Binding of agonists and antagonists to the porcine adipose tissue β-adrenergic receptor(s)

1998

400

311

The beta-adrenergic receptors (beta-AR) are present on the surface of almost every type of mammalian cell. These receptors are stimulated physiologically by the neurotransmitter, norepinephrine and the adrenal medullary hormone, epinephrine. There are three subtypes of beta-AR, namely, beta1-AR, beta2-AR, and beta3-AR; the pharmacological and physiological responses of an individual cell result from the particular mixture of the three beta-AR subtypes present on that cell. Species-specific structure (amino acid sequence) also causes modification of the function of a given beta-AR subtype. Knowledge of the beta-AR subtypes present in various cell types, coupled with knowledge of receptor structure (sequence), will allow an understanding of the complexity of physiological function regulated by beta-AR. Oral administration of some beta-AR agonists increases muscle and decreases fat accretion in cattle, pigs, poultry, and sheep. The large number of physiological functions controlled by beta-AR suggests that the mechanism(s) for the observed changes in carcass composition may be extremely complex. Any proposed mechanism must begin with the possibility of direct effects of the agonist on skeletal muscle and adipocyte beta-AR. However, many other mechanisms, such as modification of blood flow, release of hormones, or central nervous system control of feed intake may contribute to the overall effects observed with a given beta-AR agonist in a given species. Furthermore, the pharmacodynamic properties of a particular agonist are complex and expected to vary among species as well as within the same species at different ages or when fed different diets.

Section: Modulation Of Subtypesmentioning

confidence: 99%

Overview of the effects of beta-adrenergic receptor agonists on animal growth including mechanisms of action.

1998

400

311

“…The purported b1AR and b2AR antagonist and partial b3AR agonist, CGP 12,177, stimulates lipolysis in white fat cells from rats, hamsters, dogs, and rabbits; however, it is a poor lipolytic agonist for human and guinea pig white adipose cells (Langin et al, 1991;Lafontan and Berlan, 1993). Porcine adipocyte membranes bind CGP 12,177; this b1AR and b2AR antagonist inhibits isoproterenol-stimulated porcine adipocyte lipolysis (Mersmann et al, 1993). In a study of porcine adipose slices, CGP 12,177 acts as a partial agonist at concentrations greater than 500 nM in the presence of theophylline; its efficacy was only 35% of the efficacy of isoproterenol plus theophylline (Mersmann, 1996).…”

Section: Quantitative Measurements Of βAr Subtype Transcriptsmentioning

confidence: 99%

“…Multiple bAR binding sites are present on porcine adipocytes, but identity of the subtypes is unclear (Coutinho et al, 1992;Mersmann et al, 1993;Spurlock et al, 1993). We have used the physiological measurement of stimulation or inhibition of lipolytic activity (Mersmann, 1984(Mersmann, , 1992 and the more direct measurement of ligand binding to the receptor (Mersmann and McNeel, 1992a,b;Mersmann et al, 1993;Mersmann, 1996) but have not produced a clear indication of the bAR subtypes in porcine adipose tissue.…”

Section: Introductionmentioning

confidence: 99%

Distribution and quantification of beta1-, beta2-, and beta3-adrenergic receptor subtype transcripts in porcine tissues.

McNeel

1999

Distribution and concentration of beta-adrenergic receptor (betaAR) subtype transcripts (beta1AR, beta2AR, and beta3AR) were investigated in porcine and rat tissues. Reverse transcription (RT) coupled with PCR indicated the presence of beta1AR and beta2AR transcripts in porcine left ventricle, lung, longissimus muscle, and subcutaneous adipose tissue. The RT-PCR indicated that beta3AR transcripts were present in porcine subcutaneous adipose tissue, and transcripts were suggested in left ventricle and lung, but they were not detected in longissimus muscle. Quantitative ribonuclease protection assays indicated that the porcine beta1AR, beta2AR, and beta3AR transcript concentrations (amol/microg of total RNA) were, respectively as follows: heart left ventricle = 6.1, 2.4, and .02; lung = 3.8, 1.9, and .01; liver = 1.7, 2.1, and undetectable; skeletal muscle = 1.7, 1.1, .02; subcutaneous adipose tissue = 1.3, .4, .14. The proportions of porcine betaAR subtypes (beta1AR:beta2AR:beta3AR) were as follows: heart left ventricle = 72:28:.25; lung = 67:33:.2; liver = 45:55:0; skeletal muscle = 60:39:.7; and subcutaneous adipose = 73:20:7. Normalization of data to beta-actin transcript concentration did not change these relationships. Rat perigonadal adipose tissue beta1AR and beta3AR transcript concentrations were, respectively, .59 and 1.84 amol/microg of total RNA. The rat perigonadal adipose tissue concentration of beta3AR transcript was tenfold that of porcine subcutaneous adipose tissue. The predominant betaAR transcript in rat adipose tissue was beta3AR, and the predominant betaAR transcript in pig adipose tissue was beta1AR.

Evidence of classic beta 3-adrenergic receptors in porcine adipocytes.

1996

Adipocytes from several mammalian species have predominant beta 3-adrenergic receptors (beta 3-AR). Attempts to classify porcine adipocyte beta-adrenergic receptors (beta-AR) into subtypes have not been successful. Selectivity of agonists and antagonists for stimulation of lipolysis and for ligand binding is considerably more restrictive than for the classic rat and guinea pig beta-AR subtypes. The unique pattern for activity of agonists and antagonists in porcine beta-AR precludes analogy to classic receptors and consequently there is no conclusive evidence regarding porcine beta-AR subtypes. Porcine adipocyte membranes were used in ligand binding experiments designed specifically to demonstrate beta 3-AR. Equilibrium saturation curves with dihydroalprenolol, CGP 12,177, or iodocyanopindolol indicated saturation at low concentrations with a single binding site. Equilibrium competitive ligand binding with iodocyanopindolol as radioligand and isoproterenol or propranolol as competitive ligands indicated both ligands totally inhibited radioligand binding; propranolol was more potent than isoproterenol. Nonradioactive CGP 12,177 also competed with iodocyanopindolol. Ligand binding experiments provided no evidence of a low-affinity beta-AR (binding at high concentrations of ligand), the beta 3-AR. Positive evidences of a beta 3-AR were that CGP 12,177, a beta 1- and beta 2-adrenergic receptor antagonist but a beta 3-AR agonist, partially stimulated porcine adipocyte lipolysis. Furthermore, transcripts for a beta 3-AR, as well as a beta 1- and beta 2-AR, have been demonstrated in RNA from porcine adipocytes in other studies. The beta-AR subtypes expressed and functional in porcine adipocytes remain unknown. The multiple ligand binding sites cannot be attributed to classic beta-AR subtypes. The porcine beta-AR may be a single unique receptor to impart atypical binding properties, or more likely, multiple subtypes, each different enough from classic subtypes to impart the unique properties observed.