Potency, affinity constants and receptor reserves for noradrenaline and adrenaline on aortae from aged normo- and hypertensive rats.

Previously we have determined the potency, affinity constants (K(A) values), and alpha1-adrenoceptor reserves for noradrenaline and adrenaline on the thoracic aortae of 20-week-old Wistar Kyoto normotensive (WKY) and spontaneously hypertensive rats (SHRs). This study has investigated whether these parameters were altered on the thoracic aortae by ageing, and in hypertension/heart failure. The effects of phenoxybenzamine on the contractile responses of the aortae of 20-month-old WKYs and SHRs were determined. The pD2 values for noradrenaline and adrenaline were 7.1 and 7.0, respectively, on the aortae of 20-month-old WKYs, and similar values were obtained on age-matched SHRs. On the aortae of 20-month-old WKYs, the K(A) values for noradrenaline and adrenaline were 1.85 and 1.95 x 10(-6) M, and the receptor occupancies required for 50% maximum responses were 16 and 24%, respectively. There were lower affinities, by approximately twofold, but similar receptor reserves for noradrenaline and adrenaline on the aortae of age-matched SHRs. In comparison with the aortae of 20-week-old WKYs and SHRs, there was a 5-fold loss of sensitivity to noradrenaline and adrenaline between 20 weeks and 20 months. Between 20 weeks and 20 months there was a 50-fold loss of affinity with ageing and a further twofold loss with hypertension/heart failure, and an increase in alpha1-adrenoceptor reserves for noradrenaline and adrenaline between 20 weeks and 20 months. There were no differences in the sensitivity and affinity, and minor changes in the alpha1-adrenoceptor reserves for noradrenaline and adrenaline between the aortae of 20-month-old WKYs and SHRs. In contrast there were major changes in these parameters in the ageing of the WKY aorta from 20 weeks to 20 months. There were no additional changes in the sensitivity and alpha1-adrenoceptor reserves, but a small additional change in affinity for noradrenaline and adrenaline in hypertension/heart failure on the aortae of 20-month-old SHRs.