Characterization of the alpha-1 adrenergic receptors in the thoracic aorta of control and aldosterone hypertensive rats: correlation of radioligand binding with potassium efflux and contraction

Abstract

Pharmacologic analysis of functional and radioligand binding studies were used to determine whether alterations in adrenergic receptors contribute to the catecholamine supersensitivity observed in the thoracic aorta of aldosterone hypertensive rats (AHR). Adrenergic function was evaluated using receptor-mediated contraction and rate of 42K efflux. The pA2 for phentolamine was the same in AHR (7.9 +/- 0.1 and 8.0 +/- 0.1) and control (7.9 +/- 0.2 and 8.1 +/- 0.1) rats whether measured by contraction or 42K efflux. The pA2 value for the selective alpha-1 antagonist prazosin (9.8 +/- 0.1 to 10.7 +/- 0.2) and the alpha-2 antagonist yohimbine (6.6 +/- 0.2 to 7.4 +/- 0.2) was similar in AHR and control groups using both norepinephrine and phenylephrine as agonists. The rank order of potency was prazosin greater than phentolamine greater than yohimbine in both groups. The KD for [3H]prazosin binding to AHR (26 +/- 3 pM) and control tissue (34 +/- 6 pM) agreed with the prazosin KB obtained by measurements of contraction and 42K efflux. The alpha-1 receptor density was also unaltered: 39 +/- 1 fmol/mg in AHR and 44 +/- 3 fmol/mg in control. Assessment of the NE dissociation constant (Ka) by method of fractional receptor inactivation indicated that the Ka was 4.8 X 10(-7) M in both AHR and control tissues. However, at the same receptor occupancy, the NE-induced increase in 42K efflux was elevated markedly in aorta from AHR. We conclude that alteration in adrenergic receptor density, affinity or type is not the cause of catecholamine supersensitivity in the aorta from AHR and that postreceptor events mediate this phenomenon.