Effects of angiotensin II and 1-sar., 8-isoleu. angiotensin II on electrical and mechanical properties of the portal vein from rats of different ages

The effects of angiotensin II (Ag-11) and 1-sar., 8-isoleu. angiotensin 11 (anti-Ag-11) on the membrane and mechanical properties of smooth muscle cells of the rat portal vein were investigated in three different age groups (6--8 weeks, immature rat; 3--5 months old, young rat and 13--15 months old, adult rat). Application of Ag-11 (10(-10) g/ml) did not depolarize the membrane, but did increase the spike frequency and potentiated the frequency and amplitude of twitch contraction. In a concentration of more than 10(-9) g/ml Ag-II, the membrane was depolarized and a phasic contracture was developed in three age groups. Depolarization of the membrane produced by Ag-II could be classified into two components, i.e. phasic and tonic depolarizations. In adult rats, Ag-II produced the highest amplitude of the depolarization in three age groups but tonic depolarization showed nearly the same amplitude of smooth muscle cell membrane to Ag-II increases with age up to 3--5 months and that an increased sensitivity is accompanied by a generation of desensitization. Tachyphylaxis to Ag-II was also observed by repetitive applications, but the appearance depended on the stimulus conditions. Anti-Ag-II, itself slightly increased the spike frequency and the amplitude of twitch contraction. However, under pretreatment with anti-Ag-II (10(-9) or 10(-7) g/ml), the actions of Ag-II on the electrical and mechanical activities were markedly suppressed in the three age groups. When the dose-response curve was obtained from the mechanical response produced by Ag-II, the relation shifted to the right in the presence of anti-Ag-II in all age groups. In the presence of 10(-7) g/ml anti-Ag-II, no contracture was evoked by application of 10(-6) g/ml Ag-II in three age groups. Anti-Ag-II seems to possess a higher affinity to the angiotensin receptor than does Ag-II.