The role of HCO3(-)-dependent mechanisms in pHi regulation during O2 deprivation

We have reported in our previous work that, in the absence of HCO(3)(-), Na(+)/H(+) exchanger is responsible for an anoxia-induced alkalinization in hippocampal CA1 neurons. HCO(3)(-)-dependent mechanisms have been reported to play a key role in pH(i) regulation in nerve cells, but how their function is affected by O(2) deprivation has not been well studied. In this work, pH(i) measurements (obtained from dissociated neurons loaded with carboxy-seminaphthorhodafluor-1 and using confocal microscopy) and whole-cell patch clamp recording techniques were used to investigate the role of HCO(3)(-)-dependent membrane exchangers on CA1 neurons during O(2) deprivation. Anoxia (5 min) induced a small acidification in neurons in the presence of HCO(3)(-) and this acidification was changed to a significant alkalinization when neurons were bathed with Hepes buffer or when 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid was applied in a HCO(3)(-) solution, indicating that HCO(3)(-)-dependent mechanisms were involved. A marked anoxia-induced acidification (0.33+/-0.11 pH unit) was seen when the Na(+)/H(+) exchange was blocked with 3-(methylsulfonyl-4-piperidino-benzoyl)-guanidine methanesulfonate in the presence of HCO(3)(-), but the same anoxia did not cause a significant pH(i) change in a Na(+) free, HCO(3)(-) solution, suggesting that the anoxia-induced acidification in the presence of 3-(methylsulfonyl-4-piperidino-benzoyl)-guanidine methanesulfonate is dependent on both Na(+) and HCO(3)(-). Furthermore, anoxia did not cause a significant pH(i) change when both 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid and 3-(methylsulfonyl-4-piperidino-benzoyl)-guanidine methanesulfonate were present. Current clamp recordings showed a significant membrane depolarization following anoxia in HCO(3)(-) solution but not in Hepes buffer. Our data suggest that, in hippocampal neurons: a) pH(i) regulation during O(2) deprivation is affected not only by metabolism but also by membrane exchangers, and b) besides the activation of Na(+)/H(+) exchange, anoxia activates a 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid-sensitive, Na(+)-dependent acid loader (possibly electrogenic).

Effects and interaction, of cariporide and preconditioning on cardiac arrhythmias and infarction in rat in vivo

1. Although Na+-H+ exchange (NHE) inhibitors are reported to protect the myocardium against ischaemic injury, NHE activation has also been proposed as a potential mechanism of ischaemic preconditioning-induced protection. This study was performed to test any modifiable effect of cariporide, an NHE inhibitor, on cardioprotective effects of preconditioning. 2. Anaesthetized rats were subjected to 30 min of coronary artery occlusion and 150 min of reperfusion. The preconditioning (PC) was induced by 3 min of ischaemia and 10 min of reperfusion (1PC) or three episodes of 3 min ischaemia and 5 min reperfusion (3PC). Cariporide (0.3 mg kg(-1)) an NHE inhibitor, was administered 30 min (cari(30)) or 45 min (cari(45)) before coronary ligation (n=8-11 for each group). 3. Ventricular arrhythmias during 30 min ischaemia and infarct size (measured by triphenyltetrazolium (TTC) and expressed as a per cent area at risk (%AAR)) were determined. Cari(30) reduced ventricular fibrillation (VF) incidence and infarct size (from 45 to 0% and 34+/-4 to 9+/-2%; each P<0.05), whereas cari(45) did not. Likewise, 3PC reduced these variables (to 0% and 10+/-2%; P<0.05 in each case) whereas 1PC did not. Moreover, subthreshold preconditioning (1PC) and cariporide (cari(45)), when combined, reduced VF incidence and infarct size (to 0% and 15+3%; each P<0.05 ). 4. In conclusion, changes in NHE activity do not seem to be responsible for the cardioprotective action of ischaemic preconditioning. Protective effects of NHE inhibition and subthreshold preconditioning appear to act additively.