The difference in nifedipine sensitivity between hypertensive and normotensive rat aortae is dependent on age.

1. In order to investigate further whether the increased sensitivity to inhibition by nifedipine of the responses to noradrenaline of aortae from spontaneously hypertensive stroke-prone (SHRSP) rats is related to the development of the hypertension, we have compared the sensitivity to noradrenaline, potassium chloride (KCl) and nifedipine of aortae from SHRSP and control (Wistar-Kyoto) WKY rats of different age groups (young: 3-5 weeks, and adult: 13-16 weeks). 2. The sensitivity to KCl was found to be less in the aortae from the adult WKY group than in any of the other three groups. Responses to noradrenaline of the adult WKY aortae were also less sensitive to inhibition by nifedipine in comparison to each of the other three groups. 3. The changes in sensitivity were not due to the changes in the populations of alpha 1-adrenoceptor subtypes as responses of the adult SHRSP aortae to noradrenaline were more sensitive to nifedipine in the presence of either the alpha 1-adrenoceptor subtype antagonists chloroethylclonidine or WB4101 than were those of the adult WKY aortae, but aortae from the young SHRSP were not. 4. These results suggest that, rather than the SHRSP aorta becoming more sensitive to nifedipine and potassium depolarization as hypertension develops, it is the WKY aorta that becomes more resistant as it matures.