The majority of mitochondrial proteins are encoded by nuclear genes, synthesized in the cytosol and subsequently imported into mitochondria through protein translocation machineries of the outer and inner membranes. In this review, we discuss the arrangement of the various translocation complexes and the function of individual import components. We also outline the various targeting pathways that preproteins can take in order to reach their appropriate sub-mitochondrial compartment. Experimental Physiology (2003) 88.1, 57-64.

Muscle vasodilatation evoked by systemic hypoxia is adenosine mediated and nitric oxide (NO) dependent: recent evidence suggests the increased binding of NO at complex IV of endothelial mitochondria when O2 level falls leads to adenosine release. In this study on anaesthetised rats, the increase in femoral vascular conductance (FVC) evoked by systemic hypoxia (breathing 8 % O2 for 5 min) was reduced by oxypurinol which inhibits xanthine oxidase (XO): XO generates O2- from hypoxanthine, a metabolite of adenosine. By contrast, infusion of superoxide dismutase (SOD), which dismutes O2- to hydrogen peroxide (H2O2), potentiated the hypoxia-evoked increase in FVC. However, NO synthesis inhibition reduced the hypoxia-evoked increase in FVC and it was not further altered by SOD. In other studies, the spinotrapezius muscle was pre-loaded with hydroethidine (HE), or dihydrorhodamine (DHR) which fluoresce in the presence of O2- and H2O2, respectively. In muscle loaded with HE, systemic hypoxia increased fluorescence in endothelial cells of arterioles, whereas in muscle loaded with DHR, fluorescence was diffusely located in and around arteriolar endothelium. We propose that in systemic hypoxia, O2- generated by the XO degradation pathway from adenosine released by endothelial cells, and released by endothelial mitochondria by increased binding of NO to complex IV, is dismuted to H2O2, which facilitates hypoxia-induced dilatation. Experimental Physiology (2003) 88.6, 733-740.

Maintaining the extracellular K+ concentration ([K+]o) between 15 and 60 mM induced oscillations in the intracellular Ca2+ concentration ([Ca2+]i) in rat submandibular acinar cells during stimulation with acetylcholine (ACh, 1 µM). These [Ca2+]i oscillations were also induced by 1 µM thapsigargin and were inhibited by 50 µM La3+, 1 µM Gd3+, or the removal of extracellular Ca2+, indicating that the [Ca2+]i oscillations were generated by store-operated Ca2+ entry (SOC). The frequency of the ACh-evoked [Ca2+]i oscillations increased from 0.8 to 2.3 mHz as [K+]o was increased from 15 to 50 mM. TEA (an inhibitor of K+ channels) also induced [Ca2+]i oscillations at [K+]o of 4.5 or 7.5 mM in ACh-stimulated cells. These data suggest that depolarization causes [Ca2+]i to oscillate in ACh-stimulated submandibular acinar cells. Pertussis toxin (PTX, an inhibitor of G proteins) caused [Ca2+]i to be sustained at a high level in ACh-stimulated cells at 25 mM or 60 mM [K+]o. This suggests that the [Ca2+]i oscillations are generated by a periodic inactivation of the SOC channels via PTX-sensitive G proteins, which are stimulated by depolarization. Moreover, in the presence of DBcAMP or forskolin which accumulated cAMP the frequency of the [Ca2+]i oscillations remained constant (approximately 1.2 mHz) when [K+]o was maintained in the range 25-60 mM. Based on these observations in ACh-stimulated submandibular acinar cells, we conclude that depolarization stimulates the PTX-sensitive G proteins, which inactivate the SOC channels periodically ([Ca2+]i oscillation), while hyperpolarization or PTX inhibits the G proteins, maintaining the activation of the SOC channels. Accumulation of cAMP is likely to modulate the PTX-sensitive G proteins. Experimental Physiology (2003) 88.3, 369-379.

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.

The scientifically novel, but evolutionarily ancient, so-called uncoupling proteins 2 and 3 (UCP2, UCP3) are structually similar to the archetypical uncoupling protein UCP1. A series of suggestions have been forwarded for their physiological function. We discuss systematically here the pros and cons for these suggestions. We conclude that the novel UCPs do not seem to be physiologically relevant uncoupling proteins; the uncoupling property was apparently a late introduction into the subfamily through the evolution of UCP1. Physiological functions ascribed to UCP2 and UCP3 based on their purported uncoupling property may have to be revised (i.e. any type of thermogenesis, including protection against obesity, protection against the formation of reactive oxygen species and thermogenic involvement in the fever response). The presence of a mixed genetic background in most published studies of UCP2 or UCP3 gene-ablated mice also means that data concerning marked differences in diabetes propensity, infection sensitivity and production of reactive oxygen species may require confirmation in backcrossed mice. The increased expression of UCP2 and UCP3 under conditions of increased fatty acid metabolism implies an as yet undefined role in lipid metabolism. Thus, the novel UCPs should probably be considered as mitochondrial carriers, and the challenge now is to identify the transported molecule. Experimental Physiology (2003) 88.1, 65-84.

The human finger pad is highly vascularized so it might be expected that the on-going cardiac pulsations in the vicinity of a cutaneous mechanoreceptor would be reflected in its spontaneous or evoked discharge. The purpose of this study was to examine the prevalence of this potential cardiac rhythmicity in a random sample of tactile afferents from the finger pad. Unitary recordings were made from 21 rapidly ('fast') adapting (18 FA I; 3 FA II) and 44 slowly adapting (17 SA I; 27 SA II) afferents via tungsten microelectrodes in the median nerve. Skin blood flow was measured over the pulp of a contralateral finger; ECG activity, blood pressure and respiration were also recorded. Cardiac modulation, present either as a simple pulse rhythm or as modulation of an on-going discharge, was expressed by 44 % of the afferents. Only two out of 18 FA I units, and two out of three FA II units, exhibited cardiac rhythmicity, but their temporal coupling to the pulse was very tight. Modulation was more common for the slowly adapting afferents (57 %), and more prevalent among the SA II (65 %) than the SA I (47 %) classes. Nine spontaneously active SA II afferents exhibited respiratory rhythmicity, their background discharge falling during inspiration. It is concluded that cardiac modulation is common for both classes of slowly adapting tactile afferents (but less common for the rapidly adapting afferents), which may have implications for the sensory signalling of tactile information. Experimental Physiology (2003) 88.5, 617-625.

Control (normothermic) and cold-acclimated (environmental temperature gradually reduced from 20 to 5 °C for 4 weeks) groups of male rats and hamsters were compared to elucidate the nature of angiogenesis in oxidative and glycolytic muscles of these species during progressive cold exposure. Skeletal muscle capillarity and fibre cross-sectional area were measured in the tonic soleus (SOL) and phasic tibialis anterior (TA). Cryostat sections were stained for alkaline phosphatase (ALP) activity to identify all capillaries, and proliferating cell nuclear antigen (PCNA) to localise the site of cellular proliferation. Cold-induced angiogenesis, indicated by an increase in capillary to fibre ratio (C:F), occurred in SOL of rats (~20 % increase, P < 0.05) but not hamsters (~9.5 % increase, n.s.), and in TA of hamsters (~22 % increase, P < 0.01) but not rats (~1 % increase, n.s.). The change in C:F was highest in the glycolytic cortex region of TA where fibre size is larger than in the oxidative core. Capillary-specific cell proliferation (co-localised ALP and PCNA labelling) increased in parallel with C:F. The total PCNA label density within the interstitium was some 5-fold higher than that co-localised with capillaries, but where angiogenesis occurred the relative increase in capillary labelling was 2-fold greater than for other cells of the interstitium. These data suggest a significant role for endothelial cell proliferation in the angiogenic response, indicative of the sprouting form of angiogenesis. There was a tendency for fibre hypertrophy in both SOL and TA of rats, especially in the core region of TA (P < 0.01), such that capillary density (CD) and intramuscular diffusion distances (DD) were largely unchanged following cold exposure. In contrast, fibre size was maintained in hamsters, DD reduced and CD increased compared to control TA (P < 0.01). In conclusion, cold acclimation stimulated angiogenesis in muscle of hamsters more than in rats, possibly due to a higher metabolic rate in the smaller species. Angiogenesis was also seen in SOL of rat, where oxidative capacity and muscle activity is higher than the TA. Thus, a combination of oxidative capacity, muscle activity, and fibre size may determine the degree of angiogenesis in response to low environmental temperature. Experimental Physiology (2003) 88.6, 741-746.

Elemental diets are associated with intestinal atrophy and reduced intestinal integrity. Growth factors such as keratinocyte growth factor (KGF) and epidermal growth factor (EGF) have considerable potential for the therapeutic reversal of such atrophy and may have greater actions if given in combination. We examined the effects of recombinant human KGF (rHuKGF), EGF and their combination on tissue mass, cell proliferation and crypt fission throughout the intestine of mice fed elemental diets. rHuKGF significantly increased the relative wet weight of the intestine, with EGF having a lesser effect. Cell proliferation of the stomach, small intestine and colon were significantly increased by rHuKGF, but EGF only increased proliferation in the small intestine. Crypt fission in the small intestine and colon was significantly decreased by rHuKGF. An interactive effect of rHuKGF and EGF on the weight of stomach and the proliferation of the fundus and antrum was observed. Moreover, an interactive effect of the agents was also seen on crypt fission in the colon. We concluded that (1) rHuKGF and EGF have significant trophic effects on the stomach, small intestine and colon, (2) these actions vary between different sites in the gastrointestinal tract, and (3) interactive effects occur. Experimental Physiology (2003) 88.2, 261-267.

Earlier work in this laboratory showed that noradrenaline (NA) induces apoptosis in primary cultures of alveolar epithelial cells (AECs). Apoptosis of alveolar epithelial cells may promote the collapse of lung barrier function. On this basis we hypothesized that exogenous NA, administered by intratracheal (I.T.) instillation, might induce AEC apoptosis in vivo followed by acute lung injury. Delivery of NA (10 µM) I.T. into male Wistar rats increased labelling of both fragmented DNA, measured by in situ end labelling (ISEL), and the active form of caspase 3 (anti-Casp3) 6 and 20 h after administration (P < 0.05), but instillation of the vehicle alone (PBS) had no effect. Both ISEL and anti-Casp3 labelling were attenuated by concurrent I.T. delivery of the broad-spectrum caspase inhibitor ZVADfmk. After 6 h, most ISEL- and Casp3-positive cells were located in the surfaces of alveolar walls, but after 20 h more were found in alveolar spaces (P < 0.05). Instillation of NA also increased the bronchoalveolar lavage (BAL) content of fluorescent albumin (BODIPY-alb), which had previously been injected intravenously; the increase was reversed by concurrent ZVADfmk administration. These data suggest that NA-induced apoptosis of AECs in vivo is sufficient to invoke transient collapse of AEC barrier function that is rapidly repaired. Experimental Physiology (2003) 88.2, 269-275.

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.

Pituitary hormones have an important role during exercise yet relatively little is known about the stimulus for their release. Body temperature progressively increases during prolonged steady-state exercise in the heat and we have investigated the role that this may play in the release of prolactin, growth hormone and cortisol (as an indicator of adrenocorticotropic hormone) into the circulation. Fit young male subjects exercised at 73 %O2,max until volitional fatigue at 20 °C and at 35 °C (30 % relative humidity at both temperatures). Rectal temperature and mean skin temperature were monitored and blood samples analysed for lactate, glucose, cortisol, growth hormone and prolactin concentrations. During the first 20 min, core temperature rose continuously and to a similar extent at both temperatures, while mean skin temperature was approximately 4 °C lower during exercise in the cool. Blood glucose concentration was essentially constant throughout the period of exercise while lactate concentration increased in the first 10 min and then remained constant with very similar changes in the two exercise conditions. Prolactin and growth hormone concentrations both increased during the exercise period while the concentration of cortisol declined slightly before rising slightly over the 40 min period. Prolactin release was significantly greater when exercise was carried out in the heat while there was no difference in the release of growth hormone or cortisol in the two conditions. When plotted as a function of rectal temperature, growth hormone concentration showed a linear relationship which was the same at ambient temperatures of 35 °C and 20 °C. Prolactin concentration had a curvilinear relationship with rectal temperature and this differed markedly at the two ambient temperatures. Cortisol concentration showed no dependence on any measure of body temperature. Our results are consistent with some aspect of body temperature being a stimulus for growth hormone and prolactin secretion; however, the precise mechanism clearly differs between the two hormones and we suggest that skin temperature modulates prolactin release, but does not affect the release of growth hormone. Experimental Physiology (2003) 88.5, 627-635.

The basal in vitro release of amylase was similar from rat parotid lobules of innervated and chronically denervated glands and was unaffected by the inhibitors used in this study. The secretion of amylase induced by isoprenaline or vasoactive intestinal peptide (VIP) was reduced by one-third to one-half from the lobules of the innervated glands and even more so from the lobules of the denervated glands by ODQ, an inhibitor of soluble guanyl cyclase which is activated by nitric oxide (NO) and catalyses the cGMP production. The use of N ω-propyl-L-arginine (N-PLA) revealed that the evoked secretion of amylase in the denervated glands depended on the activity of neuronal type NO synthase to synthesize NO. Since the denervated gland is virtually devoid of NO synthase-containing nerve fibres, the neuronal type NO synthase was most probably of a non-neuronal source. NO-dependent amylase secretion was agonist related, since amylase secretion evoked by bethanechol and neuropeptide Y was not reduced by ODQ or N-PLA. Hence, under physiological conditions, activation of β-adrenoceptors (sympathetic activity) and VIP receptors (parasympathetic activity) is likely to cause secretion of parotid amylase partly through a NO/cGMP-dependent intracellular pathway involving the activity of neuronal type NO synthase, possibly of acinar origin. Experimental Physiology (2003) 88.3, 381-387.

Apoptosis is an evolutionary-conserved physiological mechanism to remove cells from an organism. Cellular apoptosis is mediated via an intracellular signalling programme that involves a variety of signalling molecules and cellular organelles including caspases, sphingomyelinases, Bcl-2-like proteins and proteins to cleave the DNA and mitochondria. Mitochondria contain several pro-apoptotic molecules that activate cytosolic proteins to execute apoptosis, block anti-apoptotic proteins in the cytosol and directly cleave nuclear DNA. Mitochondria trap these pro-apoptotic proteins and physically separate pro-apoptotic proteins from their cytoplasmic targets. Apoptosis is then initiated by the release of mitochondrial pro-apoptotic proteins into the cytosol. This process seems to be regulated by Bcl-2-like proteins and several ion channels, in particular the permeability transition pore (PTP) that is activated by almost all pro-apoptotic stimuli. Experimental Physiology (2003) 88.1, 85-90.

Previous studies have shown that prolactin administration to pregnant rats results in offspring with enhanced abundance of the brown adipose tissue-specific uncoupling protein (UCP) 1. The present study therefore aimed to determine whether a similar effect was observed after birth and if the sex of the animal further influenced the responsiveness of brown adipose tissue mitochondria to prolactin administration. Daily prolactin injections were therefore commenced at 15, 35 or 60 days of age and continued for 4 days. The abundance of UCP1 was unchanged with age in females but decreased between 15 and 35 days in males and was lower in males than females by 60 days of age. Cytochrome c abundance remained unchanged with postnatal age in both males and females and was consistently higher in males at each sampling age. Prolactin decreased the abundance of UCP1 and cytochrome c when administered to female rats at 35 and 60 days of age, but had no effect at 15 days. In contrast, prolactin had no effect on UCP1 in male rats at any age, but did stimulate the abundance of cytochrome c at 15 days of age. In conclusion, the administration of prolactin to postnatal rats over the period in which maturation of the hypothalamic-pituitary axis and brown adipose tissue function is occurring did not enhance UCP1 abundance. In females, prolactin actually caused a reduction in UCP1 suggesting that in rats it is only prior to birth that prolactin has a stimulatory role on brown adipose tissue development. Experimental Physiology (2003) 88.4, 527-531.

We investigated the effects of sepsis, through the lipopolysaccharide (LPS)-induced inflammatory response, on plasma corticosterone and prolactin (PRL) levels during acute immobilization stress in normal and thyroidectomized rats. Thyroidectomized (TX) or sham-operated (N) rats were subjected to 120 min of immobilization stress. Rats were treated with an intraperitoneal injection of either LPS (250 µg (100 g body wt)-1) or the same volume of vehicle (saline solution), 90 min before the induction of stress. Blood samples for hormone assays were collected before sepsis and stress induction for baseline measures (-90 min), and during sepsis and immobilization stress for the measurement of prolactin and corticosterone levels by radioimmunoassay. Our results show that the thyroid hormones are necessary for a proper response of PRL and corticosterone release during immobilization stress. Although sepsis enhanced PRL secretion, this was not true of corticosterone release in either group of rats. Low levels of thyroid hormones partially block the release of PRL, but do not block corticosterone secretion during sepsis. Experimental Physiology (2003) 88.6, 755-760.

Magnesium deficiency is generally associated with an impaired ability to dispose of glucose. In order to test whether or not increasing the peripheral free Mg concentration ([Mg2+]) would enhance glucose disposal, we have carried out glucose infusions in sheep with and without simultaneous infusion of Mg. Basal plasma glucose levels were higher in lactating sheep ('lactators') than in non-lactating sheep (controls) (P < 0.05). The glucose disposal rate (KG) with no added Mg was greater in lactators than controls (P < 0.05). When Mg was added to the infusate, KG in lactators was reduced (P < 0.005). Infusion of Mg depressed basal insulin levels in controls and lactators (P < 0.0001 for both). The insulin response to the intravenous glucose tolerance test (IVGTT) was lower in lactators compared with controls (P < 0.0001); however, after correcting for the reduced basal insulin level when Mg was included during the IVGTT, there was no difference between the two groups. We conclude that intravenous Mg at the doses used in this study leads to a decrease in basal insulin secretion, and that increasing serum [Mg2+] reduces glucose disposal in lactating sheep.Experimental Physiology (2003) 88.4, 533-540.

We hypothesised that maternal uterine artery vascular dysfunction could contribute to cardiovascular dysfunction in offspring of rats fed a diet rich in fat. Sprague-Dawley rats were fed for 10 days prior to pregnancy and throughout gestation either: (a) a control breeding diet, or (b) the same diet supplemented with 20 % w/w lard, vitamins, essential micronutrients and protein to control values. At 20 days gestation vascular function was assessed in uterine arteries and third-order mesenteric arteries. Vascular reactivity in response to application of potassium, noradrenaline, the thromboxane analogue U46619, acetylcholine and nitric oxide was assessed. Maternal plasma concentrations of factors likely to contribute to endothelial dysfunction were measured. Maximum acetylcholine-induced relaxation was impaired in the mesenteric arteries of the lard-fed dams (max % relaxation: lard-fed, 69.7 ± 6.48; control, 85.37 ± 2.69, P = 0.03). Uterine artery vascular function was similar in the two groups (max % acetylcholine-induced relaxation: lard-fed, 73.7 ± 4.01; control, 77.5 ± 4.72, P = 0.98). Concentrations of plasma lipids, 8-epi-PGF2α and leptin were normal, whereas insulin and corticosterone concentrations were raised in the lard-fed group (insulin (ng ml-1): lard-fed, 8.04 ± 0.47; control, 1.35 ± 0.37, P < 0.0001; corticosterone (ng ml-1): lard-fed, 1164.0 ± 170.9; control, 541.9 ± 96.3, P = 0.005). Fetal and placental weights were reduced in lard-fed dams (fetus (g): lard-fed, 4.27 ± 0.38; control, 2.96 ± 0.40, P = 0.025; placenta (g): lard-fed, 0.72 ± 0.06; control, 0.57 ± 0.04, P = 0.05). Cardiovascular dysfunction in offspring is not associated with reduced uterine artery endothelial function but is associated with activation of the hypothalamic-pituitary-adrenal axis, hyperinsulinaemia and fetoplacental growth retardation. Experimental Physiology (2003) 88.3, 389-398.

It has been known for a long time that isolated mitochondria are able to accumulate large amounts of calcium ions. Before the discovery that endoplasmic reticulum (ER) was the main Ca2+-storing cellular organelle, mitochondria were thought to play a major role in cytoplasmic Ca2+ homeostasis (Carafoli, 2002). After IP3 was discovered and it was shown that IP3 receptors were localized in ER membrane and that Ca2+-binding proteins such as calsequestrin could store large amounts of Ca2+ in the ER, the role of mitochondria in the regulation of cytoplasmic Ca2+ was questioned. However, in recent years, mostly due to the development of new methods, there has been increasing evidence that mitochondria could be an important cytoplasmic Ca2+ sink, especially under conditions of a high cellular Ca2+ load. Experimental Physiology (2003) 88.1, 91-97.

Chronic stimulation of β2-receptors with β2-agonists causes desensitisation, which in skeletal muscle is accompanied by myosin heavy chain (MHC) remodelling, similar to that observed in heart failure patients. However, the mechanisms for this skeletal muscle remodelling are not well established. G protein-coupled receptor kinases (GRKs) specifically phosphorylate and desensitise G protein-coupled receptors during periods of agonist activation. However, desensitisation associated with prolonged agonist activation alters β-adrenergic signalling, and downstream affects gene expression. We hypothesised that skeletal muscle remodelling induced by β2-agonist administration could be regulated by GRK expression. Therefore the aim of this study was firstly to characterise which, if any, of the six known isoforms of GRK were expressed in skeletal muscle and then secondly to determine whether remodelled skeletal muscle induced by chronic β2-agonist administration was accompanied by altered expression of GRK isoforms. Male Wistar rats were administered a β2-agonist daily for 8 weeks, and the expression of MHC and GRKs examined in gastrocnemius and soleus muscles. Treatment with β2-agonist caused a change in MHC in soleus from types I to IIA, and in gastrocnemius from MHC types IIA/IIX to IIB. Western blotting revealed that GRK2 and GRK5 were expressed in skeletal muscle. Furthermore, despite changes in MHC and differential muscle-specific expression of GRK isoforms, there was no significant change in expression of GRK2 and GRK5 in soleus or gastrocnemius following β2-agonist administration. In conclusion the level of GRK expression is unlikely to be responsible for MHC switching following chronic β2-receptor stimulation. Experimental Physiology (2003) 88.2, 277-284.