Cardiac alpha1 and beta adrenoceptors in rabbits: effects of dietary sodium and cholesterol

To investigate the effects of dietary sodium and cholesterol on alpha1 and beta adrenoceptors in hearts, male Japanese white rabbits were treated with either deoxycorticosterone acetate (DOCA) (50 mg/week sc) plus 1% saline in drinking water, frusemide (10 mg/2 days im), or cholesterol enriched diet (2%, w/w) for eight weeks. Using myocardial membrane preparations, characteristics of alpha1 and beta adrenoceptors were assessed by a radioligand binding assay using 3H-prazosin and 125I-iodocyanopindolol (125I-CYP) as radioactive ligands respectively. Although the treatment with DOCA and salt induced a small but significant increase in the mean arterial blood pressure, other treatments did not affect blood pressure. None of these interventions affected body weight, ventricular weight, ratio of ventricular weight to body weight, or protein yield of membrane preparations. Neither the number nor the affinity of cardiac alpha1 adrenoceptors (Bmax 15.1(1.7) fmol·mg protein−1, KD 0.63(0.13) nmol·litre−1 for controls) was affected by any of the treatments given. In contrast to alpha1 adrenoceptors, the number of beta adrenoceptors (Bmax) decreased significantly from 109.9(9.6) fmol·mg protein−1 for controls to 54.8(5.2) and 52.7(6.9) for rabbits treated with DOCA salt and cholesterol respectively. KD values of beta adrenoceptors for 125I-CYP decreased significantly with cholesterol treatment from 22.5(2.5) to 13.6(2.7) pmol·litre−1. Salt depletion produced by frusemide administration did not affect either the number or the affinity of beta adrenoceptors. These results suggest that changes in the sensitivity of tissues to catecholamines induced by the administration of DOCA salt and cholesterol enriched diet are due to alterations in beta adrenoceptors in the heart.