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This study examined the impact of age on contractile function, [Ca.sup.2+] homeostasis, and cell viability in isolated myocytes exposed to simulated ischemia and reperfusion. Ventricular myocytes were isolated from anesthetized young adult (3 too) and aged (24 too) male Fischer 344 rats. Cells were fieldstimulated at 4 Hz (37[degrees]C), exposed to simulated ischemia, and reperfused with Tyrode solution. Cell shortening and intracellular [Ca.sup.2+] were measured simultaneously with an edge detector and fura-2. Cell viability was assessed by Trypan blue exclusion. Ischemia (20-45 min) depressed amplitudes of contraction equally in isolated myocytes from young adult and aged animals. The degree of postischemic contractile depression (stunning) was comparable in both groups. [Ca.sup.2+] transient amplitudes were depressed in early reperfusion in young adult and aged cells and then recovered to preischemic levels in both groups. Cell viability also declined equally in reperfusion in both groups. In short, some cellular responses to simulated ischemia and reperfusion were similar in both groups. Even so, aged myocytes exhibited a much greater and more prolonged accumulation of diastolic [Ca.sup.2+] in ischemia and in early reperfusion compared with myocytes from younger animals. In addition, the degree of mechanical alternans in ischemia increased significantly with age. The observation that there is an age-related increase in accumulation of diastolic [Ca.sup.2+] in ischemia and early reperfusion may account for the increased sensitivity to ischemia and reperfusion injury in the aging heart. The occurrence of mechanical alternans in ischemia may contribute to contractile dysfunction in ischemia in the aging heart. calcium transients; cell shortening; senescence

Endothelin (ET)-I regulates the contractility and growth of the heart by binding G protein-coupled receptors of the ET type A receptor (E[T.sub.A])/ET type B (E[T.sub.B]) receptor family. E[T.sub.A], the predominant ET-1 receptor subtype in myocardium, is thought to localize preferentially within cardiac T tubules, but the consequences of mislocalization are not fully understood. Here we examined the effects of the overexpression of ETA in conjunction with T-tubule loss in cultured adult rat ventricular myocytes. In adult myocytes cultured for 3 to 4 days, the normally robust positive inotropic effect (PIE) of ET-1 was lost in parallel with T-tubule degeneration and a decline in ETA protein levels. In these T tubule-compromised myocytes, an overexpression of ETA using an adenoviral vector did not rescue the responsiveness q to ET-1, despite the robust expression in the surface sarcolemma. The 1 inclusion of the actin polymerization inhibitor cytochalasin D (CD) j during culture prevented gross morphological changes including a 1 loss of T tubules and a rounding of intercalated discs, but CD alone did not rescue the responsiveness to ET-1 or prevent ETA down regulation. The rescue of a normal PIE in 3- to 4-day cultured myocytes T required both an increased expression of ETA and intact T tubules ' (preserved with CD). Therefore, the activation of ETA localized in T tubules was associated with a strong PIE, whereas the activation of q ETA in surface sarcolemma was not. The results provide insight into i the pathological cardiac conditions in which ETA is upregulated and 2 T-tubule morphology is altered. cytochalasin D; inotropism; endothelin type A receptor; heart failure 1