Imbalances emerge in cardiac autonomic cell signaling after neonatal exposure to terbutaline or chlorpyrifos, alone or in combination

During early neonatal development, the future reactivity of the heart to cardiac autonomic stimulation is programmed by the timing and intensity of the arrival of parasympathetic and sympathetic inputs. In neonatal rats, we examined the effects of exposure to terbutaline, a β-adrenoceptor (βAR) agonist used to arrest preterm labor, and chlorpyrifos (CPF), a widely used organophosphate pesticide that acts in part through inhibition of cholinesterase, using scenarios mimicking the likely developmental stages corresponding to peak human exposures: postnatal days (PN) 2–5 for terbutaline and PN11–14 for CPF. Terbutaline evoked a progressive deficit in cardiac βAR binding but did not interfere with the ability of the receptors to stimulate adenylyl cyclase (AC). Terbutaline also reduced expression of m₂ muscarinic acetylcholine receptors and suppressed their ability to inhibit AC. Surprisingly, CPF produced similar actions, a decrement in βAR and m₂ muscarinic receptor binding and a loss of the cholinergic AC response, and also augmented the ability of βARs to stimulate AC. The effects of CPF are thus unlikely to reside in cholinergic hyperstimulation resulting from cholinesterase inhibition but instead involve other actions converging on receptors and cell signaling. Exposure to both agents, terbutaline followed by CPF, produced a summation of the two individual effects. Our findings at the level of cell signaling thus indicate that neonatal exposure to terbutaline or CPF, or sequentially to both agents, results in an imbalance of cardiac autonomic inputs favoring increased excitability, an outcome that may have an impact on cardiovascular responses.