Natriuretic response to homologous heart extract in aglomerular toadfish

Although the renal effects of the atrial natriuretic factor (ANF) have been widely investigated, the mechanism of its natriuretic and diuretic effect is still controversial. The purpose of this study was to investigate whether ANF has a specific tubular action distinct from any renal or systemic hemodynamic effects. Renal responses to homologous heart extract (HE) or synthetic ANF were examined in the aglomerular toadfish. A significant natriuresis and diuresis were observed following an intra-arterial injection of either HE (amount derived from 1 heart) or ANF (atriopeptin III, 20 micrograms). However, neither affected urinary excretion of K+. Synthetic ANF did not alter the excretion of Mg2+ or Ca2+. In addition HE showed a hypotensive effect, and ANF did not alter the mean arterial pressure significantly. Neither HE nor ANF affected heart rate. Injection of skeletal muscle extract or saline vehicle alone was without effect on renal function, mean arterial pressure, or heart rate. These results support the hypothesis that ANF has a specific tubular action to increase urinary Na+ and water excretion. The presence of a natriuretic factor was demonstrated in the heart of the fish.