Regulation of beta-adrenergic receptors in acute myocardial ischemia: subtype-selective increase of mRNA specific for beta 1-adrenergic receptors.

Acute myocardial ischemia leads to a rapid increase of cardiac beta-adrenergic receptors in plasma membranes despite the release of large and desensitizing amounts of endogenous catecholamines. Part of this increase has been shown to occur at the expense of intracellular receptors. To investigate whether an additional expressional regulation of beta-adrenergic receptors due to an increase of mRNA levels is involved, the mRNA levels specific for beta 1- and beta 2-adrenergic receptors were determined after various periods of global ischemia in isolated perfused rat hearts. The subtype-specific quantification of mRNA for beta 1- and beta 2-adrenergic receptors was determined using reverse-transcription followed by PCR (RT-PCR) and RNA protection assays. RT-PCR resulted in single amplification products of the expected sizes (159 bp for beta 1-adrenergic receptors and 240 bp for beta 2-adrenergic receptors). The specificity of these amplification products was confirmed by specific restriction digests. Southern blot hybridizations with internal oligonucleotides and sequencing using the dideoxy chain termination method. For quantification purposes, the mRNAs of housekeeping gene GAPDH and of cardiac alpha-actin were determined as internal standards. Additionally, cRNAs specific for beta 1- and beta 2-adrenergic receptors were used as external standards. Brief periods of global ischemia induced a rapid increase in the steady state level of mRNA for beta 1-adrenergic receptors. There was a statistically significant rise already after 15 min by 57% compared to controls. After 30 min of ischemia the mRNA levels had almost doubled. After 60 min of ischemia, the mRNA levels specific for beta 1-adrenergic receptors tended to decrease, but remained significantly above normoxic controls. In contrast, the mRNA levels specific for beta 2-adrenergic receptors remained constant up to 60 min of global myocardial ischemia. To investigate, whether agonist occupancy of the receptors may contribute to this regulation, the effect of preperfusion with the beta-blocker alprenolol was determined. Contrary to expectation, beta-blockade did not influence the ischemia-induced increase of mRNA levels specific for beta 1-adrenergic receptors. These data demonstrate for the first time, that acute myocardial ischemia induces a rapid, and subtype-selective regulation of mRNA levels for beta 1-adrenergic receptors. However, occupation or activation of beta-adrenergic receptors by an agonist is not involved in this newly characterized regulation of mRNA for beta 1-adrenergic receptors in acute myocardial ischemia.