PubMed · 5088362
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T V Thomas. 1971. Aortic declamping shock.. https://doi.org/10.1016/0002-8703(71)90092-5
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Electrical muscle stimulation (Mstim) at a low or high frequency is associated with failure of force production, but the exact mechanisms leading to fatigue in this model are still poorly understood. Using 31P magnetic resonance spectroscopy (31PMRS), we investigated the metabolic changes in rabbit tibialis anterior muscle associated with the force decline during Mstim at low (10 Hz) and high (100 Hz) frequency. We also simultaneously recorded the compound muscle mass action potential (M-wave) evoked by direct muscle stimulation, and we analyzed its post-Mstim variations. The 100-Hz Mstim elicited marked M-wave alterations and induced mild metabolic changes at the onset of stimulation followed by a paradoxical recovery of phosphocreatine (PCr) and pH during the stimulation period. On the contrary, the 10-Hz Mstim produced significant PCr consumption and intracellular acidosis with no paradoxical recovery phenomenon and no significant changes in M-wave characteristics. In addition, the force depression was linearly linked to the stimulation-induced acidosis and PCr breakdown. These results led us to conclude that force failure during 100-Hz Mstim only results from an impaired propagation of muscle action potentials with no metabolic involvement. On the contrary, fatigue induced by 10-Hz Mstim is closely associated with metabolic changes with no alteration of the membrane excitability, thereby underlining the central role of muscle energetics in force depression when muscle is stimulated at low frequency. Finally, our results further indicate a reduction of energy cost of contraction when stimulation frequency is increased from 10 to 100 Hz.
End-stage renal disease (ESRD) is a major cause of morbidity in children. Besides its high cost to society, ESRD carries significant mortality. Chronic renal insufficiency (CRI) often precedes ESRD. Identifying factors that correlate with the rate of progression to ESRD is beneficial in the management of children with CRI. Since 1994 the North American Pediatric Renal Transplant Cooperative Study (NAPRTCS) has extended its registry to include children with CRI, defined as creatinine clearance ( C(Cr)) <75 ml/min per 1.73 m(2). As of January 2001, our database registered 4,666 children (<20 years of age) with CRI. Data analysis showed that at least 40% of patients entered had congenital urological anomalies; 39% of patients were followed for at least 3 years. Follow-up data showed that 31% of all registered patients progressed to ESRD by the end of the reporting period. There was a correlation between CRI and several co-morbid clinical factors: low hematocrit, hypoalbuminemia, hypocalcemia, hyperphosphatemia, and hyperparathyroidism, and the rate of progression to ESRD. Primary clinical diagnosis and the age at entry into registry were additional factors that correlated with the rate of progression to ESRD. The main cause of hospitalization in this registry was infection, which accounted for 45% of hospital admissions. Growth delay measured by standard deviation score at baseline was -1.40 at the time of registration. Our data suggest potential areas of improved care that could impact the onset of ESRD.
Activity of the Na+/H+ exchanger (NHE) isoform 1 (NHE1) is increased by intracellular acidosis through the interaction of intracellular H+ with an allosteric modifier site in the transport domain. Additional regulation is achieved via kinase-mediated modulation of the NHE1 regulatory domain. To determine if intracellular acidosis stimulates NHE1 activity solely by the allosteric mechanism, we subjected cultured neonatal rat ventricular myocytes (NRVM) with native NHE1 expression to intracellular acidosis (pHi approximately 6.6) for up to 6 min by transient exposure to NH4Cl and its washout in the presence of NHE inhibition (by zero [Na+]o or the NHE1 inhibitor cariporide) in HCO3- -free medium. After the desired duration of acidosis, NHE was reactivated (by reintroduction of [Na+]o or removal of cariporide), and the rate of recovery of pHi (dpHi/dt) was measured as the index of NHE activity. Regardless of the method used when intracellular acidosis was sustained for > or =3 min, subsequent NHE activity was significantly increased (>4-fold). Similar NHE stimulatory effects of sustained acidosis were observed in adult rat ventricular myocytes and COS-7 cells. Sustained (3 min) intracellular acidosis activated several NHE1 kinases in NRVM, in an in-gel kinase assay using as substrate a glutathione S-transferase fusion protein of the NHE1 regulatory domain. Detailed investigation of ERK and its downstream effector p90RSK, two putative NHE1 kinases, revealed time-dependent activation of both by intracellular acidosis in NRVM. Furthermore, inhibition of MEK1/2 by pretreatment of NRVM with two structurally distinct inhibitors, PD98059 (30 microM) or UO126 (3 microM), inhibited the activation of ERK and p90RSK and abolished the stimulation of NHE activity by sustained (3 min) intracellular acidosis. Our data show that not only the extent but also the duration of intracellular acidosis regulates NHE1 activity and suggest that the stimulatory effect of sustained intracellular acidosis occurs through a novel mechanism mediated by activation of the ERK pathway.