INS(1,4,5)P-3 DURING MYOCARDIAL-ISCHEMIA AND ITS RELATIONSHIP TO THE DEVELOPMENT OF ARRHYTHMIAS

Reperfusion of globally ischemic rat hearts in vitro causes release of inositol(1,4,5)trisphosphate (Ins(1,4,5)P-3) which is associated with the development of reperfusion arrhythmias. Both of these responses r equire the presence of a receptor agonist, either norepinephrine or th rombin, and both responses are inhibited by the aminoglycoside, gentam icin and the polyamine, spermine. In the current study, the role of In s(1,4,5)P, in the development of arrhythmias under ischemic conditions was addressed. Arrhythmias [ventricular premature beats, ventricular tachycardia and ventricular fibrillation (VF)] occurring over 25 min s ubsequent to coronary artery ligation were shown to be independent of endogenous norepinephrine or adrenergic receptor stimulation but were effectively inhibited by gentamicin (0.15-1.5 mM, 95% VF in controls c ompared with 0% VF, at 1.5 mM, P < 0.01) and spermine (5 mM, 40% VF, P < 0.01). Depletion of Ca2+ stores, including Ins(1,4,5)P-3-sensitive Ca2+ stores, with thapsigargin (300 nM) reduced the incidence of ische mic arrhythmias (40% VF, P < 0.01). [H-3]-Inositol-labeled right atria incubated under conditions of simulated ischemia retained the ability to respond to norepinephrine by releasing inositol phosphates. Under ischemic conditions, gentamicin (1.5 mM) caused a reduction in [H-3]In s(1,4,5)P-3 without any effect on the other inositol phosphates, Simil ar effects of gentamicin were observed under ischemic conditions in th e absence of norepinephrine (95 +/- 8 cpm/mg, mean +/- S.E.M., n = 4, v 29 +/- 4. P < 0.01 for 1.5 mM gentamicin). Agonist independent relea se of [H-3]Ins(1,4,5)P-3 under ischemic conditions required extracellu lar Ca2+ suggesting the operation of a Ca2+-activated phospholipase C. In agreement with this, release of [H-3]Ins(1,4,5)P-3 could be initia ted by Ca2+ overload under normoxic conditions and this was inhibited by gentamicin. These findings show that Ca2+ overload can enhance rele ase of Ins(1,4,5)P-3 under ischemic conditions and provide evidence th at this release is involved in the genesis of arrhythmias under these conditions. (C) 1996 Academic Press Limited