Fetal and newborn lambs differ in their cardiopulmonary responsiveness to adenosine agonists

We investigated cardiorespiratory responses to adenosine analogs in the pre- and postnatal periods in an unanesthetized chronic animal preparation. Heart rate, blood pressure, breathing movements and blood gases were measured in 8 fetal lambs (117-130 days gestation) and 9 newborn lambs (4-30 days). Various doses of analogs were given by intravenous infusion. L-N6-5'-Phenylisopropyladenosine (L-PIA), 5-N-ethyl carboxamidoadenosine (NECA) and cyclohexyladenosine (CHA) were studied. All analogs produced dose-dependent bradycardia and hypotension in the fetus. However, in the newborn, NECA produced a dose-dependent tachycardia, whereas PIA and CHA produced a dose-dependent bradycardia. Fetal breathing movements were interrupted by all the analogs, but they did not produce apnea in the newborn. No changes in blood gases were observed. The actions of the adenosine agonists were blocked by caffeine.