Developmental changes in ryanodine- and IP₃-sensitive Ca²⁺ pools in ovine basilar artery

To explore the hypothesis that cerebrovascular maturation alters ryanodine- and inositol 1,4,5-trisphosphate (IP₃)-sensitive Ca²⁺ pool sizes, we measured total intracellular Ca²⁺ with ⁴⁵Ca and the fractions of intracellular Ca²⁺ released by IP₃ and/or caffeine in furaptra-loaded permeabilized basilar arteries from nonpregnant adult and term fetal (139-141 days) sheep. Ca²⁺ mass (nmol/mg dry weight) was similar in adult (1.60 ± 0.18) and fetal (1.71 ± 0.16) arteries in the pool sensitive to IP3 alone but was significantly lower for adult (0.11 ± 0.01) than for fetal (1.22 ± 0.11) arteries in the pool sensitive to ryanodine alone. The pool sensitive to both ryanodine and IP3 was also smaller in adult (0.14 ± 0.01) than in fetal (0.85 ± 0.08) arteries. Because the Ca²⁺ fraction in the ryanodine-IP₃ pool was small in both adult (5 ± 1%) and fetal (7 ± 4%) arteries, the IP₃ and ryanodine pools appear to be separate in these arteries. However, the pool sensitive to neither IP₃ nor ryanodine was 10-fold smaller in adult (0.87 ± 0.10) than in fetal (8.78 ± 0.81) arteries, where it accounted for 72% of total intracellular membrane-bound Ca²⁺. Thus, during basilar artery maturation, intracellular Ca²⁺ mass plummets in noncontractile pools, decreases modestly in ryanodine-sensitive pools, and remains constant in IP₃-sensitive pools. In addition, age-related increases in IP₃ efficacy must involve factors other than IP₃ pool size alone.