We sought to investigate whether atrial myocyte contraction and secretion of the atrial natriuretic peptide (ANP) are affected in the same manner by intervention in intracellular Ca²⁺ handling by acidosis. The effects of propionate (20 mM)-induced intracellular acidosis on the stretch-induced changes in ANP secretion, contraction force, and intracellular Ca²⁺ concentration ([Ca²⁺]_i) were studied in the isolated rat atrium. The stretch of the atrium was produced by increasing the intra-atrial pressure of the paced and superfused preparation. Contraction force was estimated from pressure pulses generated by the contraction of the atrium. Intracellular Ca²⁺ was measured from indo 1-AM-loaded atria, and ANP was measured by radioimmunoassay from the perfusate samples collected during interventions. Intracellular pH of the atrial myocytes was measured by a fluorescent indicator (BCECF)-based imaging system. Intracellular acidification caused by 20 mM propionic acid (0.18 pH units) potentiated the stretch-induced (intra-atrial pressure from 1 to 4 mmHg) ANP secretion, causing a twofold secretion compared with nonacidotic controls. Simultaneously, the responsiveness of the atrial contraction to stretch was reduced (P < 0.05, n = 7). Stretch augmented the systolic indo 1-AM transients in acidic (P < 0.05, n = 6) and nonacidic atria (P < 0.05, n = 6). However, during acidosis this was accompanied by an increase of the diastolic indo 1-AM ratio (P < 0.05, n = 6). Cooccurrence of stretch and acidosis caused an increase in systolic and diastolic [Ca²⁺]_i and potentiated the stretch-induced ANP secretion, whereas the contraction force and its stretch sensitivity were decreased. This mechanism may be involved in ischemia-induced ANP secretion, suggesting a role for ANP secretion as an indicator of contractile dysfunction.