Is the intracellular pH threshold an anaerobic threshold from the view point of intracellular events?: a brief review

Fluoxetine induces alkalinization of astroglial cytosol through stimulation of sodium-hydrogen exchanger 1: dissection of intracellular signaling pathways

Clinical evidence suggest astrocytic abnormality in major depression (MD) while treatment with anti-psychotic drugs affects astroglial functions. Astroglial cells are involved in pH homeostasis of the brain by transporting protons (through sodium-proton transporter 1, NHE1, glutamate transporters EAAT1/2 and proton-lactate co-transporter MCT1) and bicarbonate (through the sodium-bicarbonate co-transporter NBC or the chloride-bicarbonate exchanger AE). Here we show that chronic treatment with fluoxetine increases astroglial pH_i by stimulating NHE1-mediated proton extrusion. At a clinically relevant concentration of 1 μM, fluoxetine significantly increased astroglial pH_i from 7.05 to 7.34 after 3 weeks and from 7.18 to 7.58 after 4 weeks of drug treatment. Stimulation of NHE1 is a result of transporter phosphorylation mediated by several intracellular signaling cascades that include MAPK/ERK_1/2, PI3K/AKT and ribosomal S6 kinase (RSK). Fluoxetine stimulated phosphorylation of ERK_1/2, AKT and RSK in a concentration dependent manner. Positive crosstalk exists between two signal pathways, MAPK/ERK_1/2 and PI3K/AKT activated by fluoxetine since ERK_1/2 phosphrylation could be abolished by inhibitors of PI3K, LY294002 and AKT, triciribine, and AKT phosphorylation by inhibitor of MAPK, U0126. As a result, RSK phosphorylation was not only inhibited by U0126 but also by inhibitor of LY294002. The NHE1 phoshorylation resulted in stimulation of NHE1 activity as revealed by the NH₄Cl-prepulse technique; the increase of NHE1 activity was dependent on fluoxetine concentration, and could be inhibited by both U0126 and LY294002. Our findings suggest that regulation of astrocytic pH_i and brain pH may be one of the mechanisms underlying fluoxetine action.

Metabolic determinants of the onset of acidosis in exercising human muscle: a 31P-MRS study

Onset of intracellular acidosis during muscular exercise has been generally attributed to activation or hyperactivation of nonoxidative ATP production but has not been analyzed quantitatively in terms of H(+) balance, i.e., production and removal mechanisms. To address this issue, we have analyzed the relation of intracellular acidosis to H(+) balance during exercise bouts in seven healthy subjects. Each subject performed a 6-min ramp rhythmic exercise (finger flexions) at low frequency (LF, 0.47 Hz), leading to slight acidosis, and at high frequency (HF, 0.85 Hz), inducing a larger acidosis. Metabolic changes were recorded using (31)P-magnetic resonance spectroscopy. Onset of intracellular acidosis was statistically identified after 3 and 4 min of exercise for HF and LF protocols, respectively. A detailed investigation of H(+) balance indicated that, for both protocols, nonoxidative ATP production preceded a change in pH. For HF and LF protocols, H(+) consumption through the creatine kinase equilibrium was constant in the face of increasing H(+) generation and efflux. For both protocols, changes in pH were not recorded as long as sources and sinks for H(+) approximately balanced. In contrast, a significant acidosis occurred after 4 min of LF exercise and 3 min of HF exercise, whereas the rise in H(+) generation exceeded the rise in H(+) efflux at a nearly constant H(+) uptake associated with phosphocreatine breakdown. We have clearly demonstrated that intracellular acidosis in exercising muscle does not occur exclusively as a result of nonoxidative ATP production but, rather, reflects changes in overall H(+) balance.

Effects of ovariectomy on intramuscular energy metabolism in young rats: how does sports-related-amenorrhea affect muscles of young female athletes?

The purpose of this study was to evaluate the effects of ovariectomy on intramuscular energy metabolism in young rats. Twenty-four Sprague-Dawley rats (7 weeks old) were used. Twelve of them underwent ovariectomy (OVX), and the others were sham-operated on. Seven OVX rats were examined 1-week after surgery (OVX-1 group), and the other five, 4 weeks after surgery (OVX-4 group). The gastrocnemius-plantaris-soleus (GPS) muscles group was subjected to the following measurements, and the data were compared with those of the sham group (Sham-1: n = 7, or Sham-4 group: n = 5). From the 31P-MR spectra of the GPS muscles group at rest and during electric stimulation, the muscular oxidative capacity was measured. Maximum tension and wet weight of the whole GPS muscles group were also measured. Body weight in the OVX-4 group was significantly (p < 0.01) larger than that in the Sham-4 group. The weights of the whole GPS muscles group in the Sham-1, Sham-4, OVX-1 and OVX-4 groups were 1.17, 1.51, 1.25 and 1.71 (g), respectively. The muscle weight in the OVX group tended to be greater than that in the Sham group (p < 0.10). The maximum tension and oxidative capacity did not differ significantly among the groups. These data indicated that in young rats, ovariectomy induced an increase in body and muscle weight, but did not affect the maximum tension nor oxidative capacity.