ET-1 activates Ca²⁺ sparks in PASMC: local Ca²⁺ signaling between inositol trisphosphate and ryanodine receptors

<jats:p> Ca<jats:sup>+</jats:sup> sparks originating from ryanodine receptors (RyRs) are known to cause membrane hyperpolarization and vasorelaxation in systemic arterial myocytes. By contrast, we have found that Ca<jats:sup>2+</jats:sup> sparks of pulmonary arterial smooth muscle cells (PASMCs) are associated with membrane depolarization and activated by endothelin-1 (ET-1), a potent vasoconstrictor that mediates/modulates acute and chronic hypoxic pulmonary vasoconstriction. In this study, we characterized the effects of ET-1 on the physical properties of Ca<jats:sup>2+</jats:sup> sparks and probed the signal transduction mechanism for spark activation in rat intralobar PASMCs. Application of ET-1 at 0.1-10 nM caused concentration-dependent increases in frequency, duration, and amplitude of Ca<jats:sup>2+</jats:sup> sparks. The ET-1-induced increase in spark frequency was inhibited by BQ-123, an ET<jats:sub>A</jats:sub>-receptor antagonist; by U-73122, a PLC inhibitor; and by xestospongin C and 2-aminoethyl diphenylborate, antagonists of inositol trisphosphate (IP<jats:sub>3</jats:sub>) receptors (IP<jats:sub>3</jats:sub>Rs). However, it was unrelated to sarcoplasmic reticulum Ca<jats:sup>2+</jats:sup> content, activation of L-type Ca<jats:sup>2+</jats:sup> channels, PKC, or cADP ribose. Photorelease of caged-IP<jats:sub>3</jats:sub> indicated that Ca<jats:sup>2+</jats:sup> release from IP<jats:sub>3</jats:sub>R could cross-activate RyRs to generate Ca<jats:sup>2+</jats:sup> sparks. Immunocytochemistry showed that the distributions of IP<jats:sub>3</jats:sub>Rs and RyRs were similar in PASMCs. Moreover, inhibition of Ca<jats:sup>2+</jats:sup> sparks with ryanodine caused a significant rightward shift in the ET-1 concentration-tension relationship in pulmonary arteries. These results suggest that ET-1 activation of Ca<jats:sup>2+</jats:sup> sparks is mediated via the ET<jats:sub>A</jats:sub> receptor-PLC-IP<jats:sub>3</jats:sub> pathway and local Ca<jats:sup>2+</jats:sup> cross-signaling between IP<jats:sub>3</jats:sub>Rs and RyRs; in addition, this novel signaling mechanism contributes significantly to the ET-1-induced vasoconstriction in pulmonary arteries. </jats:p>