Long-Chain Acylcarnitines Mediate the Hypoxia-induced Increase in aj-Adrenergic Receptors on Adult Canine Myocytes

To elucidate the mechanisms responsible for the increase in a,-adrenergic receptors during ischemia in vivo, we developed a procedure for measuring a,-adrenergic receptors in isolated, calcium-tolerant adult canine myocytes. Specific pH]prazosin binding was rapid, saturable, reversible, and demonstrated the expected order of potency and stereospecificity for the a,-adrenergic receptor. Myocytes exposed to 30 minutes of hypoxia at 25° C or only 10 minutes at 37° C exhibited a twofold to threefold increase in the number of a,-adrenergic receptors with no significant change in receptor affinity. This hypoxia-induced increase in receptor number was reversible by 10 minutes of reoxygenation at 37° C. In contrast, more prolonged hypoxia of 80 minutes or hypotonic shock actually decreased receptor number below normoxic, control values. The concentration of long-chain acylcarnitines in myocytes also increased threefold on exposure to 30 minutes of hypoxia. Sodium 2-[5-(4-chlorophenyl)-pentyl]oxirane-2-carboxylate (POCA, 10 /AM), a potent inhibitor of carnitine acyltransferase I, not only abolished the accumulation of long-chain acylcarnitines but also the increase in a,-adrenergic receptor number induced by 30 minutes of hypoxia. Likewise, incubation of normoxic cells with exogenous palmltoyl carnitine (1 fiM) for 10 minutes also increased a,-adrenergic receptor number in the presence or absence of POCA. Thus, hypoxia results in an increase in a,-adrenergic receptors associated with an increase in endogenous long-chain acylcarnitines. Furthermore, inhibition of carnitine acyltransferase I prevents not only the sarcolemmal accumulation of long-chain acylcarnitines but also the exposure of the a,-adrenergic receptor, indicating that accumulation of endogenous long-chain acylcarnitines is critical to the hypoxia-induced increase in a,-adrenergic receptors on adult myocytes. {Circulation Research 1987;61:735-746)