摘要:

Several factors are potentially able to change the glomerular filtration rate (GFR) and thereby participate in its regulation, but only a few factors seem to be physiologically important. The variable nature of proximal tubular pressure should be recognized as important in the regulation of GFR. It is argued that a distinction should be made between the terms ‘autoregulation of GFR’ and ‘regulation of GFR’ The tubuloglomerular feedback mechanism (TGF) is an important factor for autoregulatory control of GFR. When perturbation result in major increases in tubular flow, the TGF saturates, Proximal tubular pressure then increases and becomes the major factor responsible for the stabilization of GFR. Changes in the proximal reabsorption rate (APR) are important for long‐term variations in GFR (regulation of GFR). Small changes in the APR cause near paralled changes in the GFR mainly through the TGF mechanism, while larger changes in the APR cause near parallel changes in the GFR mainly because of the effect on tubular pressure. The hydraulic resistance in the distal nephro segments is an additional factor in regulating GFR, through its effect on proximal tubular pressure. The stimulus to the TGF mechanism also depresses renin release. The resulting local angiotensin II concentration has effects both on the arteriolar resistances and on the APR. The renin‐angiotensin system and TGF are therefore considered to be integrated parts of a common control system regulating GFR. According to the hypothesis advocated here, TGF‐mediated changes in afferent arteriolar resistance and angiotensin‐mediated changes in efferent arteriolar resistance and APR cooperate in counteracting perturbatins in proximal tubular pressure and Henle loop flow. However, because of the biphasic proximal effect of angiotensin II, a major unresolved question is whether physiological increases in endogenous local angiotensin II concentrations stimulate or inhibit proxim

ISSN:0001-6772    

DOI:10.1111/j.1748-1716.1995.tb09984.x

出版商:Blackwell Publishing Ltd    

年代:1995

数据来源: WILEY

摘要:

The coupling between net transepithelial Cl‐ influx and net water flow was investigated. Experiments were performed on isolated frog skin bathed in isotonic Cl‐ Ringer's solution in the presence of the Na+channel blocking agent amiloride in the mucosal solution. The skins were voltage‐clamped at ‐80 or ‐ 100 mV (with the serosal solution as reference). Under these conditions the current across the skin is carried by an influx of Cl‐. In the absence of antidiuretic hormone the correlation between current and net water flow was low, but in the presence of the antidiuretic hormone, arginine vasotocin, there was a highly significant correlation between current and net water flow. The data presented here indicate that under steady state conditions about 70 molecules of water follow each Cl‐ ion across the skin. If the water influx is driven by electroosmosis one would expect that a change in current should result in an immediate change in the water flow. There was, however, a considerable time delay between the change in current and water flow. This indicates that the observed coupling between Cl‐ flux and water flow is caused by current‐induced local osmosis and n

ISSN:0001-6772    

DOI:10.1111/j.1748-1716.1995.tb09985.x

出版商:Blackwell Publishing Ltd    

年代:1995

数据来源: WILEY

摘要:

The extracellular receptor currents evoked by step displacements of the otolithic membrane of the isolated saccular macula ofRana esculentawere recorded under transepthelial voltage clamp conditions. With the aim to depolarize the hair cells and increase the fractional resistance of the apical membrances, the basal side of the preparation was bathed in saline with an increased K+concentration (62 mm). This caused a shift in the non‐linear receptor current‐voltage relation along the voltage axis of ‐51 mVpL10 mV; (mean pL SD; n= 32) and a reduction in the transepithelial resistance of 10%. Under these conditions the electrical properties of the macula are assumed to be controlled by the apical membranes. The effects of different concentrations of Ca2+in the apical solution on the receptor current‐voltage relation were examined.Change of the apical Ca2+concentration (range 3 mM to 70 μM) varied the transepithelial voltage at which the receptor current was zero (Vrev). Fitting a modified constant field equation to the relation between the apical Ca2+concentration and the change in Vrevgave an estimate of Pca/Pkof the transduction channels of 212. Furthermore, a high relative permeability of the transduction channels for other divalent cations (Ba2+, Sr2+) was measured, whereas Mn2+inhibited the receptor current. The receptor current was inhibited by amiloride (IC503.2 μM) and nifedipine (IC501.9 μM pL0.6 μM).Reduction of the apical Ca2+concentration to 90μM in standard apical solution reduced the size of the receptor current to 67% pL 30% (n=17) compared to control but did not affect the shape of the receptor current ‐voltage relation. Subsequent substitution of K+by Na+caused a further reduction of the receptor current to 32% pL29% (n=9), changed the receptor current‐voltage relation into a linear relation and diminished the adaptation of the receptor current.These results indicate that the mechano‐electrical transduction channels of the frog saccular hair cells are highly selective to Ca2+and that the conductance of the channels may be influenced by the apical monoval

ISSN:0001-6772    

DOI:10.1111/j.1748-1716.1995.tb09986.x

出版商:Blackwell Publishing Ltd    

年代:1995

数据来源: WILEY

摘要:

The present investigation examined passive torque and electromyographic response in human skeletal muscle during passive static stretch within 60 s after maximal repetive eccentric and concentric contractions. Passive torque (Nm) offered by the hamstring muscle group was measured during passive knee extension in a modified dynamometer in 10 subjects. The distal thigh was elevated to 0.52 rad from horizontal and the backrest was positioned at 1.57 rad. The lever arm moved the leg passivelly at 0.09 rad s‐1from a starting positioned of 1.48 rad below horizontal to the final position where it remained stationary for 90s. Gross electrical activity of the human hamstring muscle group was recorded simultaneously. The effect of concentric or eccentric contraction was tested on separate days. Two stretch manoeuvres with a 45 min hiatus were administered on a control and experimental side. The experimental side performed 40 maximal effort repetitive concentric or eccentric hamstring muscle contractions at 1.05 rad s‐1prior to the second stretch. Passive torque during the 90s stretch declined 30–35% on the experimental and control side in all stretches. P<0.001 without a significant effect of prior contraction mode. Passive peak and final torques were lower on the experimental side, P<0.01 after concentric contractions. Passive peak and final torques remained unchanged after eccentric contractions on the experimental side. The low level EMG response of the hamstring muscle during the stretch was unchanged after maximal repetitive concentric or eccentric contractions. These data demonstrate acute contraction specific alteration in passive torque in human skeletal muscle, which cannot be accounted for by EMG activity. Furthermore, the lack of difference on the control side implies that one 90s stretch has no effect on passive torque of the muscle 45 min

ISSN:0001-6772    

DOI:10.1111/j.1748-1716.1995.tb09987.x

出版商:Blackwell Publishing Ltd    

年代:1995

数据来源: WILEY

摘要:

This study examined the effect of (a) creatine supplementation on exercise metabolism and performance and (b) changes in intramuscular total creatine stores following a 5 day supplementation period and a 28 day wash‐out period. Six men performed four exercise trials, each consisting of four 1 min cycling bouts, punctuated by 1 min of rest followed by a fifth bout to fatigue, all at a workload estimated to require 115 or 125% VO2, maxAfter three familiarization trials, one trial was conducted following a creatine monohydrate supplementation protocol (CREAT); the other after 28 d without creatine supplementation, in which the last 5 d involved placebo ingestion (CON). Intramuscular TCr was elevated (P<0.05) in CREAT compared with the final familiarization trial (FAM3) and CON. Concentrations of this metabolite in these latter trials were not different. In addition, a main effect (P<0.05) for treatment was observed for PCr when the data from CREAT were compared with CON. In contrast, no differences were observed in the total adenine nucleotide pool (ATP + ADP +AMP), inosine 5′‐monophosphate, ammonia, lactate or glycogen when comparing CREAT with CON. Despite the differences in TCr and PCr concentrations when comparing CREAT with other trials, no difference was observed in exercise duration in the fifth work bout. These data demonstrate that creatine supplementation results in an increase in TCr but this has no effect on performance during exercise of this nature, where the creatine kinase system is not the principal energy supplier. In addition 28 d without supplementation is a sufficient time to return intramuscular TCr stores to basal l

ISSN:0001-6772    

DOI:10.1111/j.1748-1716.1995.tb09988.x

出版商:Blackwell Publishing Ltd    

年代:1995

数据来源: WILEY

摘要:

Although the overall shift towards the V3, myosin heavy chain (MHC) has been shown to be associated with cardiac hypertrophy, quantitative evidence describing regional expression is sparse. The aim of this study was to compare and contrast the regional ventricular myosin isoform expression in two distinct haemodynamic states:pressure and volume overload. Volume overload was achieved using an aortocaval fistula (ACF) model and pressure overload by two‐kidney‐one‐clip (2KlC) hypertension. A separate group (UC‐2KlC) had the clip removed 1 week prior to investigation. Sham operated rats (SHAM) served as controls. All groups were studied 4 weeks after surgery. Ventricular tissue samples (≅ 50 mg) were taken from the walls of the right ventricle (RV), septum and left ventricular (LV) free wall. Tissue samples (excluding RV) were divided into endocardium and epicardium, and myosin expression was determined using polyacrylamide gel electrophoresis. Cardiac hypertrophy was substantial in both LV (1.7‐fold) and RV (1.9‐fold) in ACF rats. The 2K1C rats had similar LV enlargement (1.6‐fold) whereas RV hypertrophy was not as great (1.2‐fold). Blood pressure (BP) was increased 65% in 2K1C rats, whereas there was no change in ACF rats with respect to SHAM animals. After unclipping (UC‐2KIC), LV hypertrophy and BP had returned towards control levels. In general, V3, MHC expression was associated with increasing LV hypertrophy in both 2K1C and ACF models. However, there was a marked endo‐epi differential (1.5: 1) in the LV free wall and septum of 2K1C rats. In contrast, in ACF rats there was no differential V3, MHC expression in the LV or septa1 tissue, i. e. expression was similar in both endo‐and epi‐samples. Elevated expression of V3, MHC persisted despite normotension and regression of cardiac hypertrophy in UC‐2K1C rats. Taken together with published results demonstrating that relative transmural myocyte hypertrophy in ACF rats (endoendo), the present findings reveal that regional V3, myosin expression represents a distinct adaptational component of the overall cardiac hypertrophic response in both v

ISSN:0001-6772    

DOI:10.1111/j.1748-1716.1995.tb09989.x

出版商:Blackwell Publishing Ltd    

年代:1995

数据来源: WILEY

摘要:

Endothelin‐1 (ET‐1) is a strongly vasoactive polypeptide that may be involved in the regulation of the uteroplacental blood flow. In the present study we have examined the contractile response to ET‐1 in human placental arteries in the presence of several agents that interfere with storage of intracellular calcium, e. g. caffeine, ryanodine and thapsigargin. We have also compared the contractile response to ET‐1 in normal pregnancies with that of patients with foetal intrauterine growth retardation (IUGR), a condition with reduced uteroplacental blood flow. We found that the response to ET‐1 in the placental arteries from women with normal pregnancies was reduced by 20% in the absence of extracellular calcium. Caffeine relaxed the basal tone of the vessels and reduced the contractile response to ET‐1 by 51 %. Nifedipine in addition to caffeine resulted in a reduction of 700/,.Ryanodine also reduced the tone. Thapsigargin had no effect on the placental arteries at lower concentrations, but gave a progressive and slow contraction at 10‐6M. The ET‐1 induced contraction in placental arteries from IUGR patients was 67% more potent than in placental arteries from women with normal pregnancies, 129% as compared with 77% of the maximal K+induced contraction. We conclude that the ET‐1‐induced contractile response in the human placental artery is dependent on influx of extracellular calcium as well as mobilization of calcium from intracellular stores. An increased sensitivity to ET‐1 in placental arteries may contribute to the reduced uteroplacental blood flow in intrauteri

ISSN:0001-6772    

DOI:10.1111/j.1748-1716.1995.tb09990.x

出版商:Blackwell Publishing Ltd    

年代:1995

数据来源: WILEY

摘要:

This study reports on morphological and contractile properties of the bovine central adrenomedullary vein (bCAMV). Up to several layers of circularly orientated smooth muscle cells (SMC) were observed, however, without forming a continuous, closed sheath. Discrete bundles of eccentrically arranged, longitudinal SMC were also conspicuous. Chromaffin cells were in most cases located outside the SMC layers, while sometimes being in close apposition to the endothelium in areas without SMC. Circularly mounted preparations of the endothelium‐denuded vessel responded selectively to high K+, endothelins (ETs) and neuropeptide Y (NPY). The threshold for ET‐1 was 0.13 nmand the half maximally effective concentration (EC50) was 3pL1 nm (n= 9). The order of potencies was ET‐1 ≥ ET‐2 ≥ ET‐3, suggesting a vascular receptor (ETA). Concentrations at and above EC50frequently developed long‐lasting oscillations during the spontaneous relaxation of the ET‐1 evoked tension. This response was partly (21%) independent of extracellular Ca2+. A marked tachyphylaxis developed to ET‐1 (3–30 nm), resulting, on the other hand, in facilitation of the subsequent constrictor responses to high K+and NPY. Propranolol and phentolamine alone, or in combination, were without effects on the basal tension and on the above‐mentioned responses to high K+, ET‐1 or NPY, making a contribution from adrenoceptor activation unlikely. No response was obtained with exogenous cate‐cholamines, acetylcholine or serotonin, nor with a series of peptides known to occur in the adrenal medulla. This study shows that bCAMV is not a passive capacitance vessel but appears unique among mammalian veins in being s

ISSN:0001-6772    

DOI:10.1111/j.1748-1716.1995.tb09991.x

出版商:Blackwell Publishing Ltd    

年代:1995

数据来源: WILEY

摘要:

In conscious sheep, lesions involving the anterior wall of the third cerebral ventricle extinguished the dipsogenic and cortisol releasing effects of 10 min intracarotid (i. c.) infusions of angiotensin II (Ang II) (20 pmol kg‐1min‐1). The sustained hypertension obtained in response to i. c. Ang II in the intact animal was not significantly changed by the forebrain lesions. Pronounced, but short‐lasting tachycardia immediately developed in response to i. c. Ang II in control animals. After a 1 min return to initial values the heart rate (HR) rose to about 20 beats min‐1above pre‐infusion level during the remaining infusion period. After a brief post‐infusion fall, HR retook that level. It then gradually declined but did not return to initial level until about 25 min later. During isoflurane anaesthesia in the latter animals carotid blood pressure (cBP) was reduced by ˜ 30% while the HR was more than doubled. The cBP rise in response to i. c. Ang II infusion was<50% of that seen in awake animals and the pressure returned to initial level within ˜ 5 min after the end of the infusion. The infusion did not influence the HR. This study provides support for the idea that blood borne Ang II, bound at cerebral sites posterior to the hypothalamic reion, continues to have cardiovascular effects for ˜ 20 min after the concentration in the blood has returned to pre

ISSN:0001-6772    

DOI:10.1111/j.1748-1716.1995.tb09992.x

出版商:Blackwell Publishing Ltd    

年代:1995

数据来源: WILEY

摘要:

Sympathetic β‐adrenergic influences on cardiovascular responses to 50d̀ head‐up tilt were evaluated with metoprolol (β1‐blockade; 0.29 mg kg‐1) and propranolol (β1and β‐2‐blockade; 0.28 mg kg‐1) in eight males. A normotensive‐tachycardic phase was followed by a hypotensive‐bradycardic episode associated with presyncopal symptoms after 23pL3 min (control, mean pLSE). Head‐up tilt made thoracic electrical impedance (3.0pL10Ω), mean arterial pressure (MAP, 86pL4‐93pL4 mmHg), heart rate (HR, 63pL3‐99pL10 beats min‐1) and total peripheral resistance (TPR, 15pL1‐28pL4 mmHg min L‐1) increase, while central venous oxygen saturation (74pL2‐58pL4%), cardiac output (5.7pL0.1–3.1pL0.3 L min‐1), stroke volume (95pL6‐41pL5 mL) and pulse pressure (55pL4‐49pL4 mmHg) decreased (P<0.05). Central venous pressure decreased during head‐up tilt (7pL2‐0pL1 mmHg), but it remained stable during the sustained tilt. At the appearance of preswyncopal symptoms MAP (49pL3 mmHg), HR (66pL4 beats min‐1) and TPR (15pL3 mmHg min L‐1) decreased (P<0.05). Neither metoprolol or propranolo changed tilt tolerance or cardiovascular variables, except for HR that remained at 57pL2 (metoprolol) and 55pL3 beats min‐1(propranolol), and MAP that remained at 87pL5 mmHg during the first phase with metoprolol. In conclusion, sympathetic activation was crucial for the heart rate elevation during normotensive head‐up tilt, but not for tilt to

ISSN:0001-6772    

DOI:10.1111/j.1748-1716.1995.tb09993.x

出版商:Blackwell Publishing Ltd    

年代:1995

数据来源: WILEY