Abstract
A new, image‐based, tritiated ligand technique for measuring cardiac β 2 ‐adrenoceptor (β 2 ‐AR) binding characteristics was developed and validated with adult rainbow trout Oncorhynchus mykiss hearts so that the tissue limitation of traditional receptor binding techniques could be overcome and measurements could be made in hearts nearly 14‐times smaller than previously used. The myocardial cell‐surface (functional) β 2 ‐AR density of O. nerka smolts sampled at the headwaters of the Chilko River was 54·2 fmol mg protein −1 and about half of that previously found in return migrating adults of the same population, but still more than twice that of adult hatchery O. mykiss (21·1 fmol mg protein −1 ). This technique now opens the possibility of investigating cardiac receptor density in a much wider range of fish species and life stages.