1. We have examined effects of inhibition of Na+,K+-dependent ATPase in large and small arterial ring preparations from rats and guinea pigs.

2. Ouabain (1 mmol/l) caused myogenic contraction of rat aorta and tail artery, but had no long-lasting effect on 150 μm mesenteric and 150 μm femoral resistance vessels over a 3 h period. Much lower concentrations of ouabain (1 μmol/l) caused contraction of guinea pig aorta, but had no effect on the mesenteric and femoral resistance vessels.

3. In the mesenteric resistance vessels, ouabain (1 mmol/l, rat vessels; 1 μmol/l, guinea pig vessels) caused the intracellular sodium content to rise over 2 h from approx. 13 mmol/l to approx. 60 mmol/l, and in the rat mesenteric resistance vessels this was associated with membrane depolarization from approx. −54 mV to approx. −30 mV after 3 h.

4. The results suggest that whereas Na+,K+-ATPase inhibition and consequent raised intracellular sodium may cause contraction of large vessels, this does not seem to be the case for small vessels. It therefore seems that further investigation is required before it is accepted that raised intracellular sodium is in itself a factor of importance in the etiology of hypertension.

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