Administration of pharmacological doses of arginine-vasopressin, related peptides, and other pressor agents induced a profound release of atriopeptin immunoreactivity into the circulation. The stimulated release of atriopeptin apparently was related to increased arterial blood pressure. Neither the nonpressor vasopressin analog 1-deamino-D-Arg8-vasopressin nor arginine-vasopressin in the presence of a specific pressor antagonist caused atriopeptin to be released into the circulation. Urine output was correlated with the level of atriopeptin released. Physiological levels of arginine-vasopressin suppress diuresis and produced vasoconstriction. Pharmacological levels of the hormone stimulated the cardiac endocrine system to release atriopeptin, which may cause diuresis and vasodilation to physiologically antagonize the effects of vasopressin.

Antisera to atriopeptin III and to a cyanogen bromide fragment of the precursor molecule atriopeptigen were prepared and used to examine the distribution of atriopeptin-like immunoreactive material in the heart and brain of the rat. Granules of this material were seen in myocytes throughout the right and left atria and were densest in the perinuclear region. The distribution of atriopeptin-like immunoreactive material in the heart is consistent with previous reports of atrial secretory granules. In the brain neurons containing the material were observed in the hypothalamus and the pontine tegmentum. Atriopeptin in the brain may serve as a neurotransmitter in neural systems controlling blood volume and composition, the same physiological functions regulated by blood-borne atriopeptin.

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Vasopressin induces a concentration-dependent increase in atriopeptin immunoreactivity in plasma. Rat plasma, rat atrial extract, and synthetic atriopeptin III (APIII) produced parallel displacement curves of iodine-125-labeled APIII binding to specific antiserum. Fractionation of plasma atriopeptin immunoreactivity by reverse-phase high-performance liquid chromatography showed that the major portion consists of two species of low molecular weight peptides in a ratio of 10 to 1. Both peaks exhibited potent vasorelaxant activity, suggesting the presence of the carboxyl terminal Phe-Arg sequence of atriopeptin in each species. Sequence determination of the purified peptides indicated that the major peptide is Ser-Leu-Arg-Arg-APIII and the minor peptide APIII. It appears that the former is the major species of atrial peptide in the rat circulation and that it is the product of selective cleavage of the high molecular weight precursor.

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