Administration of a newly discovered second
atrial peptide,
iso-atrial natriuretic peptide (or iso-rANP(1-45) for the rat), caused
hypotension, decreased heart rate, diuresis, and increased renal excretion of Na+, K+, and Cl- in the anesthetized rat. Bolus
injections of chemically synthetic
iso-rANP(1-45) had circulatory and
diuretic activity equal to or greater than rANP(99-126) but, at low doses, a lesser effect on renal
electrolyte excretion. The synthetic
peptide fragment, iso-rANP(17-45), analogous in structure to rANP(99-126), had attenuated activity on the circulation, and at low doses, attenuated activity on the kidney. At higher doses, where renal responses to rANP(99-126) were less (downside of a biphasic response), both
iso-rANP(1-45) and (17-45) had greater effects on water and
electrolyte excretion than rANP(99-126).
Injections of
iso-rANP(1-45) and (17-45) increased hematocrit, whereas rANP(99-126) did not; furthermore, unlike rANP(99-126), iso-rANP did not affect arterial plasma Na+ concentration. The heart produces at least two genetically different
atrial natriuretic peptides which affect the circulation and
salt and water balance.