The myocardial interstitium is important in regulating cardiac function. Between the atrial lumen and pericardial space are transmural pathways, and the movement of interstitial fluid (ISF) through these pathways is one of the main driving forces regulating the translocation of substances from the interstitium into the blood. To define how the translocation of ISF from the interstitial space into the lumenal space is regulated by each component of atrial hemodynamics, we devised a new rabbit atrial model in which each physical parameter could be controlled independently. Using this system, we also defined the physiological role of the cardiac Na⁺/Ca²⁺ exchanger on the secretion of atrial natriuretic peptide (ANP) by depletion of extracellular Na⁺ ([Na⁺]_o). Increases in stroke volume and atrial end-systolic volume caused increases in both ISF translocation and ANP secretion. However, an increase in atrial rate did not influence the translocation of ISF, but caused an increase in ANP secretion. Gradual depletion of [Na⁺]_o caused gradual increases in ANP secretion and intracellular Ca²⁺ ([Ca²⁺]_i), which were blocked in the presence of Ca²⁺-free buffer and Ni²⁺, but not in the presence of KB-R7943, diltiazem, mibefradil, caffeine, or monensin. Amiloride and its analogue blocked an increase in ANP secretion, but not an increase in [Ca²⁺]_i by [Na⁺]_o depletion. Therefore, we suggest that the secretion of ANP and the translocation of ISF may be differently controlled by each physical factor. These results also suggest that the increase in ANP secretion in response to [Na⁺]_o depletion may involve inhibition of both the Na⁺/Ca²⁺ exchanger and Na⁺/H⁺ exchanger, but may not involve an increase in [Ca²⁺]_i.