No Confirmation for a Causal Role of Volume-regulated Chloride Channels in Ischemic Preconditioning in Rabbits

Abstract

Volume-regulated chloride channels have recently been proposed to be end-effectors in ischemic preconditioning. The present study attempted to confirm this hypothesis by looking both at cardioprotection and channel activity. In isolated rabbit cardiomyocytes, hypo-osmotic stress (167 mosm/l) induced a current with a magnitude of 2–5 pA/pF at 60 mV. That current could be blocked by the selective chloride channel blockers 5-nitro–2–(3-phenylpropylamino) benzoic acid (NPPB) or indanyloxyacetic acid 94 (IAA-94), but only at 100 μM and 1 m M respectively. Lower concentrations were not effective. Because the channel-blocking concentrations were toxic in isolated perfused rabbit hearts, as evidenced by cessation of cardiac contraction and massive infarction, neither agent could be tested against preconditioning's anti-infarct effect. NPPB and IAA-94 at 1 μM and 10 μM, respectively (the doses used in a previous report), did not affect coronary flow, heart rate and developed pressure, and also did not prevent the infarct size reduction of ischemic preconditioning with 5 min global ischemia/10 min reperfusion preceding 30 min of regional ischemia and 120 min of reperfusion [11.4(±3.6) and (11.1(±3.7)% infarction of risk area, respectively]. The volume-regulated chloride and organic osmolyte channel blocker 4,4-diisothiocyanostilbene-2,2-disulfonic acid (DIDS) at 100 μM blocked the hypo-osmotically induced current in myocytes, but again could not be used, since it induced total cessation of cardiac contraction and reduced infarct size in non-preconditioned hearts. Our data do not confirm a prior study on a causal role for volume-regulated chloride channels in the protection of ischemic preconditioning. This hypothesis remains to be adequately tested.