1. We have investigated the noradrenaline-activated calcium sensitivity of 150 μm mesenteric resistance blood vessels from spontaneously hypertensive and control Wistar-Kyoto rats.

2. Under control conditions the spontaneously hypertensive rat blood vessels had a greater calcium sensitivity than the Wistar-Kyoto rat vessels.

3. In the presence of 1 mmol of ouabain/l, a treatment known to inhibit the sodium-potassium-dependent ATPase, the responses of the spontaneously hypertensive rat blood vessels were reduced more than those of the Wistar-Kyoto rat blood vessels, so that the responses of spontaneously hypertensive rat and Wistar-Kyoto rat blood vessels were then similar.

4. Similar results were obtained by removing external potassium, a procedure which should also inhibit the sodium-potassium-ATPase.

5. The results suggest that the greater noradrenaline-activated calcium sensitivity of spontaneously hypertensive rat blood vessels may be associated with an increased sodium-potassium-ATPase activity.

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