Comparative capacitative calcium entry mechanisms in canine pulmonary and renal arterial smooth muscle cells

Experiments were performed to determine whether capacitative Ca²⁺ entry (CCE) can be activated in canine pulmonary and renal arterial smooth muscle cells (ASMCs) and whether activation of CCE parallels the different functional structure of the sarcoplasmic reticulum (SR) in these two cell types. The cytosolic [Ca²⁺] was measured by imaging fura-2-loaded individual cells. Increases in the cytosolic [Ca²⁺] due to store depletion in pulmonary ASMCs required simultaneous depletion of both the inositol 1,4,5-trisphosphate (InsP₃)- and ryanodine (RY)-sensitive SR Ca²⁺ stores. In contrast, the cytosolic [Ca²⁺] rises in renal ASMCs occurred when the SR stores were depleted through either the InsP₃ or RY pathways. The increase in the cytosolic [Ca²⁺] due to store depletion in both pulmonary and renal ASMCs was present in cells that were voltage clamped and was abolished when cells were perfused with a Ca²⁺-free bathing solution. Rapid quenching of the fura-2 signal by 100 μM Mn²⁺ following SR store depletion indicated that extracellular Ca²⁺ entry increased in both cell types and also verified that activation of CCE in pulmonary ASMCs required the simultaneous depletion of the InsP₃- and RY-sensitive SR Ca²⁺ stores, while CCE could be activated in renal ASMCs by the depletion of either of the InsP₃- or RY-sensitive SR stores. Store depletion Ca²⁺ entry in both pulmonary and renal ASMCs was strongly inhibited by Ni²⁺ (0.1-10 mM), slightly inhibited by Cd²⁺ (200-500 μM), but was not significantly affected by the voltage-gated Ca²⁺ channel (VGCC) blocker nisoldipine (10 μM). The non-selective cation channel blocker Gd³⁺ (100 μM) inhibited a portion of the Ca²⁺ entry in 6 of 18 renal but not pulmonary ASMCs. These results provide evidence that SR Ca²⁺ store depletion activates CCE in parallel with the organization of intracellular Ca²⁺ stores in canine pulmonary and renal ASMCs.