1. Sodium movement across the porcine aortic arch in vitro is greater from the intimal to the medial surface than in the reverse direction.

2. A short-circuit current can be measured across the aortic arch and the calculated energy is greater than the energy required to support the net outward sodium movement, suggesting the possibility of active transport of other ions.

3. Net sodium movement is not significantly altered by an adverse gradient on the outside (medial side) but is reduced by a gradient in the opposite direction.

4. Sodium retention within the tissue is increased by a gradient on the medial side.

5. The movement of [14C]inulin is greater from the medial to the intimal surface, suggesting greater porosity on the medial side. A slight rise in both total tissue fluid and extracellular fluid occurred when the sodium concentration on the medial side of the preparation was increased. This did not occur with the sodium concentration increased on the intimal side. The changes were only significant statistically for total tissue fluid.

6. The short-circuit current can be enhanced by adrenaline and reduced by a variety of β-adrenoreceptor-blocking agents.

7. It appears that aortic tissue can actively transport sodium and it is suggested that this mechanism could play a part in blood pressure control.

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