The Effect of Prenatal Cocaine Exposure on Myometrial α- andβ-Adrenergic Receptor Kinetics and G-Protein Expression in Guinea Pigs† 1078

Maternal cocaine abuse is associated with increased uterine contractions which may lead to preterm labor and prematurity. One mechanism of cocaine action is through uptake inhibition of norepinephrine (an α- andβ-adrenergic agonist). As adrenergic stimulation plays an important role in uterine contractility, we hypothesized that cocaine exposure during pregnancy alters adrenergic receptor kinetics and its G-protein-mediated transmembrane signal transduction. Therefore, we studied the effect of cocaine exposure on α- and β-adrenergic receptor kinetics and G-protein expression in pregnant guinea pig myometrium. Methods: Pregnant guinea pigs were injected daily with cocaine (5mg/kg SC) or saline (control) throughout the latter half of gestation (term: 65 days). Guinea pigs were killed at 90% gestation, the uteri removed, and membrane protein fraction obtained. Saturation binding assays were performed for β,α₁, and α₂ adrenergic receptors using the tritiated radioligands DHA, prazosin, and rauwolscine, respectively, to determine receptor concentration, B_max, in fmol/mg of protein, and affinity, k_D, in nmol/mg of protein. G_i α and G_s α subunit expression was determined by immunoblotting techniques. Membrane protein fractions were resolved by SDS-PAGE and probed using a polyclonal antibody against the inhibitory (i) and stimulatory (s) Gα-protein subunits and compared using optical densitometry. Results: Myometrium from cocaine-exposed guinea pigs showed a significant decrease in the concentration of β-adrenergic receptors as compared with the control group (B_max 78.5±6.97 (cocaine) vs 143±23.1(control), p<0.02) without significant difference in k_D (2.58 vs 3.47). There was no significant difference in the concentration or affinity ofα₁-adrenergic receptors (B_max: 98.7(cocaine) vs 91.5(control) and k_D: 0.523 (cocaine) vs 2.34 (control) orα₂-adrenergic receptors (B_max: 162 (cocaine) vs 133(control) and k_D: 4.06 (cocaine) vs 2.55 (control)). Optical densitometry measurements failed to demonstrate a significant difference in either G_iα or G_s α protein subunit expression.Conclusion: This study demonstrates that prenatal cocaine exposure down-regulates myometrial β-adrenergic receptor concentration in pregnant guinea pigs without significant effect on α-adrenergic receptor kinetics or G-protein expression. Decreased β-adrenergic activity may result in unopposed α-adrenergic stimulation and increased uterine contractions. These findings provide a mechanism for the relationship between maternal cocaine abuse and prematurity.