The effects of bromoacetylalprenololmenthane on the responses to isoprenaline of the left ventricle from normo-, prehyper- and hypertensive rats

1 We have studied the effects of bromoacetylalprenololmenthane (BAAM), a very slowly reversible β-adrenoceptor antagonist, on the responses of the left ventricle of 5 and 22 week old Wistar Kyoto (WKY) rats and spontaneously hypertensive rats (SHRs) to isoprenaline. At 5 weeks the SHRs were prehypertensive and at 22 weeks they had hypertension-induced hypertrophy of the left ventricle. 2 The mean potency of isoprenaline on the left ventricle from 5 week old WKY was similar to that obtained on left ventricles from 22 week old WKY, and 5 and 22 week old SHRs. 3 Following treatment of the left ventricle with BAAM at 10^?6m for 30 min, there was a parallel rightward shift of the isoprenaline concentration–response curves. Treatment with a higher concentration of BAAM (10^?5m) caused non-parallel rightward shifts of the concentration–response curves, with a depression of the isoprenaline maximum responses. These data were used to derive affinity (K_A) values for isoprenaline. The mean isoprenaline K_A value on the left ventricle from 5 week old WKY was 2.44 × 10^?6m , and similar K_A values were obtained on the left ventricles from 22 week old WKY and 5 and 22 week old SHRs. On all the left ventricles tested, isoprenaline produced a half maximal response by occupying less than 1%, and a near maximal response by occupying about 5% of the available β₁-adrenoceptors. 4 This study has shown that there are no differences in the cardiac responses to isoprenaline at β₁-adrenoceptors, isoprenaline K_A values or the β₁-adrenoceptor reserve for isoprenaline on the SHR left ventricle in prehypertension or in the early stages of hypertension-induced hypertrophy.