CALCIUM DEPENDENCY OF FREQUENCY-STIMULATED ATRIAL-NATRIURETIC-PEPTIDE SECRETION

In this study we examined the mechanism whereby atrial natriuretic peptide secretion is increased when the frequency of contraction is raised from 2 to 5 Hz. We tested the hypothesis that calcium plays a significant role in the frequency-stimulated response. Using superfused rat left atria, we found that lowering the superfusate calcium concentration from 1.8 to 0.2 mmol/L abolished the frequency-stimulated atrial natriuretic peptide secretory response. Superfusion with ryanodine (1 mu mol/L), an inhibitor of sarcoplasmic reticulum calcium release, resulted in a minimal inhibitory effect. Superfusion with 50 mu mol/L nitrendipine or 10 mu mol/L diltiazem inhibited the frequency-stimulated response by 46% to 48%. The lack of total inhibition suggested that an additional mechanism of calcium influx was involved, namely, inward calcium movement carried by Na+-Ca2+ exchange. As intracellular sodium has been reported to rise with an increase in beat frequency, a fall in the sodium gradient would favor inward calcium movement by Na+-Ca2+ exchange. Because we could not directly assess the role of Na+-Ca2+ exchange in this experimental paradigm, we examined the effect of lowering the transmembrane sodium gradient on atrial natriuretic peptide secretion by superfusion with the sodium channel activator veratridine or the sodium ionophore monensin. Superfusion with 1 mu mol/L veratridine increased atrial natriuretic peptide secretion by 2.3-fold, and 1, 5, and 10 mu mol/L monensin increased secretion by 1.1-, 2.1-, and 15.7-fold, respectively. In addition, we examined the possibility that the reported rise in intracellular sodium associated with increased beat frequency was due to enhanced Na+-H+ antiporter activity. Superfusion with the Na+-H+ antiporter inhibitor 5-(N,N-hexamethylene)-amiloride (25 mu mol/L) abolished the frequency-stimulated secretory response. We conclude the following: (1) Frequency-stimulated atrial natriuretic peptide secretion is dependent on calcium influx; (2) calcium influx through L-type calcium channels plays a significant role in the frequency-stimulated response; (3) by inference, Na+-Ca2+ exchange is also an important mechanism of calcium influx; and (4) calcium release from the sarcoplasmic reticulum plays only a minor role in the frequency-stimulated response. These results lend further support to the concept that calcium is an important second messenger in regulated secretion of atrial natriuretic peptide.