Cardiac β‐adrenoceptor binding characteristics with age following adrenal demedullation

1 The role of adrenal medullary catecholamines in the regulation of cardiac function becomes more important when adrenergic neural influences in the heart are decreased. Since adrenergic nervous input to the heart decreases with age, it would be expected that catecholamine influence on cardiac neuroeffector junction would increase. 2 Fischer‐344 rats of 6‐, 12‐ and 24‐months were adrenal demedullated or sham‐operated and the animals were killed at the end of two weeks. β‐Adrenoceptors were studied in the membrane preparations from the ventricles of rat hearts. [ 125 I]‐iodopindolol was used as the radioligand. The density of β‐receptors ( B max ), dissociation constant ( K D ) and the ratio of cardiac β‐adrenoceptor subtypes were studied. The relative percentages of β‐receptor subtypes were determined by use of ICI 89,406 (β 1 ‐selective antagonist) and ICI 118,551 (β 2 ‐selective antagonist). 3 In 24‐month‐old animals which were adrenal demedullated, hydrocortisone replacement was employed for one week; the animals were killed one week later. 4 The data indicate that there was a diminution of the B max following adrenal demedullation at all ages but that the ratios of β 1 :β 2 ‐adrenoceptors remain the same as in the controls (67:33). The effect of adrenal medullary catecholamines on cardiac β‐receptor binding characteristics did not seem to be influenced by age.