Effect of Anoxia on Cyclic Nucleotides and Inositol Phosphate Turnover in Cardiac Myocytes

The loss of 5′-nucleotides (especially ATP and GTP) from cardiac muscle cells is a distinguishing feature of myocardial ischemia. Isolated adult rat cardiac myocytes were used as a model system to determine whether GTP depletion could affect (1) the ability of the myocytes to synthesize cyclic GMP (cGMP), or (2) the ability of the myocytes to respond to α-adrenergic challenge. Myocytes were made anoxic for 30- or 60-min periods, then challenged with either 1 m msodium nitroprusside (NaNP) for 1 min or 40 μ mnorepinephrine (NE) for 20 min. The cells were extracted and the extracts assayed for cyclic GMP (NaNP challenge) or phosphoinositides (NE challenge). When challenged with NaNP, anoxic myocytes made up to five-fold more cGMP than aerobic controls (1401±353 fmol cGMP/mg cell protein in anoxic cells v121±23 fmol/mg in aerobic controls). Phosphoinositide turnover was reduced in anoxic cells vaerobic controls. Stimulation of this pathway by NE was reduced two-fold after 30 min of anoxia, and abolished after 60 min of anoxia. Similar results were obtained with 30 μ mand 60 μ mphenylephrine. The authors concluded that nucleotide depletion under anoxic conditions has no effect on the production of cyclic GMP, but may interfere with the linkage of α-adrenergic receptors to phosphatidylinositol breakdown.