Pulmonary and systemic vascular smooth muscle mechanical characteristics in newborn sheep.

To investigate the hypothesis that the higher pulmonary vascular resistance in newborn sheep is the result of developmental differences in the vascular muscle mechanical properties, we evaluated pulmonary arteries from newborn and adult sheep and compared them with their respective systemic counterparts (common carotid arteries). The newborn pulmonary artery mechanical stress (13.0 +/- 1.4 mN/mm2) and shortening capacity (11.4 +/- 1.1% of optimal length) were lower (P less than 0.01) than in the adult (20.4 +/- 2.5 and 15.6 +/- 1.3, respectively). The adult carotid artery muscle developed a greater stress (97.6 +/- 18.5 mN/mm2) than the newborn (40.7 +/- 5.0; P less than 0.01), whereas no age differences in shortening capacity were observed (newborn = 19.4 +/- 1.7; adult = 18.4 +/- 1.5% of optimal length). The contraction half-time was similar for the pulmonary and carotid arteries and was not affected by age, whereas the relaxation half-times of the newborn pulmonary (30.7 +/- 2.9 s) and carotid artery (23.3 +/- 1.5) were greater than in the adult (24.9 +/- 2.9 and 14.6 +/- 1.4, respectively; P less than 0.01). The myosin contents of the pulmonary and carotid arteries, as an indicator of the tissue muscle mass, were similar and did not change with age. In conclusion, while the lower stress and shortening capacity of the newborn pulmonary arteries limit their maximum capacity to vasoconstrict, the significantly greater relaxation time of their vascular muscle, a new observation, may account for the higher resistance to blood flow after birth.