Increase of cardiac beta-adrenergic receptors in young spontaneously hypertensive rats

The authors studied the number of myocardial beta-adrenergic receptors and the cyclic nucleotide concentration in both male spontaneously hypertensive rats (SHR) and Wistar Kyoto rats (WKY) at 4 to 5, 10 to 15, 20 to 25 and 35 to 55 weeks of age. A potent beta-adrenergic antagonist, (125I) iodohydroxybenzylpindolol was used to estimate the number and affinity of beta-adrenergic receptors. beta-adrenergic receptors in cardiac membranes from SHR of 4 to 5 weeks and 10 to 15 weeks numbered 63.1 +/- 4.6 and 51.6 +/- 4.6 f mol/mg protein, respectively. These were significantly (p less than 0.02) greater than the number in WKY at 4 to 5 weeks and 10 to 15 weeks (42.2 +/- 5.1 and 31.5 +/- 5.4 f mol/mg protein, respectively). The dissociation constant in the membranes was the same in WKY and SHR, and no significant differences were found in the number of receptors and affinity of SHR and WKY at 20 to 25 weeks or 35 to 55 weeks of age. Also, there was no difference in the concentration of myocardial cyclic nucleotides at the various ages. Since cardiac hypertrophy in SHR had appeared before the onset of hypertension at about 7 weeks, the present results suggest that the SHR heart is hypersensitive to catecholamines and hemodynamically hyperkinetic due to the increased numbers of beta-receptors in the pre- and early stages of hypertension.