Bifunctional role of protein tyrosine kinases in late preconditioning against myocardial stunning in conscious rabbits

Although protein tyrosine kinases (PTKs) have been implicated in late preco nditioning (PC) against infarction, their role in late PC against stunning is unknown. Furthermore, it is unknown whether PTK signaling is necessary o nly to trigger late PC on day 1 or also to mediate it on day 2. Thus, consc ious rabbits underwent a sequence of six if-minute coronary occlusion/4-min ute reperfusion cycles for 3 consecutive days (days 1, 2, and 3), In the co ntrol group (group I, n=7), the recovery of systolic wall thickening after the 6 occlusion/reperfusion cycles was markedly improved on days 2 and 3 co mpared with day 1, indicating the development of late PC against stunning. Administration of the PTK inhibitor lavendustin-A (LD-A, 1 mg/kg IV) before the first occlusion on day 1 (group II, n=7) completely prevented the late PC effect against stunning on day 2. Late PC against stunning was also abr ogated when LD-A was given before the first occlusion on day 2 (group III, n=7); however, in these rabbits, the late PC effect became apparent on day 3, indicating that LD-A itself did not have any delayed deleterious actions on myocardial stunning, In group V (n=5), the sequence of 6 occlusion/repe rfusion cycles resulted in a robust increase in the activity of inducible N O synthase (iNOS [assessed as Ca2+-independent L-citrulline formation]) and nitrite+nitrate (NOx) tissue levels 24 hours later (on day 2), with no con comitant change in Ca2+-dependent NO synthase (endothelial NO synthase and/ or neuronal NO synthase) activity. Similar results were obtained on day 3 ( group VIII, n=6), indicating sustained upregulation of iNOS. Administration of LD-A either on day 1 (group VI, n=5) or on day 2 (group VIII n=6) abrog ated the increase in iNOS activity and NO, levels on day 2. LD-A had no eff ect on iNOS activity or NO, levels in the absence of PC (group X, n=5). Thi s study demonstrates that in conscious rabbits, PTK activity is necessary n ot only to trigger late PC against stunning on day 1 but also to mediate th e protection on day 2. This investigation also provides the first direct ev idence that cardiac iNOS activity is upregulated during the late phase of i schemic PC in rabbits. Furthermore, the data indicate that PTK signaling is essential for the augmentation of iNOS activity and that PTKs modulate thi s enzyme at two distinct levels: at an early stage on day 1 and at a late s tage on day 2. This bifunctional role of PTKs in late PC has broad implicat ions for the signaling mechanisms that underlie the response of the heart t o ischemic stress and, possibly, other stresses.