RESUMO
1. We have studied the effects of bromoacetylalprenololmenthane (BAAM), a very slowly reversible beta-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 ventricle with BAAM at 10(-6) M for 30 min, there was a parallel rightward shift of the isoprenaline concentration-response curves. Treatment with a higher concentration of BAAM (10(-5)M) 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 (KA) values for isoprenaline. The mean isoprenaline KA value on the left ventricle from 5 week old WKY was 2.44 x 10(-6)M, and similar KA 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 beta(1)-adrenoceptors. 4. This study has shown that there are no differences in the cardiac responses to isoprenaline at beta(1)-adrenoceptors, isoprenaline KA values or the beta(1)-adrenoceptor reserve for isoprenaline on the SHR left ventricle in prehypertension or in the early stages of hypertension-induced hypertrophy.