Pharmacological characterization of the receptors involved in the b-adrenoceptor-mediated stimulation of the L-type Ca 2+ current in frog ventricular myocytes
Abstract
The whole-cell patch-clamp was used for studying the eects of various b 1-and b 2-adrenoceptor agonists and antagonists on the L-type Ca current (I Ca) in frog ventricular myocytes. 2 Dose-response curves for the eects of isoprenaline (non selective b-agonist), salbutamol (b 2-agonist), dobutamine (b 1-agonist) on I Ca were obtained in the absence and presence of various concentrations of ICI 118551 (b 2-antagonist), metoprolol (b 1-antagonist) and xamoterol (partial b 1-agonist) to derive EC 50 (i.e. the concentration of b-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 EC 50 of 20.0 nM and an E max of 597%. ICI 118551 and metoprolol competitively antagonized the eect of isoprenaline with a K B of 3.80 nM and 207 nM, respectively. 4 Salbutamol increased I Ca with an EC 50 of 290 nM and an E max of 512%. ICI 118551 and metoprolol competitively antagonized the eect of salbutamol with a K B of 1.77 nM and 456 nM, respectively. 5 Dobutamine increased I Ca with an EC 50 of 2.40 mM and an E max of 265%. ICI 118551 and metoprolol competitively antagonized the eect of dobutamine with a K B of 2.84 nM and 609 nM, respectively. 6 Xamoterol had no stimulating eect on I Ca. However, xamoterol competitively antagonized the stimulating eects 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 b-adrenoceptor-mediated regulation of I Ca in frog ventricular myocytes. The pharmacological pattern of the response of I Ca to the dierent b-adrenoceptor agonists and antagonists tested suggests that these receptors are of the b 2subtype.
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