Distension-dependent changes in noradrenaline sensitivity in small arteries from the rat.

The effects of distension on the sensitivity to noradrenaline and potassium were assessed in ring preparations of 200 micron branches from the superior mesenteric artery of the rat. In response to noradrenaline, maximal tension development occurred at smaller luminal diameters than with potassium activation. The sensitivity to noradrenaline increased with increasing distension whereas potassium sensitivity remained largely unchanged. A distension-dependent increase in noradrenaline sensitivity was observed also in vessels depolarized with high-potassium solution. The role of the distension-dependent sensitivity to noradrenaline for isotonic contractions was also studied. Upon activation with applied noradrenaline, submaximal isotonic responses were somewhat smaller than isometric responses at the same dose. This may be ascribed to a reduction in sensitivity during the isotonic contraction. Upon neurogenic activation, no difference between isotonic and isometric responses was found. The results show that distension increases noradrenaline sensitivity in resistance vessels in vitro, but that the consequences for their function as neuroeffectors may be small.