diacetylmonoxime and Acidosis

The hypothesis that rat cardiomyocytes become susceptible to hypercontracture after anoxia/reoxygenation was investigated. The cells were gradually overloaded with Ca2+ after different periods of simulated ischemia (substrate-free anoxia, medium at pH 6.4) followed by 20 minutes of reoxygenation. The cytosolic Ca2+ concentration (measured with fura 2) at which the cells developed maximal hypercontracture (Camax) was used as an index for their susceptibility to hypercontracture (SH). SH was increased in cardiomyocytes after prolonged periods of simulated ischemia; ie, these cells developed hypercontracture at significantly lower cytosolic Ca2+ levels than did normoxic cells (Camax, 0.80 +/- 0.05 mumol/L versus 1.27 +/- 0.05 mumol/L; P < .01). To find the possible cause of increased SH, the influence of Ca2+ overload, acidosis, and protein dephosphorylation were studied. Prevention of cytosolic Ca2+ overload in anoxic cardiomyocytes or imitation of ischemic acidosis in normoxic cells did not influence Camax. In contrast, use of 10 mumol/L cantharidin (inhibitor of protein phosphatases 1 and 2A) during anoxic superfusion prevented the reduction of Camax. Furthermore, treatment of normoxic cardiomyocytes with 20 mmol/L of the chemical phosphatase 2,3-butanedione monoxime reduced Camax. Therefore, prolonged simulated ischemia increases susceptibility of cardio-myocytes to hypercontracture. This seems to be due to protein dephosphorylation.

Topics: Acidosis; Animals; Calcium; Diacetyl; Energy Metabolism; Heart; Hypoxia; Male; Myocardial Contraction; Myocardial Ischemia; Myocardium; Phosphoprotein Phosphatases; Protein Phosphatase 2; Rats; Rats, Wistar; Time Factors

Effects of the Ca2+ sensitizer N-hydroxy-5,6-dimethoxy-benzo[b]thiophene-2-carboximidamide hydrochloride (Org-30029) on the myocardial contractile depression induced by acidosis and 2,3-butanedione monoxime (BDM) were investigated in aequorin-loaded canine ventricular myocardium. The peak Ca2+ transient-peak force relation during administration of Org-30029 (10(-4) to 10(-3) M) was shifted to the left and upward compared with the relation for elevation of the extracellular Ca2+ concentration ([Ca2+]o) (2.5-12.5 mM). Acidosis (pH 6.6) depressed the force with a small increase in the peak Ca2+ transient. BDM (3 mM) depressed the force with no change in the peak and duration of the Ca2+ transient, indicating that BDM may inhibit selectively the cross-bridge interaction. During acidosis or in the presence of BDM, elevation of [Ca2+]o increased the peak Ca2+ transient to the same extent as that in the control, but the force was inhibited. In contrast, Org-30029 increased the force to a level equivalent to the control with a slight change in the peak Ca2+ transient. In addition, during acidosis, Org-30029 (10(-3) M) increased the force in association with a slight decrease in the peak Ca2+ transient. Thus Org-30029 can reverse the myocardial contractile depression induced by a decrease in the Ca2+ sensitivity of myofilaments, as occurs in pathophysiological situations such as acidosis in cardiac ischemia. Org-30029 may exert the Ca(2+)-sensitizing effect by an increase in the affinity of troponin C for Ca2+ and by a direct action on the cross-bridge interaction.

Topics: Acidosis; Animals; Calcium; Cardiotonic Agents; Diacetyl; Dogs; Extracellular Space; Female; In Vitro Techniques; Male; Myocardial Contraction; Myocardium; Organic Chemicals; Osmolar Concentration