Rapid effects of hypoxia on H+ homeostasis in articular chondrocytes

Abstract

Articular chondrocytes experience low oxygen (O(2)) levels compared with many other tissues, and values fall further in disease states. Chondrocyte intracellular pH (pH(i)) is a powerful modulator of matrix synthesis and is principally regulated by Na(+)-H(+) exchange (NHE). In equine chondrocytes, NHE is inhibited when cells are incubated for 3 h at low O(2), leading to intracellular acidosis. O(2)-dependent changes in reactive oxygen species (ROS) levels appear to underlie this effect. The present study examines whether hypoxia can influence chondrocyte NHE activity and pH(i) over shorter timescales using the pH-sensitive fluoroprobe BCECF in cells isolated not only from equine cartilage but also from bovine tissue. O(2) levels in initially oxygenated solutions gassed with N(2) fell to approximately 1% within 2 h. A progressive fall in pH(i) and acid extrusion capacity was observed, with statistically significant effects (P < 0.05) apparent within 3 h. For equine and bovine cell populations subjected to step change in O(2) by resuspension in hypoxic (1%) solutions, a decline in acid extrusion and pH(i) was observed within 10 min and continued throughout the recording period. This effect represented inhibition of the NHE-mediated fraction of acid extrusion. Cells subjected to hypoxic solutions supplemented with CoCl(2) (100 microM) or antimycin A (100 microM) to raise levels of ROS did not acidify. The conserved nature and rapidity of the response to hypoxia has considerable implications for chondrocyte homeostasis and potentially for the maintenance of cartilage integrity.

Histological evaluation of the possible role of Na+/ H+ entiporter and anion exchanger in endochondral ossification activities of secondary bone healing in rats

“…The joining between bone development and bone regeneration has been further strengthened through a topical understanding of the role of transport proteins in the chondrocyte plasma membrane, especially the NHE-1 and AE-2 in endochondral ossification of secondary bone healing. Inhibited NHE in isolated chondrocytes was similar to the specific NHE-1 inhibitor HOE694, supporting an indication that NHE-1 is the essential isoform in chondrocytes (11). The adding of IGF-1 to chondrocytes motivates Na + /H + exchanger-1 (12).…”

“…Pharmacological inhibition of NHE-1 immediate short interfering RNA (siRNA) inhibited hypertrophy and proliferation chondrocytes (15). The inhibitory result of 5-(N-ethyl-N-isopropyl) -Amiloride (EIPA) on growth of bone is due to a decrease in chondrocytes volume and this cause delay of the chondrocytes differentiation and proliferation (11). Diisothiocyanatostilbene-2,2'-disulfonic acid disodium salt hydrate (DIDS) and EIPA reduced the size of the hypertrophic chondrocytes zone in growth plate chondrocyte hypertrophy and bone lengthening in bone rudiments of Sprague-Dawley seven days old rats through inhibition AE-2 and NHE-1 respectively (6).…”

Transient hypoxia improves matrix properties in tissue engineered cartilage

“…2C). We hypothesize that under extended hypoxia, chondrocytes may preferentially maintain cellular metabolism and viability via increased glucose uptake, 49 maintenance of pH homeostasis, 50,51 and reduction of oxygen consumption rate 52 rather than undergoing the heavy metabolic demands of ECM synthesis. Mobasheri et al 3 presented the dual model of oxygen and glucose sensing, which proposed that transcription of hypoxia-responsive glucose transporter is mediated by HIF-1α during oxygen deprivation in combination with low intracellular ATP and low extracellular glucose.…”

Mechanisms by which TLR4 and endotoxin promote hepatocellular carcinoma require further investigation

“…There is no strong evidence for NHE2 in chondrocytes [Tattersall et al, 2003]. Furthermore, careful studies by Gibson et al [2009] showed that NHE activity of isolated chondrocytes was inhibited to the same extent by amiloride (1 mM) or the specific NHE1 inhibitor HOE694 (10 µM), supporting the notion that NHE1 is the principal isoform in chondrocytes. We therefore feel that it is unlikely that EIPA exerts unexpected effects on cells by acting on other NHE subtypes, and we are not aware of any inhibitory effects of EIPA on other chondrocyte membrane transporters other than NHE.…”