Beta-adrenoreceptors in the trout (Oncorhynchus mykiss) heart: characterization, quantification, and effects of repeated catecholamine exposure.
Specific binding of the hydrophilic radioligand [3H]CGP-12177 to cell surface (functional) beta-adrenoreceptors was quantified in ventricular micropunches (2 mm diameter, 350 microns thickness) from seawater-acclimated rainbow trout held at 7-9 degrees. Binding was stereospecific, saturable, of high affinity, and displaceable by appropriate agonists and antagonists. Phentolamine failed to displace [3H]CGP at concentrations up to 10(-4) M, indicating an absence of [3H]CGP binding to alpha-adrenergic receptors. Trout ventricular beta-adrenoreceptors are exclusively of the beta 2 type. This conclusion is based on (1) the IC50 value for the beta 2-antagonist ICI 118551 (2.9 x 10(-6) M); (2) the inability of the beta 1-antagonist atenolol to displace [3H]CGP from beta-adrenoreceptors; and (3) the order of agonist-binding affinity (isoproterenol > epinephrine >> norepinephrine). The Bmax and Kd values for [3H]CGP binding to myocardial tissue were approximately 0.04 fmol micrograms protein-1 and 0.25 nM, respectively. The Bmax value indicates that the density of cell surface (functional) beta-adreno-receptors in the ventricle was 12,000 sites per cell or 3.38 sites per microns 2 of sarcolemma. The Kd and Bmax values for [3H]CGP binding to ventricular beta-adrenoreceptors were unaffected by the in vivo administration of five bolus catecholamine injections (4.0 micrograms kg-1 epinephrine, 2.0 micrograms kg-1 norepinephrine). This suggests that stress-induced increases in plasma catecholamines are unlikely to cause the down-regulation of heart beta-adrenoreceptors in fish. The method described here represents a simple but powerful technique for the quantification and characterization of adrenergic receptors in the fish heart.
Department of Biology, Dalhousie University, Halifax, Nova Scotia, Canada.
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Gamperl
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Wilkinson
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Boutilier
D000319:Adrenergic beta-Antagonists / Q000493:pharmacokinetics; D000818:Animals; D001667:Binding, Competitive / Q000187:drug effects; D002395:Catecholamines / Q000494:pharmacology*; D015536:Down-Regulation / Q000187:drug effects; D005615:Freezing; D006321:Heart / Q000187:drug effects; D006352:Heart Ventricles / Q000187:drug effects / Q000378:metabolism; D009206:Myocardium / Q000378:metabolism*; D017686:Oncorhynchus mykiss / Q000378:metabolism*; D011412:Propanolamines / Q000493:pharmacokinetics; D011943:Receptors, Adrenergic, beta / Q000032:analysis / Q000187:drug effects / Q000378:metabolism*; D013237:Stereoisomerism; D013315:Stress, Psychological / Q000378:metabolism; D013999:Timolol / Q000493:pharmacokinetics
D000319:Adrenergic beta-Antagonists; D002395:Catecholamines; D011412:Propanolamines; D011943:Receptors, Adrenergic, beta; D013999:Timolol; C033394:CGP 12177