EXPRESSION OF THE ALPHA-SUBUNITS AND BETA-SUBUNITS OF THE STIMULATORY AND INHIBITORY G-PROTEINS IN BETA(1)-ADRENOCEPTOR-BLOCKED AND NON-BETA-ADRENOCEPTOR-BLOCKED HUMAN ATRIUM

1. Chronic beta(1)-adrenoceptor blocker therapy induces hyperresponsiveness of the beta(2)-adrenoceptor in human atrium. To investigate whether the beta(2)-adrenoceptor sensitization induced by beta(1)-adrenoceptor blockade is associated with altered gene expression of G-proteins, which couple the receptors to adenylate cyclase, we determined the mRNA expression of the alpha- and beta-subunits of the stimulatory G-protein, G(s), and inhibitory G-protein, G(i), in human right atrial appendage by polymerase chain reaction and by enhanced chemiluminescence Northern blot analysis. 2. The polymerase chain reaction revealed bands of predicted size of G(s) alpha, both short form and long form, all three G(i) alpha subtypes and three G beta subtypes. In Northern blots, the digoxigenin-labelled antisense cRNA probe specific for G(i) alpha 2 hybridized to a predominant band at 2.3 kb, whereas the G(i) alpha 3 cRNA probe detected a message of 1.8 kb in total RNA extracted from human atrium. The cRNA probe encoding G(s) alpha revealed one major band at 1.9 kb and one minor band at 1.7 kb. The G beta cRNA probes detected messages of 3.4 kb for G beta 1, 1.8 kb for G beta 2 and 1.9 kb for G beta 3 in human atrium. 3. The mRNA levels of G(s) alpha in B beta(1)-adrenoceptor-blocked atria (n=12) were not significantly different from those in non-beta-adrenoceptor-blocked atria (n=12), nor were there any significant differences in the G(i) alpha 2 mRNA levels between atria from patients treated with beta(1)-adrenoceptor blockers and untreated patients. The ratios of 1.9-kb G(s) alpha mRNA to 1.7-kb G(s) alpha mRNA and of 1.9-kb G(s) alpha mRNA to 2.3-kb G(i) alpha 2 mRNA in beta(1)-adrenoceptor-blocked patients were almost identical to those in non-beta-adrenoceptor-blocked patients. Neither G beta 1 mRNA nor G beta 2 mRNA expression in beta(1)-adrenoceptor-blocked atria differed significantly from that in non-beta-adrenoceptor-blocked atria. 4. We conclude that the previously observed sensitization following beta(1)-adrenoceptor-blockade of beta(2)-adrenoceptors in human atria is unlikely to be mediated by altered gene expression of the alpha- and beta-subunits of G-proteins.