β-Adrenoceptor-mediated cell signaling in the neonatal heart and liver: responses to terbutaline

Abstract

Terbutaline, a beta(2)-adrenoceptor (beta(2)-AR) agonist, is a widely used tocolytic that also crosses the placenta to stimulate fetal beta-ARs. The current study examines the effects of terbutaline administered to neonatal rats. Terbutaline (10 mg/kg sc) given on postnatal day (PN) 2-5 or PN 11-14 elicited significant downregulation of both cardiac and hepatic beta-ARs, with a much greater effect in the liver. Despite the reduction in cardiac beta-ARs, receptor desensitization was absent as evidenced by the maintained ability of isoproterenol to stimulate adenylyl cyclase (AC) in membrane preparations. The underlying mechanism was dissected by using stimulants that operate at different points in the AC signaling pathway, NaF, forskolin, and Mn(2+). When administered in the early neonatal period, terbutaline failed to evoke any changes in cardiac AC activity; however, treatment on PN 11-14 evoked heterologous sensitization downstream from the receptor, evidenced by increases in the response to NaF and forskolin. In the liver, neonatal terbutaline administration elicited a small (approximately equal to 10%) decrease in the AC response to isoproterenol, an effect much smaller than the downregulation of beta-ARs (>40%). In this tissue, desensitization was again offset by heterologous sensitization of AC signaling. These results indicate that, in the developing organism, beta-AR-mediated cell signaling responses are maintained in the face of receptor downregulation through heterologous induction of downstream signaling elements. These unique responses serve to sustain beta-AR signaling in the perinatal period.

Critical periods for chlorpyrifos-induced developmental neurotoxicity: alterations in adenylyl cyclase signaling in adult rat brain regions after gestational or neonatal exposure.

“…In addition to measuring basal AC activity, we assessed the response to βAR stimulation (100 µM isoproterenol), as well as the response to the direct AC stimulants forskolin (100 µM) and Mn 2+ (10 mM). These concentrations of each stimulant produce maximal responses, as assessed in previous studies (Auman et al 2000(Auman et al , 2001Slotkin et al 2001b;Zeiders et al 1997Zeiders et al , 1999.…”

“…All of the assay methodologies used in this study have been described previously (Aldridge et al 2003;Auman et al 2000Auman et al , 2001Meyer et al 2003;Slotkin et al 2001b;Zeiders et al 1997Zeiders et al , 1999, so only brief descriptions are provided here. Tissues were thawed and homogenized with a Polytron (Brinkmann Instruments, Westbury, NY), and cell membranes were prepared and washed by sequential sedimentation at 40,000 × g. The membrane pellets were dispersed with a smooth-glass homogenizer and used for ligand binding and AC assays.…”

Ontogenesis of β-Adrenoceptor Signaling: Implications for Perinatal Physiology and for Fetal Effects of Tocolytic Drugs

“…acting at each step in the pathway (Zeiders et al, 1997(Zeiders et al, , 1999a(Zeiders et al, ,b, 2000Auman et al, 2001bAuman et al, , 2002Garofolo et al, 2002). The concentration of ␤ARs can be assessed with radioligands to provide a measure of receptor up-or downregulation.…”

“…Finally, as NaF stimulates both G s and G i , a change in the net response to NaF reflects a shift in the balance of expression or function of these two opposing signaling proteins. Figure 2 summarizes studies on the effects of repeated ␤AR stimulation evoked by isoproterenol administration given to neonatal rats on postnatal days (PN) 2 to 5, with evaluations conducted on PN 6, 24 h after the last dose (Zeiders et al, 1997(Zeiders et al, , 1999a(Zeiders et al, ,b, 2000Auman et al, 2001bAuman et al, , 2002Garofolo et al, 2002). The determinations were then conducted on cardiac membrane preparations isolated after the in vivo treatments.…”

“…F, shows the heterologous down-regulation and loss of responsiveness to m 2 AChRs, receptors that mediate inhibition of AC. Data were compiled from previously published studies (Zeiders et al, 1997(Zeiders et al, , 1999a(Zeiders et al, ,b, 2000Auman et al, 2001bAuman et al, , 2002Garofolo et al, 2002). (Fig.…”