Responses of small intrapulmonary arteries to vasoactive compounds in the fetal and neonatal lamb: norepinephrine, epinephrine, serotonin, and potassium chloride.

Despite considerable study, the mechanisms responsible for the transition of the pulmonary circulation from the fetal to newborn life remain obscure. We compared the responses of third and fourth generation pulmonary arteries to norepinephrine, epinephrine, serotonin, and KCl from lambs 7 d preterm and 1, 7, and 21 d of age to assess differences between age and third and fourth generations of the pulmonary arterial tree during the transitional period. Preterm vessels were significantly smaller in internal diameter than all other aged vessels for both generations. Fourth generation vessel response to KCl increased with age (94 +/- 15 mg/mm2, preterm; 259 +/- 31 mg/mm2, 21 d). Third generation vessel response to KCl did not change with age (135 +/- 15 mg/mm2, preterm; 158 +/- 18 mg/mm2, 21 d). There were no differences in maximum response to norepinephrine and epinephrine between ages; however, third generation vessel response to these compounds was significantly greater (30-60% of maximum KCl response) than fourth generation vessel response (0-10%). Third and fourth generation vessels had the same maximum response to serotonin regardless of age or generation. Third generation vessels were significantly reduced in sensitivity (log molar concentration which produced 50% of the maximum response--EC50) to norepinephrine at d 1 (5.89 +/- 0.12 log molar) and 7 (5.90 +/- 0.21) compared to preterm (6.48 +/- 0.10) and 21 d of age (6.50 +/- .03).(ABSTRACT TRUNCATED AT 250 WORDS)