Different receptors for angiotensin II at pre- and postjunctional level of the canine mesenteric and pulmonary arteries

Abstract

1. This investigation was undertaken to compare pre- and postjunctional receptors involved in the responses of the canine mesenteric and pulmonary arteries to angiotensin II.

2. In the mesenteric artery, angiotensin II caused an enhancement of tritium overflow evoked by electrical stimulation (EC_30%=5 nM), the maximal effect representing an increase by about 45%. Postjunctionally, angiotensin II caused concentration-dependent contractions (pD₂=8.57). Saralasin antagonized both pre- and postjunctional effects of angiotensin II, but it was more potent at post- than at prejunctional level (pA₂ of 9.51 and 8.15, respectively), while losartan antagonized exclusively the postjunctional effects of angiotensin II (pA₂=8.15). PD123319 had no antagonist effect either pre- or postjunctionally.

3. In the pulmonary artery, angiotensin II also caused an enhancement of the electrically-evoked tritium overflow (EC_30%=1.54 nM), its maximal effect increasing tritium overflow by about 80%. Postjunctionally, angiotensin II caused contractile responses (pD₂=8.52). As in the mesenteric artery, saralasin antagonized angiotensin II effects at both pre- and postjunctional level and it was more potent postjunctionally (pA₂ of 9.58 and 8.10, respectively). Losartan antagonized only the postjunctional effects of angiotensin II (pA₂=7.96) and PD123319 was ineffective.

4. It is concluded that in both vessels: (1) pre- and postjunctional receptors belong to a different subtype, since they are differently antagonized by the same antagonists; (2) postjunctional receptors belong to AT₁ subtype, since they are blocked by losartan but not by AT₂ antagonists; (3) prejunctional receptors apparently belong to neither AT₁ or AT₂ subtype since they are blocked by neither AT₁ nor AT₂ antagonists.

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