ACTIVATION OF CALCIUM CURRENTS IN CARDIAC MYOCYTES BY EMPTY BETA-ADRENOCEPTORS

Background. The cardiac calcium channel is known to be modulated by ca techolamines via beta-adrenoceptors acting through intermediary GTP-bi nding regulatory proteins (G proteins). In biochemical studies on isol ated membranes and reconstituted systems, it has been demonstrated tha t various G protein-coupled receptors, including beta-adrenoceptors, c an activate G proteins and also intracellular second messengers like c yclic AMP (cAMP) even in the absence of an agonist and that antagonist s can block this empty receptor action. We examined electrophysiologic ally whether agonist-free beta-adrenoceptors can modulate C-type calci um currents (I-Ca) in intact cardiac myocytes. Methods and Results. Ca rdiomyocytes were isolated from ventricles of guinea pig and human hea rts and from human right atrial appendage. The patch-clamp technique w as applied in the single electrode mode to measure whole-cell I-Ca. Mo dulation of calcium currents by beta-adrenoceptor antagonists, without addition of an agonist, was studied in the absence and presence of th e direct adenylyl cyclase activator forskolin and the cAMP analog aden osine cyclic 3',5'-monophosphorothioate (Sp-cAMPS). In the presence of forskolin (0.5 mu mol/L), an agent known to sensitize the adenylyl cy clase signal transduction system for receptor regulation, addition of the beta(1)-selective antagonist atenolol and the nonselective antagon ist propranolol (but not of the beta(2)-selective antagonist ICI 118,5 51) caused a marked reduction of I-Ca in a concentration-dependent and stereoselective manner. The inhibitory effect of atenolol was reversi ble after washing out and was found to be half maximal and maximal (50 % reduction) at about 50 and 300 nmol/L, respectively. In the absence of forskolin, inhibition of I-Ca by atenolol was markedly less (18% at 10 mu mol/L atenolol). Finally, in contrast to forskolin-stimulated c urrents, atenolol (1 mu mol/L) did not reduce calcium currents activat ed by the protein kinase A activator Sp-cAMPS (0.1 mmol/L), causing by itself a similar increase in calcium currents as forskolin. Conclusio ns. In isolated guinea pig and human cardiomyocytes, agonist-free beta -adrenoceptors are functionally active and can stimulate L-type calciu m currents, an effect blocked by receptor-specific antagonists.