Long-chain acylcarnitines mediate the hypoxia-induced increase in alpha 1-adrenergic receptors on adult canine myocytes

To elucidate the mechanisms responsible for the increase in alpha 1-adrenergic receptors during ischemia in vivo, we developed a procedure for measuring alpha 1-adrenergic receptors in isolated, calcium-tolerant adult canine myocytes. Specific [3H]prazosin binding was rapid, saturable, reversible, and demonstrated the expected order of potency and stereospecificity for the alpha 1-adrenergic receptor. Myocytes exposed to 30 minutes of hypoxia at 25 degrees C or only 10 minutes at 37 degrees C exhibited a twofold to threefold increase in the number of alpha 1-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 degrees 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 microM), a potent inhibitor of carnitine acyltransferase I, not only abolished the accumulation of long-chain acylcarnitines but also the increase in alpha 1-adrenergic receptor number induced by 30 minutes of hypoxia. Likewise, incubation of normoxic cells with exogenous palmitoyl carnitine (1 microM) for 10 minutes also increased alpha 1-adrenergic receptor number in the presence or absence of POCA. Thus, hypoxia results in an increase in alpha 1-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 alpha 1-adrenergic receptor, indicating that accumulation of endogenous long-chain acylcarnitines is critical to the hypoxia-induced increase in alpha 1-adrenergic receptors on adult myocytes.