Regional Differences in Vascular Tachyphylaxis to Angiotensin in the Cat

Abstract

The denervated resistance vessels of the cat''s intestine and skeletal muscles were studied with respect to their constrictor responses to prolonged noradrenaline [norepinephrine] and angiotensin administration. As is the case with noradrenaline [norepinephrine], angiotensin induced a sustained and dose-dependent resistance response in skeletal muscles. The corresponding responses of the intestinal resistance vessels were more complex. The noradrenaline vasoconstriction was dose-dependent, but partly counteracted by the characteristic autoregulatory escape* of these vessels. However, the presence of a continued noradrenaline influence was revealed by a considerable post-infusion hyperemia. In contrast to this, the angiotensin response rapidly vanished, could not be reestablished by increased dosage, no post-infusion hyperemia was seen and the angiotensin administration had to be interrupted for several minutes before responses could again be induced. Moreover, angiotensin could be intraarterially infused in gradually increasing amounts to the intestine with almost no effect on intestinal flow resistance, even when such high angiotensin concentrations were reached that their recirculation produced marked increases in blood pressure and muscle flow resistance. The results suggest that the well-known vascular tachyphylaxis to angiotensin is developed to a much greater extent in the intestinal vascular bed than in that of the skeletal muscles, while neither bed shows any tachyphylaxis to noradrenaline.