1. We have compared the noradrenaline sensitivity of 150 μm arterial resistance vessels taken from a specific place in the mesenteric bed of spontaneously hypertensive (SH) rats and of control Wistar—Kyoto (WK) rats at three ages: 6 weeks, 12 weeks and 24 weeks.

2. The noradrenaline sensitivity of the vessels under normal conditions was the same at all ages in both SH and WK rats (ED50 about 3 μmol/l).

3. After addition of cocaine (which inhibits the uptake of noradrenaline in the nerve terminals) all vessels became more sensitive to noradrenaline, but at all ages the increase in sensitivity was greater in the vessels of SH rats, suggesting that the smooth muscle cells in these vessels had a greater intrinsic noradrenaline sensitivity than the vessels of WK rats.

4. Since elevation of the blood pressure in the SH rats occurs mainly between the ages of 6 and 12 weeks, the results suggest that the greater intrinsic smooth muscle cell sensitivity of the SH rat vessels is a factor which is amongst the primary factors responsible for the development of hypertension in the SH rat.

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