Myocardial depression is a recognized com- plication encountered after recovery from hy- pothermic preservation of neonatal hearts. As many factors may be implicated in this adverse outcome (e.g., myocardial contracture, coronary vasospasm, intrinsic myocytic injury, and circu- lating factors such as vasopressin, leukocytes, interleukins, and platelet activating factors), an in-depth understanding of each pathological process involved is warranted to conceive a solid strategy directed toward each contributing element. Some groups advocate the adjunction of different substrates to the perfusate (10, 11) or the use of anticirculating agents (1, 14), some investigate ultra-profound hypothermia and acellular substitution (2, 18, 19), and others lean toward the use of warm cardioplegia as opposed to cold cardioplegia (3). To some extent, warm cardioplegia has the advantage of avoiding un- desired mechanisms related to cold exposure. However, hypothermia remains an effective means to achieve myocardial preservation. With this perspective, we have been exploring the physiological and molecular mechanisms in- volved in the response of vascular tissues to hypothermia (5, 21–24). Previous studies from this laboratory have implicated protein tyrosine kinase (PTK)-/protein tyrosine phosphatase (PTP)-dependent signaling in the newborn lamb cerebral arteries (22). We have also demon- strated a cold-induced contraction in the iso- lated coronary arteries (CA) of the newborn lamb (21) possibly associated with an abnormal response to anoxia upon rewarming. In the pre- sent experiment we investigated the hypothesis Phosphorylation in Coronary Artery Cold-Induced Contraction in the Newborn Lamb Nagib S. Dahdah,* Pierantonio Russo,† , ‡ and L. Craig Wagerle† , ‡ *Department of Pediatrics, Division of Pediatric Cardiology, MetroHealth Medical Center, Case Western Reserve University, Cleveland, Ohio 44109, U.S.A.; and †Department of Surgery and ‡Department of Pediatrics, Thomas Jefferson University, Philadelphia, Pennsylvania 19107, U.S.A. Myocardial dysfunction after hypothermic protection has been linked to various mechanisms. Coronary vasospasm in particular may be responsible for ischemic injury during reperfusion. Herein we hypothesized that coronary arteries (CA) sustain a cold-induced contraction during hypothermia mediated by a protein ty- rosine kinase (PTK)-/protein tyrosine phosphatase (PTP)-dependent pathway. Isolated newborn lamb CA rings were studied in a tissue bath for isometric contraction during 2-h profound (17°C) or ultra-profound (7°C) hypothermia. In parallel, protein tyrosine phosphorylation was evaluated by use of the Western blot technique. Na–orthovanadate (SOV) and genestein (GEN) were used separately and in combination to evalu- ate the effect of PTK/PTP activation on CA contraction and tyrosine phosphorylation during cooling (17 or 7°C) vs 37°C. Cooling from 37 to 7°C induced transient contraction at 17°C (29% KCl response), which was more prominent during rewarming to 37°C (36% KCl). Cooling to 17°C resulted in sustained contraction (7–10% KCl), which was reversible upon rewarming. Cold-induced contraction was significantly enhanced by SOV (7- to 10-fold at 17°C; 2-fold at 7°C) and abolished by GEN. Concurrently, tyrosine phosphorylation of 33-, 45-, and 104-kDa proteins increased during cooling (35–100% at 17°C; 46–66% at 7°C). Tyrosine phosphorylation was similarly enhanced by SOV (1.7- to 2.3-fold at 17°C; 2.9- to 3.9-fold at 7°C) and abol- ished by GEN in the presence or absence of SOV. These results support a prominent role for the PTK/PTP signal transduction pathway in the coronary artery cold-induced contraction. This information provides one possible biomolecular mechanism linked to ischemia/reperfusion pathophysiology of CA in neonatal hearts exposed to hypothermic myocardial protection. © 2001Academic Press Key Words: hypothermia; myocardial protection; protein tyrosine kinase/phosphatase; signal transduction. 40 Cryobiology 41, 40–48 (2001) doi:10.1006/cryo.2001.2299, available online at http://www.academicpress.com on 0011-2240/01 $35.00 Copyright © 2001 by Academic Press All rights of reproduction in any form reserved. Received August 30, 2000; accepted January 4, 2001. This study was supported in part by the Commonwealth of Pennsylavania (Grant 08031000) and the Nello’s Fund, Philadelphia, Pennsylvania.