1. The role of Na+-K+-activated ATPase in the regulation of resistance vessel reactivity to barium chloride and noradrenaline was investigated with a pair-perfused hindquarter technique in spontaneously hypertensive rats and Wistar-Kyoto rats (6 and 12 week old). Ouabain (10−4 mol/l) was used to inhibit the sodium pump.

2. In all groups studied, dose-response curves to agonists were shifted to the left when ouabain was added to the perfusion medium. This potentiation in vascular reactivity produced by ouabain was expressed as ‘index for sodium-pump inhibition’ (ISPI = ED50 of an agonist/ED50 + ouabain).

3. In mature rats index for sodium-pump inhibition calculated for both agonists was significantly greater in spontaneously hypertensive rats in comparison with Wistar-Kyoto rats, but not so in young spontaneously hypertensive rats. There was, however, a trend towards increased noradrenaline sensitivity in spontaneously hypertensive rats although the difference from Wistar-Kyoto rats was not statistically significant.

4. The data may suggest that there is some increase in the activity of the sodium pump in the resistance vessels of mature spontaneously hypertensive rats, perhaps to compensate for an increased passive sodium permeability.

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