Maturation of intracellular calcium homeostasis in sheep pulmonary arterial smooth muscle cells

Cytosolic Ca²⁺ signaling dynamics are important to pulmonary arterial reactivity, and alterations are implicated in pulmonary vascular disorders. Yet, adaptations in cellular Ca²⁺ homeostasis and receptor-mediated Ca²⁺ signaling with maturation from fetal to adult life in pulmonary arterial smooth muscle cells (PASMCs) are not known. The present study tested the hypothesis that cytosolic Ca²⁺ homeostasis and receptor-generated Ca²⁺ signaling adapt with maturation in sheep PASMCs. Digitalized fluorescence microscopy was performed using isolated PASMCs from fetal and adult sheep that were loaded with the Ca²⁺ indicator fura 2. The results show that basal cytosolic and sarcoplasmic reticulum Ca ²⁺ levels are attained before birth. Similarly, Ca²⁺ efflux pathways from the cytosol and basal as well as capacitative Ca ²⁺ entry (CCE) are also developed before birth. However, receptor-mediated Ca²⁺ signaling adapts with maturation. Prominently, serotonin stimulation elicited Ca²⁺ elevations in very few fetal compared with adult PASMCs; in contrast, phenylephrine elevated Ca²⁺ in a similar percentage of fetal and adult PASMCs. Serotonin and phenylephrine elicited Ca²⁺ increases of a similar magnitude in reactive cells of fetus and adult, supporting the assertion that inositol trisphosphate signaling is intact. Caffeine and ATP elevated Ca²⁺ in equivalent numbers of fetal and adult PASMCs. However, the caffeine-induced cytosolic Ca²⁺ increase was significantly greater in fetal PASMCs, whereas the ATP-elicited increase was greater in adult cells. Overall, the results of this study demonstrate selective adaptations in receptor-mediated Ca²⁺ signaling, but not in cellular Ca²⁺ homeostasis.