The density of beta-adrenoreceptors in cardiac muscle membranes of muskrats and guinea pigs.

1. Cardiac membranes were prepared and then incubated with a range of concentrations of the beta-adrenergic ligand, dihydroalprenolol. Specific binding to the receptor was measured and receptor density and binding affinities were determined. 2. The receptor density was about 40% greater in the guinea pig heart than in the muskrat heart. The reduced beta-receptor density in the muskrat heart helps explain the previous findings of a lesser pumping ability and decreased responsiveness to isoproterenol in the muskrat compared with guinea pig hearts. 3. The decreased beta-receptor density may aid the muskrat in diving by reducing the oxygen consumption of the heart and thus prolonging diving time.