Ca<SUP>2&#8290;</SUP>-induced Ca<SUP>2&#8290;</SUP> Release from Sarcoplasmic Reticulum Negatively Regulates Myocytic ANP Release in Beating Rabbit Atria

Ca<SUP>2&#8290;</SUP>-induced Ca<SUP>2&#8290;</SUP> Release from Sarcoplasmic Reticulum Negatively Regulates Myocytic ANP Release in Beating Rabbit Atria

It is not clear whether Ca<SUP>2&#8290;</SUP>-induced Ca<SUP>2&#8290;</SUP> release from the sarcoplasmic reticulum (SR) is involved in the regulation of atrial natriuretic peptide (ANP) release. Previously, we have shown that nifedipine increased ANP release, indicating that Ca<SUP>2&#8290;</SUP> entry via voltage-gated L-type Ca<SUP>2&#8290;</SUP> channel activation decreases ANP release. The purpose of the present study was two-fold: to define the role of SR Ca<SUP>2&#8290;</SUP> release in the regulation of ANP release and whether Ca<SUP>2&#8290;</SUP> entry via L-type Ca<SUP>2&#8290;</SUP> channel is prerequisite for the SR-related effect on ANP release. Experiments were performed in perfused beating rabbit atria. Ryanodine, an inhibitor of SR Ca<SUP>2&#8290;</SUP> release, increased atrial myocytic ANP release (8.69&#8273;3.05, 19.55&#8273;1.09, 27.31&#8273;3.51, and 18.91&#8273;4.76% for 1, 2, 3, and 6μM ryanodine, respectively; all P＜0.01) with concomitant decrease in atrial stroke volume and pulse pressure in a dose-dependent manner. In the presence of thapsigargin, an inhibitor of SR Ca<SUP>2&#8290;</SUP> pump, ryanodine-induced increase in ANP release was not observed. Thapsigargin attenuated ryanodine-induced decrease in atrial dynamic changes. Blockade of L-type Ca<SUP>2&#8290;</SUP> channel with nifedipine abolished ryanodine-induced increase in ANP release (0.69&#8273;5.58% vs. 27.31&#8273;3.51%; P＜0.001). In the presence of thapsigargin and ryanodine, nifedipine increased ANP release and decreased atrial dynamics. These data suggest that Ca<SUP>2&#8290;</SUP>-induced Ca<SUP>2&#8290;</SUP> release from the SR is inversely involved in the regulation of atrial myocytic ANP release.