REGULATION OF BETA-ADRENERGIC RECEPTORS IN ACUTE MYOCARDIAL-ISCHEMIA - SUBTYPE-SELECTIVE INCREASE OF MESSENGER-RNA SPECIFIC FOR BETA(1)-ADRENERGIC RECEPTORS

Acute myocardial ischemia leads to a rapid increase of cardiac beta-ad renergic receptors in plasma membranes despite the release of large an d desensitizing amounts of endogenous catecholamines, Part of this inc rease has been shown to occur at the expense of intracellular receptor s. To investigate whether an additional expressional regulation of bet a-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 b eta(1)- and beta(2)-adrenergic receptors was determined using reverse- transcription followed by PCR (RT-PCR) and RNA protection assays. RT-P CR resulted in single amplification products of the expected sizes (15 9 bp for beta(1)-adrenergic receptors and 240 bp for beta(2)-adrenergi c receptors). The specificity of these amplification products was conf irmed by specific restriction digests, Southern blot hybridizations wi th internal oligonucleotides and sequencing using the dideoxy chain te rmination method. For quantification purposes, the mRNAs of housekeepi ng gene GAPDH and of cardiac alpha-actin were determined as internal s tandards. Additionally, cRNAs specific for beta(1)- and beta(2)-adrene rgic receptors were used as external standards. Brief periods of globa l ischemia induced a rapid increase in the steady state level of mRNA for beta(1)-adrenergic receptors, There was a statistically significan t 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 ischem ia, the mRNA levels specific for beta(1)-adrenergic receptors tended t o decrease, but remained significantly above normoxic controls. In con trast, the mRNA levels specific for beta(2)-adrenergic receptors remai ned constant up to 60 min of global myocardial ischemia, To investigat e, whether agonist occupancy of the receptors may contribute to this r egulation, the effect of preperfusion with the beta-blocker alprenolol was determined. Contrary to expectation, beta-blockade did not influe nce 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 regul ation of mRNA levels for beta(1)-adrenergic receptors. However, occupa tion or activation of beta-adrenergic receptors by an agonist is not i nvolved in this newly characterized regulation of mRNA for beta(1)-adr energic receptors in acute myocardial ischemia.