2500 and 5500 m) in 46 control and 41 trained rats. Trained rats were exercised on two running schedules: 4 weeks (4-trained) and 6 weeks (6-trained). Both these groups of trained rats were exercised for the same length of running time each day. The increase in resting mean pulmonary arterial pressure with increasing equivalent altitude was lower in the two trained groups than in the control group. The increase in with progressive intensity of exercise was lower in the 6-trained than in the 4-trained and control groups at 610 and 2500 m. The 6-trained rats showed higher pH,P _{a CO 2} and O₂ saturation in their blood than did the control group, whereas theP _{a O 2} was less. Lung tissue cyclic AMP concentration at rest was higher in the 6-trained than in the control group. Finally, it may be noted that exercise-induced lung tissue vasodilator responses seem to be enhanced in well-trained rats under both normobaric normoxia and hypobaric hypoxia. This study indicates that exercise training may be useful in preventing pulmonary hypertension resulting from both hypoxia and exercise.     

Relationship between plasma potassium and ventilation during successive periods of exercise in men

During and after two successive incremental cycle ergometer tests (tests A and B), plasma potassium concentration ([K+]p), plasma pH (pHp), plasma partial pressure of carbon dioxide, blood lactate concentration ([Lac-]b) and ventilation (VE) were measured. While there was a good correlation between the increase in [K+]p and VE or pHp, respectively, in test A, in test B a close correlation was found only between the increase in VE and [K+]p (r greater than 0.9 for nearly all single cases; r was 0.84 and 0.89 for all (pooled) cases in tests A and B, respectively; the correlation coefficients between changes in pHp and VE in tests A and B were r = 0.74 and r = 0.28, respectively, and r = 0.89 and r = 0.10 between the changes in [Lac-]b and VE in tests A and B). The close relationship for individuals between VE and [K+]p in tests A and B supported the hypothesis that the extracellular increase in [K+] may contribute to the ventilatory drive during exercise. The comparison of the results of tests A and B further indicated that the relationship between pHp and VE was dependent on the experimental design, and that pHp and VE changes are unlikely to be cause and effect.     

Influence of cold exposure on blood lactate response during incremental exercise

This study examined the effect of acute exposure of the whole body to cold on blood lactate response during incremental exercise. Eight subjects were tested with a cycle ergometer in a climatic chamber, room temperature being controlled either at 24 degrees C (MT) or at -2 degrees C (CT). The protocol consisted of a step increment in exercise intensity of 30 W every 2 min until exhaustion. Oxygen consumption (VO2) was measured at rest and during the last minute of each exercise intensity. Blood samples were collected at rest and at exhaustion for estimations of plasma norepinephrine (NE), epinephrine (E), free fatty acid (FFA) and glucose concentrations, during the last 15 s of each exercise step and also during the 1st, 4th, 7th, and the 10th min following exercise for the determination of blood lactate (LA) concentration. The VO2 was higher during CT than during MT at rest and during nearly every exercise intensity. At CT, lactate anaerobic threshold (LAT), determined from a marked increase of LA above resting level, increased significantly by 49% expressed as absolute VO2, and 27% expressed as exercise intensity as compared with MT. The LA tended to be higher for light exercise intensities and lower for heavy exercise intensities during CT than during MT. The E and NE concentrations increased during exercise, regardless of ambient temperature. Furthermore, at rest and at exhaustion E concentrations did not differ between both conditions, while NE concentrations were greater during CT than during MT. Moreover, an increase off FFA was found only during CT.(ABSTRACT TRUNCATED AT 250 WORDS)     

Hormonal regulation of hepatic glycogenolysis inAmphibolurus nuchalis,the western netted dragon: an in vitro study

Summary In mammals hepatic glycogenolysis is controlled by several hormones using cyclicAMP, Ca²⁺ and/or diacylglycerol as intracellular messengers. In contrast, in teleost fish, lungfish and amphibians fewer hormones promote hepatic glycogenolysis, and cyclicAMP is the sole intra-cellular messenger. This suggests that the -adrenergic mechanism became associated with the liver after amphibians separated from the vertebrate line. Reptiles separated later, and the aim of this study is to elucidate the hormonal control of hepatic glycogenolysis in a reptile,Amphibolurus nuchalis, and especially to determine which adrenergic receptor system is operative.InA. nuchalis liver pieces cultured in vitro, adrenaline and glucagon stimulated glycogen breakdown and glucose release, glycogen phosphorylase activity and accumulation of cyclicAMP in the tissue. Neurohypophysial peptides did not affect these parameters. These actions of adrenaline were completely blocked by the -adrenergic antagonist, propranolol and slightly reduced by the -adrenergic antagonist, phentolamine. Removal of Ca²⁺ from the medium and addition of the Ca²⁺ chelator, EGTA, did not block the actions of adrenaline, and the Ca²⁺ ionophore A23187 did not mimic these actions.The -adrenegic ligand [¹²⁵I]-iodocyanopindolol (ICP) bound specifically to an isolated membrane preparation fromA. nuchalis liver with a calculated K_D of 100 pM and a Bmax of 37.6 fmol·mg protein^–1. The adrenergic ligands propranolol, isoprenaline, adrenaline, noradrenaline, phenylephrine and phentolamine displaced ICP with K_D's of 20 nM, 1 M, 4.5 M, 32 M, 35 M and 500 M, respectively. The ₂-adrenergic ligand yohimbine did not bind specifically to the membrane, but at 1 nM and 100 pM, specific binding of the ₁-adrenergic ligand prazosin was 45% of total with a mean of 11.3 fmoles·mg protein^–1 specifically bound.These findings indicate that the glycogenolytic actions of adrenaline are mediated primarily via -adrenergic receptors inA. nuchalis, but that -adrenergic receptors may also play some role in the control of hepatic metabolism.     

Acid-base balance and electrolyte concentration in blood during graded exhausting exercise in non-trained subjects

Venous acid-base balance and electrolyte concentration during step-graded and exhausting exercise with a two-minute steady-state has been studied in a group of non-trained young men. The results showed a significant decrease in pH, pCO2 and bicarbonate and a significant increase in lactate, potassium, inorganic phosphate and proteins during the exercise. The supervening acidosis had a large anion gap that was of proportion with the increase in lactate values. We suggest that the total sum of other anions such as proteins, phosphate, pyruvate, citrate, free fatty acids and aminoacids, as well as sodium-hydrogen exchange could account for this acidosis.     

Metabolic and hormonal responses during exercise at 20°, 0° and −20°C

This study was designed to clarify the effects of cold air exposure on metabolic and hormonal responses during progressive incremental exercise. Eight healthy males volunteered for the study. Informed consent was obtained from every participant. The following protocol was administered to each subject on three occasions in a climatic chamber in which the temperature was 20°, 0° or –20°C with relative humidity at 60%±1%. Exercise tests were conducted on an electrically braked ergocycle, and consisted of a propressive incremental maximal exercise. Respiratory parameters were continuously monitored by an automated open-circuit sampling system Exercise blood lactate (LA), free fatty acids (FFA), glucose levels, bicarbonate concentration (HCO ₃ ^– ), acidbase balance, plasma epinephrine (E) and norepinephrine (NE) were determined from venous blood samples obtained through an indwelling brachial catheter. Maximal oxygen uptake was significantly different between conditions: 72.0±5.4 ml kg^–1 min^–1 at 20°C; 68.9±5.1 ml kg^–1 min^–1 at 0°C and 68.5±4.6 ml kg^–1 min^–1 at –20°C. Workload, time to exhaustion, glucose levels and rectal Catecholamines and lactate values were not significantly altered by thermal conditions after maximal exercise but the catecholamines were decreased during rest. Bicarbonate, respiratory quotient, lactate and ventilatory thresholds increased significantly at –20°C. The data support the contention that metabolic and hormonal responses following progressive incremental exercise are altered by cold exposure and they indicate a marked decrease in maximal oxygen uptake, time to exhaustion and workload.This study was supported by grants from CSR, Univesité du Québec; FIR, Université du Québec à Trois-Rivières and NATO no, 86.0435.     

Recovery of the torque-velocity relationship after short exhausting cycling exercise

The effects of fatigue upon the torque-velocity (T-omega) relationship in cycling were studied in 11 subjects. Fatigue was induced by short exhausting exercise, on a cycle ergometer, consisting of 4 all-out sprints without recovery. The linear (T-omega) relationship was determined during each all-out sprint, before, during and after the exhausting exercise. The kinetics of the T-omega relationship had permitted the study of the recovery of optimal torque, optimal velocity and their corresponding maximal power outputs (Pmax), 30 s or 1 min after the short exhausting exercise. Fatigue induced a parallel shift to the left of the T-omega relationship which was partly reversed by a parallel shift to the right during recovery. After 30 s recovery optimal velocity, optimal torque and Pmax were slightly lower than the corresponding values before the exhausting exercise; after 1-min optimal velocity and optimal torque had recovered 99% and 97% of their initial values. These mechanical data suggested that the causes of exhaustion were processes that allowed fast recovery of both optimal velocity and optimal torque.     

Adenosine and Cyclic AMP in Rat Cerebral Cortical Slices: Effects of Adenosine Uptake Inhibitors and Adenosine Deaminase Inhibitors

The endogenous levels of adenosine functionally linked to cyclic AMP systems in rat cerebral cortical slices are regulated by both adenosine deaminase and adenosine uptake systems. 2'-Deoxycoformycin (2'-DCF), an adenosine deaminase inhibitor, slightly increased basal, adenosine, and norepinephrine-elicited accumulations of cyclic AMP, whereas dipyridamole, an uptake inhibitor, had an even greater effect on cyclic AMP accumulations under the same conditions. Combinations of 2'-DCF and dipyridamole elicited a greater effect than either compound alone. Neither 2'-DCF nor dipyridamole significantly augmented accumulations of cyclic AP elicited by a depolarizing agent, veratridine, suggesting that the adenosine "released" during neuronal depolarization of brain slices is not as subject to inactivation by uptake or deamination as endogenous adenosine in control brain slices. The accumulation of cyclic AMP elicited by a combination of norepinephrine and veratridine was greater than additive. The response to a pure beta-adrenergic agonist, isoproterenol, was not potentiated by 2'-DCF, dipyridamole, or veratridine, consonant with minimal interaction of endogenous adenosine with beta-adrenergic systems.     

Hormonal Regulation of Adipose S-100 Protein Release

The release of S-100 protein from epididymal fat pads was enhanced by epinephrine in vitro, and about 50% of S-100 protein in the tissue was released into the medium after 2-h incubation at 37 degrees C with 10 microM epinephrine. Similar results were obtained with the incubation of isolated adipocytes. The S-100 protein release was also enhanced by isoproterenol, norepinephrine, ACTH, and dibutyryl cyclic AMP, which all increase the lipolysis by increasing cyclic AMP levels in the tissue. Propranolol, a beta-adrenergic blocker, could block the increase of S-100 protein release by catecholamines, indicating that the release was mediated by the beta-adrenergic effect of catecholamines. However propranolol had no suppressive effect on the enhancement of S-100 protein release by ACTH or dibutyryl cyclic AMP. Insulin had an inhibitory effect on the epinephrine-enhanced S-100 protein release. Epinephrine or ACTH could not stimulate the S-100 protein release in the absence of Ca2+, whereas the epinephrine-enhanced glycerol release was not affected under the same conditions. The increase in S-100 protein release was induced by only a pretreatment of the tissue with epinephrine. However, the lipolysis in the tissue was not enhanced by the pretreatment alone. These results indicate that the release of S-100 protein from adipocytes is regulated by the hormones that have been known to control the lipolysis with a manner slightly different from that of lipolysis.     

Hormonal regulation of glycogen synthase: Insulin decreases protein kinase sensitivity to cyclic AMP

Rat hemidiaphragms incubated with epinephrine exhibited increases in cyclic AMP content and protein kinase activity which were proportional to the logarithm of the hormone concentration from 0.1–2 μM. The fraction of glycogen synthase made independent of glucose-6-P for activity (%I) decreased concomitantly, but correlated only with epinephrine concentrations up to 0.2 μM. Insulin (0–100 mU/ml) increased glycogen synthase %I in a dose-dependent manner with no change in cyclic AMP concentration. Protein kinase activity increased slightly at the lowest insulin concentration, then decreased slightly as glycogen synthase %I increased. Insulin was without effect when administered with a supramaximal dose of epinephrine. In the presence of submaximal epinephrine, insulin produced a dose-dependent increase in glycogen synthase %I which correlated with a decrease in protein kinase activity, without changing cyclic AMP. Insulin had no effect on the increases in cyclic AMP produced by varying levels of epinephrine. However, the activation of protein kinase activity by endogenous cyclic AMP was inhibited in the presence of insulin. The glycogen synthase %I response to epinephrine also was less sensitive in the presence of insulin. Insulin antagonizes the activation of cyclic AMP-dependent protein kinase by epinephrine without altering cyclic AMP levels.     

Adrenergic versus VIPergic control of cyclic AMP in human colonic crypts

The actions of catecholamines on VIP-induced cyclic AMP is studied in human colon. We show that: (1) Epinephrine in the 10(-7)-10(-3) M concentration range (ED50 = 11.10(-6) M) inhibits VIP-induced cyclic AMP rise in isolated colonic epithelial cells; the maximal inhibition reaches 30% of VIP effect; epinephrine alters the efficacy of the peptide and does not modify its potency; epinephrine also reduces the basal cyclic AMP level. (2) The inhibition is found with other alpha adrenergic agonists with the order of potencies epinephrine greater than norepinephrine greater than phenylephrine. Clonidine has a poor intrinsic activity but antagonizes the action of epinephrine. (3) The inhibition of VIP action by epinephrine is reversed by the alpha antagonists dihydroergotamine, phentolamine and the alpha 2 antagonist yohimbine, while unaffected by the beta antagonist propranolol and the alpha 1 antagonist prazosin, (4) Epinephrine inhibits VIP-stimulated adenylate cyclase activity in preparations of colonic plasma membranes. Thus catecholamines exert through an alpha 2 adrenoreceptor a negative control on basal and VIP-stimulated cyclic AMP formation in human colon. We suggest that colonic cyclic AMP metabolism undergoes a dual control: VIPergic, activator and adrenergic, inhibitor.     

Norepinephrine Suppresses Inducible Nitric Oxide Synthase Activity in Rat Astroglial Cultures

Abstract: Exposure of primary rat astrocyte cultures to bacterial endotoxin lipopolysaccharide (LPS) causes expression of a Ca²⁺-in-dependent form of nitric oxide synthase (NOS). In these cells, the presence of norepinephrine (NE) caused a dose-dependent inhibition of the LPS induction of NOS activity, with an IC₅₀ value of 100 nMand significant suppression at 100 pAf. Short incubations (5-40 min) with NE were as effective as 24-h continuous exposure, and inhibition was observed up to the longest incubation period measured (56 h). In contrast, previously induced NOS activity was not affected by exposure to NE. The effects of NE were mediated primarily by binding to β-adrenergic receptors (β-ARs) because (a) the β-AR antagonist propranolol, but not the n-AR antagonist phentol-amine, could reverse the effects of NE; (b) the β-AR agonist isoproterenol. but not the a-AR agonist phenylephrine, was as effective as NE in blocking the effects of LPS; and (c) incubation with the cyclic AMP analogue dibutyryl cyclic AMP replicated the effects of NE. In contrast to astroglial cultures, LPS induction of NOS activity in RAW 264.7 macrophage cells was not affected by NE or dibutyryl cyclic AMP. These results indicate that in brain, inducible NOS in astrocytes can be regulated by neurotransmitter binding to glial receptors.     

Kinetics of heart rate and catecholamines during exercise in humans The effect of heart denervation

To elucidate the role of factors other than the nervous system in heart rate (f _c) control during exercise, the kinetics off _c and plasma catecholamine concentrations were studied in ten heart transplant recipients during and after 10-min cycle ergometer exercise at 50 W. Thef _c did not increase at the beginning of the exercise for about 60 s. Then in the eight subjects who completed the exercise it increased following an exponential kinetic with a mean time constant of 210 (SEM 22) s. The two other subjects were exhausted after 5 and 8 min of exercise during whichf _c increased linearly. At the cessation of the exercise,f _c remained unchanged for about 50 s and then decreased exponentially with a time constant which was unchanged from that at the beginning of exercise. In the group of eight subjects plasma noradrenaline concentration ([NA]) increased after 30 s to a mean value above resting of 547 (SEM 124) pg · ml^–1, showing a tendency to a plateau, while adrenaline concentration ([A]) did not increase significantly. In the two subjects who became exhausted an almost linear increase in [NA] occurred up to about 1,300 pg · ml^–1 coupled with a significant increase in [A]. During recovery an immediate decrease in [NA] was observed towards resting values. The values of thef _c increase above resting levels determined at the time of blood collection were linearly related with [NA] increments both at the beginning and end of exercise with a similar slope, i.e. about 2.5 beats · min^–1 per 100 pg · ml^–1 of [NA] change. These findings would seem to suggest that in the absence of heart innervation the increase inf _c depends on plasma [NA].     

Changes in membrane fluidity and lipid peroxidation of skeletal muscle mitochondria after exhausting exercise in rats

We studied the effects of exhausting exercise and exercise training on skeletal muscle mitochondrial membrane fluidity and lipid peroxidation in rats. The first part of the study involved 60 untrained rats divided into six equal groups. Of the total number 10 rats were sedentary and acted as controls. The remaining 50 rats exercised to exhaustion and were sacrificed at 0-h, 24-h, 48-h, 72-h, and 96-h post-exercise. The second part of the study involved 40 rats which were divided into four equal groups. Of these 10 rats were sedentary and acted as controls. The remaining 30 rats underwent 8 weeks of exercise training. They were then subjected to a single period of exhausting exercise and were sacrificed at 0-h, 24-h and 48-h post-exercise. Membrane fluidity was measured using the fluorescence polarization method. Lipid peroxidation was estimated by determining the thiobarbituric acid-reactive substances (TBARS) in mitochondria. In the untrained rats, mitochondrial fluorescence polarization and TBARS contents were significantly increased post-exercise compared with the sedentary controls (P < 0.05). They did not return to near control levels until 96 h and 48 h, respectively. In the trained rats, fluorescence polarization was raised compared with the sedentary controls but this was significantly lower than those measured at the same times of the untrained group post-exercise (P < 0.05). Exhausting exercise decreased membrane fluidity and increased lipid peroxidation in rat skeletal muscle mitochondria. These effects were relieved to some extent by exercise training.     

Role of xanthine oxidase in delayed lipid peroxidation in rat liver induced by acute exhausting exercise

The aim of this study was to examine whether xanthine oxidase (XOD)-derived hepatic oxidative damage occurs in the main not during but following strenuous exercise. The degree of damage to hepatic tissue catalyzed by XOD was investigated immediately and 3 h after a single bout of exhausting exercise, in allopurinol and saline injected female Wistar rats. Allopurinol treatment resulted in increased hypoxanthine and decreased uric acid contents in the liver compared with the saline treated group, immediately and 3 h after the exercise. Analysis immediately after the exercise showed no changes in hepatic hypoxanthine, uric acid, and thiobarbituric acid-reactive substance (TBARS) contents in the saline treated group, when compared with the resting controls. However, significant increases in uric acid contents in the saline treated livers were observed 3 h after the exercise, relative to the controls. Hepatic TBARS content in the saline treated group were markedly greater than those in both the control and allopurinol treated groups after 3 h of recovery following the exercise. It was concluded that a single bout of exhausting exercise may impose XOD-derived hepatic oxidative damage, primarily during the recovery phase after acute severe exercise.     

Regulation of μ-Opioid Receptor in Neural Cells by Extracellular Sodium

Abstract: SH-SY5Y neural cells expressing μ- and δ-opioid receptors were maintained viable in isotonic, sodium-free buffer in vitro. Intracellular sodium levels were manipulated by various methods, and ligand binding to intact cells was studied. In physiological buffer containing 118 mM sodium, [³H]Tyr-d -Ala-Gly-(Me)Phe-Gly-ol ([³H]-DAMGO) and [³H]naltrexone bound to μ receptor with K_D values of 3.1 and 0.32 nM and B_max values of 94 and 264 fmol/mg of protein, respectively. Replacement of sodium by choline decreased the affinity of the antagonist and increased B_max for [³H]DAMGO, without significantly affecting the other corresponding binding parameters. Depolarizing concentrations of KCl (34 mM) in physiological buffer decreased the intracellular sodium levels by 67%, but this did not decrease the [³H]DAMGO binding to the cells. Incubation of cells with monensin and ouabain increased the intracellular sodium levels dramatically (from 78 to 250 and 300 nmol/mg, respectively), with no changes in agonist binding parameters. Ethylisopropylamiloride inhibited [³H]DAMGO and [³H]naloxone binding to intact cells with EC₅₀ values of 24 and 3,600 nM, respectively. Adenylyl cyclase activities measured in intact cells, at different concentrations of sodium, showed the physiological significance of this ion in signal transduction. Potency of DAMGO in inhibiting the forskolin-stimulated adenylyl cyclase activity was significantly higher at lower concentrations of sodium. However, inhibition reached the maximal level only at 50 mM sodium, and typical sigmoidal dose-response curves were obtained only in the presence of 118 mM sodium. Furthermore, even at low or high intracellular sodium levels, DAMGO inhibition of cyclic AMP levels was normal. These results support a role for extracellular sodium in regulating not only the ligand interactions with the receptor, but also the signal transduction through the μ receptor.     

Detection of potassium currents and regulation of multidrug resistance by potassium channels in human gastric cancer cells

The present study aimed to investigate the potassium currents and further explore the role of potassium channels in drug response of gastric cancer cells. By patch-clamp technique, potassium currents of human gastric cancer cell SGC7901 were recorded in the mode of voltage clamp. Both 4-aminopyridine (4-AP) and tetraethylammonium (TEA) could almost completely block this current. The chemotherapeutic drugs, adriamycin or 5-fluorouracil could significantly increase the K(+) current density on SGC7901 cells in a dose-dependent manner. 4-AP or TEA was found to restrain adriamycin-induced apoptosis and enhance multidrug-resistant phenotype of SGC7901 cells. Up-regulation of Kv1.5, which has been found widely expressed in gastric cancer cells including SGC7901, increased the K(+) current density and sensitivity of SGC7901 cells to multiple chemotherapeutic drugs, whereas down-regulation of Kv1.5 enhanced the drug-resistant phenotype of SGC7901 cells. In conclusion, potassium channels may exert regulatory effects on multidrug resistance by regulating drug-induced apoptosis in gastric cancer cells.     

Effect of prolonged physical exercise on intra-erythrocyte and plasma potassium

The intracellular concentrations of sodium [Na+] and potassium [K+] and the water content in human erythrocytes were investigated in 21 male runners before and after a marathon. From 2 to 5 min after the race, the intra-erythrocyte [K+] was significantly decreased (p less than 0.001) by 7% whereas the plasma [K+], intra-erythrocyte [Na+] and the erythrocyte water content were unchanged. The change in the intra-erythrocyte [K+] observed immediately after the marathon, was negatively correlated with the race time (r = -0.44; p less than 0.05). Furthermore, the change in the plasma [K+] (r = -0.64; p less than 0.001) and the amount of K+ excreted in the urine during the race (r = 0.54; p less than 0.05) were also, respectively, negatively and positively correlated with the race time. It is concluded that during prolonged physical exercise the erythrocytes could serve as a kind of K+ reservoir that is drained with increasing magnitude of body K+ loss. This might explain why in the faster marathon runners, in whom the urinary K+ loss is smaller and the K+ intake is greater than in the slower runners during race, the intra-erythrocyte [K+] is unchanged after a marathon whereas in the slower runners it is decreased.     

Cardiovascular reflexes during sustained handgrip exercise: role of muscle fibre composition, potassium and lactate

Six healthy men performed sustained static handgrip exercise for 2 min at 40% maximal voluntary contraction followed by a 6-min recovery period. Heart rate (fc), arterial blood pressures, and forearm blood flow were measured during rest, exercise, and recovery. Potassium ([K+]) and lactate concentrations in blood from a deep forearm vein were analysed at rest and during recovery. Mean arterial pressure (MAP) and fc declined immediately after exercise and had returned to control levels about 2 min into recovery. The time course of the changes in MAP observed during recovery closely paralleled the changes in [K+] (r = 0.800, P < 0.01), whereas the lactate concentration remained elevated throughout the recovery period. The close relationship between MAP and [K+] was also confirmed by experiments in which a 3-min arterial occlusion period was applied during recovery to the exercised arm by an upper arm cuff. The arterial occlusion affected MAP while fc recovered at almost the same rate as in the control experiment. Muscle biopsies were taken from the brachioradialis muscle and analysed for fibre composition and capillary supply. The MAP at the end of static contraction and the [K+] appearing in the effluent blood immediately after contraction were positively correlated to the relative content of fast twitch (% FT) fibres (r = 0.886 for MAP vs % FT fibres, P < 0.05 and r = 0.878 for [K+] vs % FT fibres, P < 0.05). Capillary to fibre ratio showed an inverse correlation to % FT fibres (r = -0.979, P < 0.01).(ABSTRACT TRUNCATED AT 250 WORDS)     

Determinants of frequency-dependent regulation of Kv1.2-containing potassium channels

Voltage-gated potassium channels are important regulators of electrical excitation in many tissues, with Kv1.2 standing out as an essential contributor in the CNS. Genetic deletion of Kv1.2 invariably leads to early lethality in mice. In humans, mutations affecting Kv1.2 function are linked to epileptic encephalopathy and movement disorders. We have demonstrated that Kv1.2 is subject to a unique regulatory mechanism in which repetitive stimulation leads to dramatic potentiation of current. In this study, we explore the properties and molecular determinants of this use-dependent potentiation/activation. First, we examine how alterations in duty cycle (depolarization and repolarization/recovery times) affect the onset and extent of use-dependent activation. Also, we use trains of repetitive depolarizations to test the effects of a variety of Thr252 (S2-S3 linker) mutations on use-dependent activation. Substitutions of Thr with some sterically similar amino acids (Ser, Val, and Met, but not Cys) retain use-dependent activation, while bulky or charged amino acid substitutions eliminate use-dependence. Introduction of Thr at the equivalent position in other Kv1 channels (1.1, 1.3, 1.4), was not sufficient to transfer the phenotype. We hypothesize that use-dependent activation of Kv1.2 channels is mediated by an extrinsic regulator that binds preferentially to the channel closed state, with Thr252 being necessary but not sufficient for this interaction to alter channel function. These findings extend the conclusions of our recent demonstration of use-dependent activation of Kv1.2-containing channels in hippocampal neurons, by adding new details about the molecular mechanism underlying this effect.