Platelet binding sites for atrial natriuretic factor in humans. Characterization and effects of sodium intake.

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Abstract
Platelets bear receptors for vasoactive peptides such as angiotensin II and vasopressin. The presence of binding sites for another vasoactive peptide, atrial natriuretic factor, was therefore investigated in human platelets. 125I-labeled synthetic atrial natriuretic factor bound specifically to human platelets. Steady-state and kinetic experiments demonstrated the existence of one class of high-affinity low-capacity binding sites for atrial natriuretic factor in platelets with a dissociation constant of 30 pM. The order of potency of atrial natriuretic factor fragments showed that the structural requirements of the high-affinity binding site detected were similar to those of receptors for atrial natriuretic factor in rat blood vessels and adrenal zona glomerulosa. To study the regulation of these binding sites by sodium, normal young men were subjected successively in random order to a low-sodium (40 mmol per day) and high-sodium (300 mmol per day) diet for 4 days. Binding of atrial natriuretic factor to platelets was higher with the low-sodium diet (10.3 +/- 1.0 sites per cell) than with the high-sodium diet (7.1 +/- 0.7 sites per cell). In conclusion, human platelets bear binding sites for atrial natriuretic factor, the density of which may be modulated by sodium intake. The platelet is a useful model for investigating atrial natriuretic factor receptors in different physiopathological conditions in humans.