The effects of phenoxybenzamine treatment on the contractile responses of the aorta from WKY and SH rats to noradrenaline and adrenaline have been determined. 2. There was no change in sensitivity of the aorta to noradrenaline or adrenaline with hypertension. 3. Phenoxybenzamine treatment caused nonparallel rightward shifts of the concentration-response curves to the lower concentrations of noradrenaline and adrenaline. 4. The KA value for noradrenaline on the WKY rat aorta was 4.15 x 10(-8) M and noradrenaline produced a 95% maximum response by occupying 95% of the available alpha 1-adrenoceptors. These parameters were significantly different on the SH rat aorta; thus the KA value for noradrenaline was 76.40 x 10(-8) M and noradrenaline produced a 95% maximum response by occupying 63% of the alpha 1-adrenoceptors. 5. The KA value for adrenaline on the WKY rat aorta was 5.03 x 10(-8) M and adrenaline produced a 95% maximum response by occupying 95% of the available alpha 1-adrenoceptors. These adrenaline parameters were not significantly different on the SH rat aorta. 6. In summary this study has demonstrated that although the sensitivities of the rat aorta to noradrenaline and adrenaline do not alter, there is a change in the KA and receptor reserve for noradrenaline, but not adrenaline in hypertension.
