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Division of Pulmonary and Critical Care Medicine, Johns Hopkins Medical Institutions, Baltimore, Maryland 21224
Ca+ spark has been implicated as a pivotal feedback mechanism for regulating membrane potential and vasomotor tone in systemic arterial smooth muscle cells (SASMCs), but little is known about its properties in pulmonary arterial smooth muscle cells (PASMCs). Using confocal microscopy, we identified spontaneous Ca2+ sparks in rat intralobar PASMCs and characterized their spatiotemporal properties and physiological functions. Ca2+ sparks of PASMCs had a lower frequency and smaller amplitude than cardiac sparks. They were abolished by inhibition of ryanodine receptors but not by inhibition of inositol trisphosphate receptors and L-type Ca2+ channels. Enhanced Ca2+ influx by BAY K8644, K+, or high Ca2+ caused a significant increase in spark frequency. Functionally, enhancing Ca2+ sparks with caffeine (0.5 mM) caused membrane depolarization in PASMCs, in contrast to hyperpolarization in SASMCs. Norepinephrine and endothelin-1 both caused global elevations in cytosolic Ca2+ concentration ([Ca2+]), but only endothelin-1 increased spark frequency. These results suggest that Ca2+ sparks of PASMCs are similar to those of SASMCs, originate from ryanodine receptors, and are enhanced by Ca2+ influx. However, they play a different modulatory role on membrane potential and are under agonist-specific regulation independent of global [Ca2+].
sarcoplasmic reticulum; ryanodine receptors; calcium channels; membrane potential; endothelin-1
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