β-Adrenergic Receptors in the Rat Mammary Gland during Pregnancy and Lactation: Characterization, Distribution, and Coupling to Adenylate Cyclase

Abstract

To investigate a possible role of catecholamines in mammary gland growth and differentiation, we have studied the characteristics of a specific beta-adrenergic receptor population during the different reproductive phases of the rat mammary gland, namely pregnancy and lactation. The functional response to mammary beta-adrenergic receptor stimulation was assessed by measurement of adenylate cyclase activity during the same physiological states of the gland [125I]Cyanopindolol (CYP) binds specifically to membranes prepared from lactating mammary glands. Scatchard analysis of the binding data shows the presence of a single class of high affinity sites, with an apparent Kd value of 25.0 +/- 0.4 pM and a binding capacity of 32.5 +/- 1.2 fmol/mg protein in lactating mammary glands at random stages of lactation. The order of potency of a series of agonists to compete for [125I]CYP binding is consistent with the interactions with a beta 2-subtype receptor. The binding of [125I]CYP to mammary glands also shows a marked stereoselectivity; the (-)isomers of isoproterenol and propranolol are more potent than their respective enantiomers. The radioautographic localization of [125I]CYP reveals the presence of specific beta-adrenergic receptors in the epithelial cells, alveoles, ducts, as well as adipocytes. [125I]CYP binding shows a 2- to 3-fold increase during pregnancy. Such a result correlates with parallel increases in stimulation of adenylate cyclase activity, the cytosolic progesterone receptor concentration, as well as plasma 17 beta-estradiol and progesterone levels. At parturition, a sharp decline in beta-adrenergic receptor concentration is observed, a finding concomitant with a drop in progesterone receptor levels as well as plasma estradiol and progesterone concentrations. During midlactation, beta-adrenergic receptors reach their maximal levels. The presence of specific beta-adrenergic receptors functionally coupled to the adenylate cyclase system and the marked changes in receptor capacity and distribution measured during the different physiological states of the mammary gland suggest that the mammary beta-adrenergic receptors are highly sensitive to changes in the hormonal milieu and provide a mechanism for a direct catecholaminergic influence on mammary gland growth and differentiation.