Effect of Atrial Natriuretic Peptide on Hemodynamics of the Stage 21 Chick Embryo

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Abstract
Atrial natriuretic peptide (ANP) is important in the modulation of hemodynamics and fluid balance in the mature subject, but its hemodynamic effects at early stages of morphogenesis are not defined. We studied the effect of rat atriopeptin III on hemodynamics in chick embryos at Hamburger-Hamilton stage 21. The cardiovascular system is not yet innervated, nor is the kidney formed in these embryos. The vitelline arterial and venous blood pressures were measured with a servo-null, micro-pressure system and the dorsal aortic blood flow was measured with a 20 MHz pulsed Doppler velocity meter. The peptide was infused into the vitelline vein with a microinjector at doses of 0.1, 1.0, and 10 ng. Doses normalized by body wt of embryos averaged 0.003, 0.035, and 0.32 ng/mg (n =1), respectively. Vitelline arterial blood pressure decreased in a dose dependent manner [y =5.8 — 9.9x; r= —0.49; p<0.01 (y= % of baseline, x = log ng/mg)], and dorsal aortic blood flow, a measure of cardiac output, decreased similarly (y = 39.6 - 16.2x; r = -0.47; p<0.01). Heart rate did not change. Ten ng of ANP increased the vitelline venous diameter, determined directly under a microscope, from 125 ± 47 (SD) v.m to 139 49 ixm (n =1; p<0.01), and decreased vitelline venous pressure from 0.34 ± 0.05 mm Hg to 0.10 ± 0.07 mm Hg (n = 5). We conclude that ANP exerts its hemodynamic effect by direct venodilation in the noninnervated and anephric circulation. We speculate that ANP modulation of vascular tone and volume could be a mechanism for the regulation of the preinnervated embryonic cardiovascular system.