A number of mitochondrial carrier systems function both in homologous and in heterologous exchange mode, which, in the case of the phosphate carrier is a homologous Pi--Pi- and a heterologous Pi--OH- exchange. In addition, we showed that mitochondrial carriers, e.g. the aspartate/glutamate and the phosphate carrier, can undergo a functional shift from coupled antiport to uncoupled uniport after modification of cysteine residues. In this transport mode a mixture of carrier- and channel-type properties is observed. To address this question on the molecular level, the phosphate carrier from yeast (S. cerevisiae) mitochondria was expressed in E. coli, solubilized, purified and functionally reconstituted. From three cysteine residues present in the yeast phosphate carrier at positions 28, 134 and 300, only one single cysteine residue (C28) was found responsible for the functional switch from antiport to uniport. Upon replacement by a serine residue, this interconversion was blocked. After incorporation of the carrier into giant liposomes, electrophysiological methods (patch clamp) were applied. Under these conditions, a fourth transport mode of the phosphate carrier was observed, namely an action as anion-selective channel, which could be reversibly blocked by phosphate.

Aside from its role as a hypothalamic stress hormone, corticotrophin releasing hormone (CRH) is also a placental hormone, at least in primates. Although the function of placentally derived CRH remains to be fully elucidated, elevated CRH levels have been associated with premature labour, suggesting that the hormone may be involved in regulating the duration of pregnancy. Indeed, pregnant human myometrium expresses functional CRH receptors (CRH R1 and CRH R2 subtypes) thought to signal predominantly via the second messenger cAMP. Thus, like other cAMP-producing hormones in the myometrium such as β2 agonists, CRH may play a part in maintaining uterine quiescence. However, several of the CRH receptor isoforms identified to date have a reduced ability to activate adenylate cyclase, raising the question as to whether they are linked to other signal transduction pathways. Here, we discuss critically the evidence for the peptide's role in regulating contractility, both directly at the myometrium and indirectly via the fetal membranes and decidua. The possibility of a role in myometrial growth modulation is also described. Experimental Physiology (2001) 86.2, 273-281.

We investigated how sympathetic nerve activity and vasoactive hormones interact in controlling renal haemodynamics in pentobarbitone-anaesthetized rabbits. Renal blood flow was progressively reduced by electrical stimulation (0.5-3 Hz) of the renal nerves, during renal arterial infusion of saline, vasoconstrictors (angiotensin II and [Phe2,Ile3,Orn8]-vasopressin), or vasodilators (acetylcholine, adrenomedullin and the nitric oxide donor methylamine hexamethylene NONOate (MAHMA NONOate). A frequency-rich stimulus was also applied to test whether the vasoactive agents affect the dynamic control of renal blood flow by sympathetic nerve activity. The vasodilators tended to increase renal blood flow, but only the effect of MAHMA NONOate was statistically significant. [Phe2,Ile3,Orn8]-vasopressin reduced medullary perfusion (by 61 ± 12 %) but not renal blood flow or cortical perfusion. Angiotensin II reduced renal blood flow (33 ± 3 %) and cortical perfusion (14 ± 5 %) but not medullary perfusion. Steady-state responses of renal blood flow and cortical perfusion during renal nerve stimulation were attenuated during infusion of acetylcholine and [Phe2,Ile3,Orn8]-vasopressin, while angiotensin II attenuated responses of medullary perfusion, and MAHMA NONOate and adrenomedullin had no significant effects. The dynamic response to sympathetic nerve activity (renal blood flow responded as a low pass filter with a pure time delay of ~664 ms) was not altered by the vasoactive agents. We conclude that some vasoactive agents can modulate steady-state renal haemodynamic responses to sympathetic nerve activity in a regionally specific manner, independent of their effects on baseline renovascular tone. However, they have little impact on the dynamic response of renal blood flow to sympathetic nerve activity. Experimental Physiology (2003) 88.2, 229-241.

Contents Page

Introduction 1

Cardiac sympathetic imbalance and arrhythmia 2

Nitric oxide-cGMP pathway and the cholinergic modulation of cardiac excitability 3

Nitric oxide-cGMP pathway and the sympathetic modulation of cardiac excitability 7

Functional significance of nitric oxide in the autonomic regulation of cardiac excitability 9

Summary 9

References 10

Experimental Physiology (2001) 86.1, 1-12.

An elevated extracellular concentration of D-glucose (i.e. hyperglycaemia) inhibits cell proliferation and incorporation of the endogenous nucleoside thymidine into DNA in human umbilical vein endothelial cells (HUVECs). Cells in their log-phase of growth (3.7 ± 0.3 days, n = 27) incubated for 30 min with 25 mM D-glucose, but not with equimolar concentrations of L-glucose or D-mannitol, exhibited reduced [3H]thymidine incorporation and cell growth rate, with no change in cell viability (> 98 %), total DNA, protein content or cell volume. Incubation with D-glucose activated protein kinase C (PKC), endothelial NO synthase (eNOS), p42 and p44 mitogen-activated protein kinases (p42/44mapk), but inhibited superoxide dismutase (SOD). Incubation with D-glucose also increased cGMP and cAMP levels. The effect of D-glucose was blocked by the PKC inhibitor calphostin C, the MAP kinase kinase 1/2 (MEK1/2) inhibitor PD-98059, the eNOS inhibitor L-NAME, the protein kinase G (PKG) inhibitor KT-5823 and the protein kinase A (PKA) inhibitor KT-5720. In the presence of 5 mM D-glucose, [3H]thymidine incorporation and cell growth were reduced by the PKC activator phorbol 12-myristate 13-acetate (PMA), the NO donor S-nitroso-N-acetyl-L,D-penicillamine (SNAP), dibutyryl cGMP, dibutyryl cAMP and the Ca2+ ionophore A-23187. The effect of A-23187 was blocked by calphostin C and PD-98059. D-Glucose-dependent inhibition of thymidine incorporation and cell proliferation is associated with increased PKC, eNOS, and MEK1/2, but decreased SOD activity, and higher intracellular levels of cGMP, cAMP and Ca2+ in HUVECs. These are cellular mechanisms which may reduce endothelial cell growth in pathological conditions such as in diabetes mellitus or hyperglycaemia. Experimental Physiology (2003) 88.2, 209-219.

As skeletal muscle fibres mostly express a single myosin isoform, they are a potential source of pure myosin isoforms. A technique is described that allows extraction and identification of pure myosin isoforms from single fibres, and testing of such myosins in an in vitro motility assay (IVMA). The results show that the extraction procedure does not alter myosin function and support the view that single fibres are reliable sources of purified myosin isoforms for IVMA.

The intracellular concentration of cholesterol is regulated by the balance between endogenous synthesis and exogenous uptake. Oestrogens have been reported to be involved in the physiological regulation of cellular cholesterol content. Relevant reports have focused on long-term responses and there is a lack of information about the relationship between the timing of the oestrogen effect and the regulation of cholesterol homeostasis. The aim of this work has been to set up a systematic picture of the short-term effects induced by oestrogen on hepatic lipid metabolism in vivo and the involvement of some relevant signal transduction pathways. At intervals after oestrogen administration (30 min to 6 h), oestrogen receptor expression and changes in liver cAMP, IP3 and protein kinase C-α (PKC-α) were followed. Changes in the expression of the low density lipoprotein receptor at mRNA and protein levels, and of hydroxy-methyl-glutaryl-CoA reductase activity have been verified. At the same time, the content of hepatic cholesterol, ubiquinone and dolichol and of plasma cholesterol have been determined. Changes of rab 5 and rab 8, small GTP-binding prenylated proteins involved in the transfer of neosynthesised proteins through the cell, have been also checked. In vivo treatment with oestradiol produced no change in cyclic AMP but a rapid increase in IP3, increased PKC-α localisation on the membranes and enhanced expression of the low density lipoprotein receptor in the liver occurred. PKC inhibition completely prevented any increase in low density lipoprotein receptor mRNA in isolated and perfused rat liver. Early changes of ubiquinone and dolichol content and a later reduction in hepatic hydroxy-methyl-glutaryl-CoA reductase activity and plasma cholesterol content were also detectable. A functional role of the IP3 -protein kinase C-α pathway in the induction of the low density lipoprotein receptor is suggested. Experimental Physiology (2001) 86.1, 39-45.

The aim of the present study was to investigate the mechanism of the retrograde transfer of ovarian steroid hormones from the ovarian lymphatic and venous effluent to the arterial blood supplying the ovary. In the first experiment, reproductive organs were collected from gilts in the luteal (n = 10) and follicular (n = 10) phase of the oestrous cycle. The ovary with the mesovarium was isolated and perfused through the ovarian artery with warmed, oxygenated autologous blood. The concentrations of progesterone and oestradiol in ovarian arterial blood increased on passing through the ovarian artery to the ovary, in the luteal phase, from 20.3 ± 2.1 to 31.4 ± 3.9 ng ml-1 (P < 0.001) and from 6.2 ± 0.8 to 11.4 ± 1.4 pg ml-1 (P < 0.001), respectively, and in the follicular phase, from 1.2 ± 0.2 to 2.2 ± 0.4 ng ml-1 (P < 0.001) and from 8.2 ± 1.8 to 13.2 ± 2.3 pg ml-1 (P < 0.001), respectively. Approximately 17.5 ± 3.9 % of the progesterone and 12.6 ± 1.7 % of the oestradiol found in the ovarian venous effluent was retrogradely transferred from the ovarian venous blood to the ovary in the luteal phase. In the follicular phase, these values were 10.1 ± 2.0 % and 8.6 ± 1.4 %, respectively. The efficiency of retrograde transfer of oestradiol and the rate of retrograde transfer of progesterone differed between phases of the oestrous cycle (P < 0.05 and P < 0.0001, respectively). A direct relationship between the concentration of the steroids in the venous effluent and the efficiency and rate of the retrograde transfer to the ovary was not found. In the second experiment (luteal phase, n = 10; follicular phase, n = 5), the concentration of progesterone and oestradiol increased in both ovarian arterial blood (P < 0.0001) and in the venous effluent (P < 0.0001) after administration of the steroids into the lymphatic vessels of the isolated mesovarium with separated ovary. In the third experiment (follicular phase, n = 5), with the mesovarium isolated after the ovary was removed and ovarian venous blood flowing out under the force of gravity (without the blood pressure in the ovarian vein), it was demonstrated that the veno-venous network covering the branches of the ovarian artery was supplied with the blood flowing out from the mesovarian tissue and that the filling of the veno-venous network was dependent on the blood pressure in the ovarian artery. We conclude that the effective retrograde transfer of steroid hormones from ovarian venous and lymphatic effluent to the ovary is accomplished not only by the classical counter-current exchange mechanism, but also as a result of complex processes that may be dependent on a specific part of the circulation of the blood and lymph in the periovarian vascular complex of the mesovarium. Experimental Physiology (2002) 87.3, 361-371.

Atherosclerosis is the leading cause of mortality in the developed world. In the United States alone there are approximately 700 000 deaths per year attributable to coronary artery disease, stroke and peripheral vascular diseases (National Center for Health Statistics, 1994). Although national data demonstrate an increase in coronary artery disease prevalence since 1970 (Morano, 1996), there has been a significant decline in mortality from atherosclerosis over the past three decades. Much of this decline may be attributable to risk factor modification. Intriguingly, many of the risk factors for atherosclerosis (hypercholesterolaemia, hypertension, homocysteinaemia, diabetes mellitus and exposure to tobacco) are associated with a reduced elaboration of nitric oxide by the endothelium (endothelium-derived nitric oxide (EDNO)) (Lüscher, Raij & Vanhoutte, 1987; Celermajer et al. 1993; Wu & Meininger, 1995). The result is a perturbation in the normal functions of the endothelium to maintain appropriate vessel calibre and resist the development of atherosclerosis. This endothelial impairment occurs well before any structural changes of atherogenesis are detected. Indeed when otherwise normal vessels are exposed in vitro to oxidized lipoproteins, an impairment in EDNO-dependent vasodilatation can be detected within minutes.It is well documented that risk factor modification has reduced the incidence of morbidity and mortality due to cardiovascular disease (Pasternak, Grundy, Levy & Thompson, 1996). More recently, modification of risk factors has also been shown to restore endothelial function and it may be by this mechanism that morbidity and mortality are curtailed. For example, cigarette smoking, which has been significantly linked to the occurrence of coronary artery disease, is associated with endothelial vasodilator dysfunction (Celermajer et al. 1993). Cessation of smoking appears to improve endothelial vasodilator dysfunction and reverses this increased risk within a few years (Rosenberg, Kaufman, Helmrich & Shapiro, 1985; Ockene, Kuller, Svendsen & Meilahn, 1990). Likewise, lowering cholesterol has been shown to improve endothelial function (Stroes, Koomans, de Bruin & Rabelink, 1995; Treasure et al. 1995) and reduce the incidence of initial and recurrent coronary artery disease events (Sacks et al. 1996). The effect of these therapies on endothelial function may explain their beneficial impact upon clinical events. Indeed, the effect of lipid-lowering therapy on mortality greatly exceeds the rather modest effect upon angiographic measures of lesion progression (Fuster, Gotto, Libby, Loscalzo & McGill, 1996). The success of risk factor modification in reversing endothelial dysfunction and clinical events serves to illustrate the importance of restoring endothelial function by other means. A greater understanding of the involvement of the nitric oxide system in the pathophysiology of vascular disease may provide the basis for new therapeutic strategies.

The specificity in location of angiogenesis to either glycolytic or oxidative fibre types, or muscle regions, was examined in the tibialis anterior (TA) and extensor digitorum longus (EDL) muscles of rat. Angiogenesis was induced by mechanical means either with (chronic muscle stimulation) or without (muscle stretch by overload) changes in blood flow, treatments which invoked only minor changes in fibre type and fibre size. Proliferation estimated by PCNA labelling of cells co-localised with capillaries was very rare in control muscles, where it occurred mainly in the glycolytic regions, but was increased in both models of angiogenesis. However, when labelled capillaries were scored according to the type of surrounding fibres, only muscle stimulation significantly accentuated proliferation of capillaries surrounded by glycolytic fibres. We conclude that while mechanical stimuli are important for proliferation in glycolytic regions in both models, capillary growth occurs specifically around glycolytic fibres in that region when the angiogenic stimulus includes increased blood flow and/or increased metabolic demand. Experimental Physiology (2003) 88.5, 565-568.

The fibre types that run in a vagal branch projecting to the rat heart are described in this study. In order to obtain spontaneous discharge in this vagal branch and optimal recording conditions, we compared the decerebrate state to urethane, urethane-chloralose and pentobarbital-chloralose anaesthesia with regard to level of chronotropic cardiac vagal tone. Administration of atropine (2 mg kg-1, I.V.) significantly decreased baseline cardiac interval only in the decerebrate and urethane-anaesthetised rat (by 0.018 ± 0.001 s and 0.019 ± 0.002 s, respectively). As a result of these experiments, urethane was chosen as the anaesthetic for all subsequent studies. Using a heart rate signal-averaging method we demonstrated that rat cardiac vagal preganglionic neurones innervating the sinoatrial node should have an expiratory discharge pattern, as reported in other species. However, only 5 % of chronotropic vagal tone was found to be subject to respiratory sinus arrhythmia. A suction microelectrode method, combined with spike-triggered averaging, was employed to record activity from a total of 58 vagal afferents that had axons in this branch. Approximately 75 % of these latter sensory fibres displayed cardiac rhythm. In a separate study we also recorded 318 preganglionic neurones with axons in the right cardiac vagal branch of the rat. Respiratory-modulated preganglionic units were statistically less common than tonically firing units. Six preganglionic subtypes were categorised according to conduction velocity and respiratory discharge pattern. Myelinated B-fibre and unmyelinated C-fibre types were found to be equally prevalent and equally likely to be reflexly excited during the pulmonary chemoreflex and the peripheral arterial chemoreflexes. The electrophysiological analysis has shown how diverse the discharge patterns of the preganglionic neurones or interneurones are whose axons course in the right cardiac vagal branch of the rat. The results of these experiments demonstrate the usefulness of combining spike discrimination with multiple spike-triggered averaging to simultaneously record B and C centrifugal vagal efferents. Experimental Physiology (2003) 88.6, 711-723.

A method is described that enables the cell membrane of isolated rat ventricular myocytes to be permeabilized and resealed while maintaining cell viability. Streptolysin O, a cholesterol-binding cytolysin, was used to form pores in the surface membrane; subsequent incubation with 5 % fetal bovine serum was used to reverse this permeabilization. The efficacy of membrane permeabilization and resealing was ascertained using a simultaneous double-staining technique using propidium iodide, a marker for cells with permeabilized membranes, and fluorescein diacetate, a marker for viable cells. This procedure allowed a distinction to be made between dead cells, unpermeabilized cells and viable cells that had been successfully permeabilized and resealed. The accessibility of the cell interior during permeabilization was investigated by including fluorescein isothiocyanate (FITC)-labelled dextrans (11, 38 and 148 kDa) and bovine serum albumin (67 kDa) in the permeabilization buffer, and localizing the FITC label using confocal microscopy following resealing. The confocal images showed that these molecules entered the cells and were retained after resealing. Following the permeabilization-resealing protocol, cells appeared to have both normal morphology and response to electrical stimulation. Thus this appears to be a cheap, simple and effective method to introduce relatively large molecules into cardiac myocytes.

The effect of anaesthetics on the relation between local cerebral blood flow (rCBF) and extracellular glucose was studied in rat striatum. Cerebral blood flow was measured using the hydrogen clearance method and extracellular glucose using an implanted glucose oxidase-based biosensor. Rats were given an intraperitoneal (I.P.) injection of either sodium pentobarbitone (60 mg kg-1) or chloral hydrate (350 mg kg-1). The effect of the I.P. injection, as demonstrated by an I.P. saline injection, was a brief increase in rCBF accompanied by a decrease in glucose. Sodium pentobarbitone produced a decrease in both rCBF and glucose, whereas chloral hydrate caused a decrease in glucose but an increase in rCBF. These findings show a dissociation between rCBF and extracellular glucose and suggest that glucose in the extracellular compartment is not derived directly from the blood vascular compartment.

In humans, it has been reported that dynamic exercise attenuates spontaneous baroreceptor reflex sensitivity (sBRS), which is an index of the gain of the baroreceptor-cardiac reflex. We demonstrated previously that endogenously produced NO from endothelial nitric oxide synthase (eNOS) within the nucleus tractus solitarii (NTS), the central terminal site of baroreceptor afferents, depressed sBRS. In this study, we investigated whether eNOS activity within the NTS plays any role in down-modulating the sBRS during dynamic exercise. In conscious Wistar rats arterial pressure and heart rate (HR) were monitored continuously and chronically using radiotelemetry before and during wheel cage running at 6 m min-1 for 10 min. sBRS was determined by a time-series method. During dynamic exercise systolic blood pressure (SBP) and HR were significantly increased (SBP: 138 ± 2 vs. 125 ± 2 mmHg, P < 0.001; HR: 447 ± 6 vs. 362 ± 8 beats min-1, P < 0.001) while sBRS was significantly decreased (0.53 ± 0.03 vs. 1.08 ± 0.08 ms mmHg-1, P < 0.001). In sino-aortic denervated rats the change in SBP in response to dynamic exercise was significantly larger than that in baroreceptor-intact rats (denervated: 21.6 ± 2.5 mmHg; intact: 12.0 ± 2.8 mmHg, P < 0.05). In contrast, denervation made no difference to the change in HR. Although disabling eNOS activity in the NTS by adenoviral-directed expression of a dominant negative mutant form of eNOS increased resting sBRS (1.48 ± 0.20 vs. 1.09 ± 0.15 ms mmHg-1, P < 0.05), the absolute level reached during dynamic exercise was identical to control. These results demonstrate that during dynamic exercise (i) the sBRS decreases around the operating point of the baroreceptor-cardiac reflex function curve in normotensive rats, (ii) the baroreceptor reflex operates to limit the rise in arterial pressure, and (iii) the attenuation of sBRS is not mediated by changes in eNOS activity within the NTS. Experimental Physiology (2003) 88.4, 517-526.

Rats have an attenuated febrile response to exogenous (e.g. bacterial endotoxin) and endogenous pyrogen (e.g. interleukin-1β) near the term of pregnancy, the mechanism of which is unknown. The present experiments were carried out on 71 non-pregnant and 181 pregnant Sprague-Dawley rats to determine if basal levels of the endogenous antipyretic substance, interleukin-1 receptor antagonist (IL-1ra), change relative to interleukin-1β (IL-1β) throughout gestation. Furthermore, we have constructed complete Escherichia coli lipopolysaccharide (LPS) dose-core temperature response curves in non-pregnant and pregnant rats on days 10, 15 and 20 of gestation (term of gestation ~21 days) to determine if the attenuated febrile response near the term of pregnancy results from a simple shift of the dose-response relationship or results from a dampening of the overall dose-response relationship. Basal IL-1β, as determined by ELISA on trunk blood from non-pregnant and pregnant rats on days 10, 15 and 20 of gestation (d10, d15, d20), did not change significantly during pregnancy. Basal IL-1ra, however, was increased significantly in d15 rats as compared to non-pregnant, and d10 and d20 rats. The attenuated febrile response near the term of pregnancy, as determined by biotelemetry, did not result from a simple shift of the E. coli LPS dose-core temperature response curve but rather a dampening of the overall dose-response relationship. The febrile responses to EC50 and EC100 doses of E. coli LPS were preceded by a period of hypothermia, and were delayed and attenuated near the term of pregnancy as compared to that observed early in pregnancy and in non-pregnant rats. Our data provide evidence that pregnant rats are more sensitive to the hypothermia-producing effects of E. coli LPS than are non-pregnant rats and allow us to speculate that elevated basal IL-1ra may play a role in mediating the attenuated febrile response to pyrogen on day 15 but not on day 20 of gestation in rats. Experimental Physiology (2003) 88.6, 747-754.

Under physiological conditions, calcium-dependent ([Ca2+]i-dependent) Cl- currents (ICl(Ca)) have been suggested to participate in the repolarizing processes. In this paper, the possible contribution of ICl(Ca) to transient inward currents and, hence to arrhythmias, has been studied in myocytes from the working myocardium and from the conductive system. Single atrial, ventricular and Purkinje cells, isolated enzymatically from rabbit heart, have been studied under whole-cell voltage-clamp and were internally perfused with the fluorescent Ca2+ indicator, fura-2 (100 µM). Ca2+ release from the sarcoplasmic reticulum was either induced by external application of caffeine or occurred spontaneously in Ca2+-overloaded cells. Membrane currents accompanying Ca2+ transients showed linear current-voltage characteristics between -60 and +80 mV as evidenced from fast voltage ramps. When intra- and extracellular Cl- concentrations were kept symmetrical in the absence of the Na+-Ca2+ exchange mechanism, transient currents had a reversal potential close to 0 mV. Reduction of external Cl- concentration shifted this reversal potential towards the new Cl- equilibrium potential. Neither the time course of the transient currents nor the shift in their reversal potentials was significantly affected by the presence of Na+. Approximately 90 % of this current was blocked by the application of the Cl- channel blocker, anthracene-9-carboxylic acid (0·5 mM) at +80 mV. It is concluded, that [Ca2+]i-activated transient membrane currents in atrial, ventricular and Purkinje cells of rabbit heart are mainly due to the activation of a [Ca2+]i-dependent Cl- current.

The properties are discussed of system y+L, a broad scope amino acid transporter which was first identified in human erythrocytes. System y+L exhibits two distinctive properties: (a) it can bind and translocate cationic and neutral amino acids, and (b) its specificity varies depending on the ionic composition of the medium. In Na+ medium, the half-saturation constant for L-lysine influx was 9·5 ± 0·67 µM and the inhibition constant (Ki) for L-leucine was 10·7 ± 0·72 µM. L-Leucine is the neutral amino acid that binds more powerfully, whereas smaller analogues, such as L-alanine and L-serine interact less strongly (the corresponding inhibition constants were Ki,Ala, 0·62 ± 0·11 mM; Ki,Ser, 0·49 ± 0·08 mM). In the presence of K+, the carrier functions as a cationic amino acid specific carrier, but Li+ is able to substitute for Na+ facilitating neutral amino acid binding. The effect of the inorganic cations is restricted to the recognition of neutral amino acids; translocation occurs at similar rates in the presence of Na+, K+ and Li+. The only structural feature that appears to impair translocation is bulkiness and substrates with half-saturation constants differing by more than 100-fold translocate at the same rate. This suggests that translocation is largely independent of the forces of interaction between the substrate and the carrier site. System y+L activity has been observed in Xenopuslaevis oocytes injected with the cRNA for the heavy chain of the 4F2 human surface antigen. 4F2hc is an integral membrane protein with a single putative membrane-spanning domain and it remains to be clarified whether it is part of the transporter or an activator protein.

Rats were submitted to 14 days of hindlimb suspension in order to examine the contractile and elastic properties of the soleus muscles under disuse conditions. The calcium/strontium activation properties, the maximal shortening velocity (V0), as well as the time behaviour of force transients following quick releases and the T1 curves characterizing the active part of the series elastic elements, were determined on single chemically skinned fibres. After the functional measurements, the fibres were subjected to sodium dodecyl sulfate-polyacrylamide gel electrophoresis in order to analyse both the myosin heavy (MHC) and light (MLC) chain isoforms. According to the MHC and MLC composition, two groups of fibres were defined after hindlimb suspension: a group of slow fibres expressing the slow set of both MHC and MLC isoforms, and a group of fast fibres co-expressing the slow and fast MHC and MLC isoforms with a predominant expression of the fast ones. For the first group, the contractile as well as the elastic properties were found to be close to those of control slow soleus fibres. For the second group, both contractile and elastic properties were modified insofar as they became close to those found in a fast muscle such as the extensor digitorum longus. We suggested that between the two populations found in the soleus muscle after hindlimb suspension the modifications in the contractile properties, as well as the alterations in the elastic characteristics, were concomitant to the changes in both MHC and MLC compositions.

There are two basic modes of communication used between neurones in the nervous system: neurotransmission and neuromodulation. When representations of neural circuits are drawn, they tend to include only fast neurotransmission. This can lead us to believe that neurotransmission conveys information through the nervous system and that neuromodulation merely modifies the neurotransmission. Even the words that we use to describe these two forms of communication imply that meaning. However, the very act of modifying neurotransmission conveys information. Therefore, in order to better understand information flow in the nervous system, we need to determine how neuromodulation is integrated into neuronal circuits.

Changes in the composition of fibre types and the capillary supply of skeletal muscle (tibialis anterior) were quantified in rats and hamsters subjected to 8-10 weeks of cold exposure and reduced photoperiod (10 °C, 1 h light-23 h dark). Muscle mass decreased in both species (by 12 % and 17 %, respectively). Following acclimation to cold there were no specific changes in fibre cross-sectional area (FCSA) in rats, whereas in hamsters there was a substantial atrophy of Type II, but not Type I fibres. In rat muscle there was little difference between the two groups in average capillary to fibre ratio (C:F) (1.76 ± 0.15, normothermia, N; 1.69 ± 0.05, hypothermia, H) and average capillary density (CD) (188 ± 14 mm-2, N; 201 ± 12 mm-2, H). Similarly, the average C:F was unaltered in hamsters (2.75 ± 0.11, N; 2.72 ± 0.15, H), although the 30 % smaller fibre size observed with hypothermia resulted in a corresponding increase in average CD, to 1539 ± 80 mm-2 (P < 0.01). However, there was a coordinated regional adaptation to cold exposure in hamsters resulting in capillary rarefaction in the glycolytic cortex and angiogenesis in the oxidative core. Following acclimation of rats to cold there was a reduction in the supply area of individual vessels (capillary domain), particularly in the cortex (9310, N; 8938 µm2, H; P < 0.05). In contrast, hypothermic hamsters showed only a small decrease in mean domain area in the cortex (948 µm2, N; 846 µm2, H; n.s.) but a marked reduction in the core (871 µm2, N; 604 µm2, H; P < 0.01). Rats showed little or no change in local capillary supply (LCFR) to fast fibres on acclimation to cold, while in hamsters the LCFR of Type IIb fibres showed a decrease in the cortex (2.7, N; 2.3, H) and an increase in the core (3.0, N; 3.3, H) during acclimation to cold. These data suggest that during a simulated onset of winter rats maintain FCSA and capillary supply as part of an avoidance strategy, whereas hamsters increase muscle capillarity in part as a consequence of disuse atrophy. Experimental Physiology (2001) 86.5, 629-639.