Morphological and functional coupling of IP3 receptors with Na pumps in adrenal chromaffin cells

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Adrenal chromaffin cells are exposed to high concentrations of cortical hormones, one of which is ouabain-like substance. Thus, the effects of ouabain on catecholamine secretion and distribution of Na^+, K^+-ATPase α subunits in chromaffin cells were examined using amperometry and immunocytochemistry. While exposure to 1 μM ouabain did not have a marked effect on resulting secretion, it induced an increase in the secretion due to mobilization of partially stored Ca^<2+>. Immunocytochemistry revealed that Na^+, K^+-ATPase α1 subunit-like immunoreactivity (IR) was distributed ubiquitously at cell periphery, whereas α2-like IR were present at part of cell periphery. Inositol trisphosphate receptor (InsP_3R) type 2-like IR was distributed perinuclearly and in the vicinity of the plasma membrane, consistent with that of BODIPY-FL-InsP_3 binding. Peripheral BODIPY-FL-InsP_3 binding sites underlay membranes with α2-like IR, indicating that the peripheral InsP_3R type 2-containing Ca^<2+> storage was juxtaposed to membranes with the α2 subunit. This close association may account for the facilitation of Ca^<2+> uptake into stores and the consequent increase in mobilization-dependent secretion in the presence of 1 μM ouabain.