Genetic evidence for functional role of ryanodine receptor 1 in pulmonary artery smooth muscle cells

Ryanodine receptor 1 (RyR1) is well-known to be expressed in systemic and pulmonary vascular smooth muscle cells (SMCs); however, its functional roles remain largely unknown. In the present study, we attempted to determine the potential importance of RyR1 in membrane depolarization-, neurotransmitter-, and hypoxia-induced Ca²⁺ release and contraction in pulmonary artery SMCs (PASMCs) using RyR1 homozygous and heterozygous gene deletion (RyR1^−/− and RyR1^+/−) mice. Our results indicate that spontaneous local Ca²⁺ release and caffeine-induced global Ca²⁺ release are significantly reduced in embryonic RyR1^−/− and adult RyR^+/− cells. An increase in Ca²⁺]_i following membrane depolarization with high K⁺ is markedly attenuated in RyR1^−/− and RyR1^+/− PASMCs in normal Ca²⁺ or Ca²⁺-free extracellular solution. Similarly, muscle contraction evoked by membrane depolarization is reduced in RyR1^+/− pulmonary arteries in the presence or absence of extracellular Ca²⁺. Neurotransmitter receptor agonists and inositol 1,4,5-triphosphate elicit a much smaller increase in Ca²⁺]_i in both RyR1^−/− and RyR1^+/− cells. We have also found that neurotransmitter-evoked muscle contraction is significantly inhibited in RyR1^+/− pulmonary arteries. Hypoxia-induced increase in Ca²⁺]_i and contraction are largely blocked in RyR1^−/− and/or RyR1^+/− PASMCs. Collectively, our findings provide genetic evidence for the functional importance of RyR1 in spontaneous local Ca²⁺ release, and membrane depolarization-, neurotransmitter-, as well as hypoxia-induced global Ca²⁺ release and attendant contraction in PASMCs.