Pharmacological characterization of the receptors involved in the beta-adrenoceptor-mediated stimulation of the L-type Ca2+ current in frog ventricular myocytes

1 The whole-cell patch-clamp was used for studying the effects of various beta(1)- and beta(2)-adrenoceptor agonists and antagonists on the L-type Ca current (I-Ca) in frog ventricular myocytes. 2 Dose-response curves for the effects of isoprenaline (non selective beta-agonist), salbutamol (beta(2)-agonist), dobutamine (beta(1)-agonist) on I-Ca were obtained in the absence and presence of various concentrations of ICT 118551 (beta(2)-antagonist), metoprolol (beta(1)-antagonist) and xamoterol (partial beta 1-agonist) to derive EC50 (i.e. the concentration of beta-agonist at which the response was 50% of the maximum) and E-max (the maximal response) values by use of a Michaelis equation. Schild regression analysis was performed to examine whether the antagonists were competitive and to determine the equilibrium dissociation constant (K-B) for the antagonist-receptor complex. 3 Isoprenaline increased I-Ca with an EC50 of 20.0 nM and an E-max of 597%. ICI 118551 and metoprolol competitively antagonized the effect of isoprenaline with a K-B of 3.80 nM and 207 nM, respectively. 4 Salbutamol increased I-Ca with an EC50 of 290 nM and an E-max of 512%. ICI 118551 and metoprolol competitively antagonized the effect of salbutamol with a K-B of 1.77 nM and 456 nM, respectively. 5 Dobutamine increased I-Ca with an EC50 of 2.40 mu M and an E-max of 265%. ICI 118551 and metoprolol competitively antagonized the effect of dobutamine with a K-B of 2.84 nM and 609 nM, respectively. 6 Xamoterol had no stimulating effect on I-Ca. However, xamoterol competitively antagonized the stimulating effects of isoprenaline, salbutamol and dobutamine on I-Ca with a K-B of 58-64 nM. 7 We conclude that a single population of receptors is involved in the beta-adrenoceptor-mediated regulation of I-Ca in frog ventricular myocytes. The pharmacological pattern of the response of I-Ca to the different beta-adrenoceptor agonists and antagonists tested suggests that these receptors are of the beta(2)-subtype.