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

Abstract

1.The whole-cell patch-clamp was used for studying the effects of various β1- and β2-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 β-agonist), salbutamol (β2-agonist), dobutamine (β1-agonist) on I(Ca), were obtained in the absence and presence of various concentrations of ICE 118551 (β2-antagonist), metoprolol (β1-antagonist) and xamoterol (partial β1-agonist) to derive EC50 (i.e. the concentration of β-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 μM 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 β-adrenoceptor-mediated regulation of I(Ca), in frog ventricular myocytes. The pharmacological pattern of the response of I(Ca), to the different β-adrenoceptor agonists and antagonists tested suggests that these receptors are of the β2-subtype.