Effects of aging on AV nodal and ventricular beta-adrenergic receptors in the Fischer 344 rat

The effects of aging on atrioventricular (AV) nodal and right and left ventricular beta-adrenergic receptor (beta-AR) characteristics were studied in Fischer 344 rat hearts using quantitative autoradiography. Twenty-micrometer-thick cardiac sections containing the compact AV node from 16 mature (4-6 mo old) rats, 6 middle-aged (12 mo old) rats, and 16 old (24 mo old) rats were incubated with [125I]iodocyanopindolol, [125I]iodocyanopindolol plus atenolol, or ICI 118551. Saturation experiments revealed a significant age-related decrease in AV nodal beta-AR density (mature: 190 +/- 46, middle-aged: 165 +/- 27, old: 133 +/- 34 amol/mm2; P < 0.01), with no change in affinity (mature: 106 +/- 62, middle-aged: 132 +/- 46, old: 128 +/- 66 pM). No age-related changes in AV nodal beta-AR subtype ratio (55% beta 1, 45% beta 2) or estimated compact AV node volume were detected (mature: 66 +/- 17, old: 65 +/- 23 microns 3). No difference in beta-AR density or affinity was detected between mature and old rats in either left (LV) or right (RV) ventricular tissue (LV, mature: 60 +/- 11, middle-aged: 59 +/- 11, old: 62 +/- 11 amol/mm2; RV, mature: 65 +/- 9, middle-aged: 65 +/- 11, old: 58 +/- 10 amol/mm2). beta-AR subtype ratios for the left ventricle (64% beta 1, 36% beta 2) and right ventricle (63% beta 1, 37% beta 2) did not significantly differ between mature and old rats. To summarize, aging from 4 to 24 mo in the Fischer 344 rat is associated with 1) a decrease in AV nodal beta-AR density with no change in affinity; 2) no change in volume of the compact region of the AV node; 3) no change in beta-AR subtype ratio in the AV node, left ventricle, or right ventricle; and 4) no change in either RV or LV beta-AR density.