Variable receptor affinity hypothesis.

Measurements of the contractile response to norepinephrine (NE) of a variety of arteries of three mammalian species that are commonly used in the laboratory provide evidence that tissue sensitivity and affinity of the alpha 1-adrenoceptor for NE covary over a range of several orders of magnitude. The quantitative relationship suggests that variation in affinity can, to a great extent, account for the variation in sensitivity found in a number of circumstances. Furthermore, it is argued that the variation in affinity appears to be continuous and thus does not provide a basis for receptor type subdivision. There is also evidence that adrenergic antagonist affinity can vary significantly in tissues. The factors that might account for this variation include differences in receptor chemical structure, in the local membrane microenvironment, and in a number of intracellular processes. A hypothesis of variable receptor affinity has been proposed. If it is correct, then variation in receptor affinity is an important functionally relevant variable that could account for selectivity of tissue responses to circulating hormones and may represent a mechanism of change in the intact organism and in disease.