Cardiovascular effects of a mu-selective opioid agonist (tyrosine-D-arginine-phenylalanine-lysine-NH2) in fetal sheep: sites and mechanisms of action.

<bold>Objective: </bold>We investigated the effects of DALDA (tyrosine-D-arginine-phenylalanine-lysine-NH2), a mu-selective opioid peptide, on heart rate and blood pressure in fetal sheep with long-term instrument implantation.<bold>Study Design: </bold>DALDA was given as an intravenous bolus in doses ranging from 0.15 to 0.5 mg/kg. A 0.5 mg/kg dose of DALDA was given in the presence of the opioid antagonist naloxone and its quaternary analog naloxone methiodide (6 mg/h); it was also given in conjunction with the beta-adrenergic antagonist propranolol (2 mg/h). Statistical analyses were performed by 1-way and 2-way analysis of variance.<bold>Results: </bold>The fetus responded to DALDA with an increase in heart rate with all doses (P <.01) but without any change in blood pressure. This response was abolished by naloxone (P <.001), naloxone methiodide (P =.003), and propranolol (P <.001).<bold>Conclusions: </bold>In the fetus intravenous DALDA increases heart rate without any change in blood pressure by way of the mu receptor and through central sympathetic activation. These effects of DALDA are different from those seen in the adult, suggesting different sites and mechanisms of action. [ABSTRACT FROM AUTHOR]