TRPML channel activation partially rescues Ca 2+ spark activity in sheep fetal pulmonary arterial myocytes following intrauterine long‐term hypoxia

Ca 2+ sparks result from the activation of ryanodine receptors located on the sarcoplasmic reticulum of pulmonary arterial smooth muscle cells. These Ca 2+ sparks activate potassium channels on the plasma membrane and are important to arterial vasodilation and increases in vascular blood flow. Previous studies showed that long‐term high altitude hypoxia restricted membrane depolarization elicited Ca 2+ spark activity in fetal lamb pulmonary arterial myocytes, but had little effect on pulmonary myocytes in adult sheep. Moreover, the studies also indicated that there was preservation of the sarcoplasmic reticulum Ca 2+ stores in pulmonary arterial myocytes of both fetuses and adults as determined through direct activation of ryanodine receptors with caffeine. Given that the sarcoplasmic reticulum Ca 2+ stores are intact and have functional ryanodine receptors we hypothesize that aberrations to the canonical membrane depolarization Ca 2+ spark triggering pathway in fetal pulmonary arterial myocytes due to intrauterine hypoxia can be resolved through activation of alternate pathways. Recent studies illustrated that Ca 2+ sparks were triggered in various vascular myocytes by releasing Ca 2+ from lysosomes through activation of TRPML (transient receptor potential cation channel, mucolipin subfamily) channels. To evaluate the potential that TRPML activation may elicit Ca 2+ sparks in fetal and adult pulmonary arterial myocytes we studied isolated pulmonary arteries from near term fetal and adult sheep from low altitude (700 m) or those that had been exposed to high altitude (3801 m) for 110+ days. Ca 2+ sparks were recorded using laser scanning confocal microscopy while 10 μM MLSA1 was used to activate TRPML channels. MLSA1 enhanced the activity of Ca 2+ sparks in myocytes of adult pulmonary arteries to a greater extent than those from fetuses, suggesting developmental changes in the communication of lysosomes with the sarcoplasmic reticulum. Of importance, TRPML activation by MLSA1 partially recovered the aberrant Ca 2+ spark activity in fetal pulmonary arterial myocytes caused by hypoxia. The findings suggest that ryanodine receptor activation through pathways alternate to membrane depolarization may hold promise for improving pulmonary vascular function following intrauterine hypoxic exposure. Support or Funding Information Imaging was performed in the LLUSM Advanced Imaging and Microscopy Core with support of NSF Grant MRI‐DBI 0923559 (SMW) and the Loma Linda University School of Medicine. Additional support provided through NIH grant HD083132 (LZ).