Endothelin-1 and IP₃ induced Ca²⁺ sparks in pulmonary arterial smooth muscle cells

Endothelin-1 has been implicated as an important modulator or mediator of acute and chronic hypoxic pulmonary hypertension. It has been shown that endothelin-1 increases [Ca²⁺]_i and contraction in pulmonary arterial smooth muscle cells. Recently, we have identified local Ca²⁺ release transients or Ca²⁺ sparks, which represent Ca²⁺ release from clusters of ryanodine receptors on the sarcoplasmic reticulum, in pulmonary arterial smooth muscle cells. These pulmonary Ca ²⁺ sparks were associated with membrane depolarization, and activated specifically by endothelin-1 via endothelin-A receptor activation of phospholipase C and inositol trisphosphate production, possibly through local Ca²⁺ signaling between inositol trisphosphate receptors and ryanodine receptors. To test this hypothesis, we measured Ca²⁺ sparks in intralobar pulmonary arterial smooth muscle cells using laser scanning microscopy, and compared the spatiotemporal properties of Ca²⁺ sparks activated by endothelin-1 and by photorelease of inositol trisphosphate. We found that both endothelin-1 and inositol trisphosphate had similar effects on Ca²⁺ sparks. They both increased spark frequency, elevated spark amplitude and prolonged duration, without any significant effect on the spatial spread or size of Ca²⁺ sparks. These results provide further support to the suggestion that inositol trisphosphate production stimulated by endothelin-1 can account for the activation of Ca²⁺ sparks in pulmonary arterial smooth muscle cells.