THE VARIETY OF EXTRACELLULAR INFO CARRIERS * * * Heart as an Endocrine Gland 1 More than pumps, the atria secrete a recently discovered hormone, atrial natriuretic factor, that interacts with other hormones to fine-tune control of blood pressure and volume. The heart is a pump : a muscular organ that contracts in rhythm, impelling the blood firs to the lungs for oxygenation and then out into the vascular system to supply oxygen and nutrients to every cell in the body. That has been known since the publication in 1628 of William Harvey=s Essay on the motion of the Heart and the Blood in Animals. Within the past few years has been discovered that the heart is something more than a pump. It is also an endocrine gland. It secrets in a powerful peptide hormone called atrial natriuretic factor (ANF). The hormone has an important role in the regulation of blood pressure and blood volume and in the exertion of water, sodium and potassium. It exerts it s effects widely: on the blood vessels themselves , on the kidneys and the adrenal glands and on a large number of regulatory regions in the brain. The discovery of ANF solved a long-standing mystery. As early as 1935 the late John Peters of the Yale University School of Medicine speculated theat there must be mechanism in or near the heart to Asense the fullness of the bloodstream@ and finetune the regulation of blood volume. During the 1950's and 1960's numerous investigators searched in vain for a hypothesized Anatriuretic hormone@. Such a hormone would explain the occurrence of natriuresis (excretion of sodium) and concomitant diuresis (excretion of water) in the absence of changes in known regulatory processes. Such unexplained natriuresis and diuresis were observed to follow distention of the atria, the two upper chambers of the heart, which receive blood from the pulmonary veins of the vena cava and delivery it to the adjoining ventricles. The putative hormone was referred to as the Athird factor@, since it would complement the activity of to known regulators of blood pressure and blood volume :the hormone aldosterone and the filtration of the blood by the kidney. The first step toward the discovery of the third factor came in the 1956, when Bruno Kisch of the American College of Cardiology noted the presence of what he called dense bodies in the cardiocytes, or guinea pig atria. In 1964 James D. Jamiseon and George E. Palade of the Yale School of Medicine reported that such bodies, whose function was still not known, seemed to be present in the atria of all mammals they examined, including human beings. Our group at the University of Montreal noted in 1974 that the granules were very similar to storage granules seen in the endocrine (hormone-secreting) cells of, for example, the pancreas or the anterior pituitary gland. We found that when radioactively labelled amino acids were introduced into animals, they rapidly appeared in the atrial granules, incorporated into newly synthesized polypeptide=s (protein chains )- just as they would in the storage granules of endocrine cells. In 1976 Pierre-Yves Hatt and his colleagues at the University of Paris correlated current knowledge about the granules with earlier findings covering the regulation of sodium and water levels. They showed the number of granules in the atrial cardiocytes increases when the amount of sodium in an animal diet is reduced..This implied that the granules must store some substance that has to do with sodium balance. A breach through was made in 1981 , when Adolfo J. de Bold, Harald Sonnenberg and their colleagues at Queen=s University at Kingston in Ontario injected homogenized rat atria into rats and observed a rapid , massive and short-lasting diuresis and natriuresis. They concluded that the atria indeed contained a Afactor@ that promotes these effects , and they named it atrial natriuretic factor. In the next three years at the firs direct evidence was reported for the location and biochemical identity of ANF. Workers found there are from two to two and a half times as many of granules in the right atrium as there are in the left atrium in rats ..The granules are highly concentrated near the surface of the heart and in the exterior regions of the atria. They have not been found in the ventricles of rats or any other mammals, and the injection of the mammalian ventricular extracts does not affected blood vessels, diuresis or natriuresis. In contrast, granules have been discovered in the ventricles as well as in the atria of nonmammalian species and can be shown to be related to diuretic and natriuretic effects. The presence of the granules in the ventricles of nonmammalian species but not in the ventricles of mammals is consistent with the fact that heart cells tend to be more specialized in higher species. Once the location of ANF was determined, the peptide was isolated and purified in June, 1983, by our group; it was synthesized two months later by Ruth F. Nutt of the Merck Sharp&Dohme Research Laboratories and her colleagues. ANF is the active part of a larger precursor molecule. When various groups determined the amino acid sequence of the polypeptide , they all found ANF has the same core of 21 amino acids .The active, circulating hormone in the rat has 28 amino acids and a molecular weight of 3,060. The active hormone is attached to an inactive peptide of 100 amino acids and 24- amino-acid signal peptide that is cleaved when the molecule is synthesized. The circulating form in the human has not yet been determined, although we strongly suspect that it too is made up of 28 amino acids. Recently the human ANF gene has been cloned and sequenced, making it possible to synthesize the hormone chemically or by inserting the gene into yeast or bacteria. By either method the hormone can be produced in quantity for studies of its activity throughout the body. In addition antibodies to the hormone have been developed , so that sensitive immunological tests can be carried out to trace the release of ANF and to find the sites where it is active. When rats are subjected to the stress of immobilization, there sis a five- to twentyfold increase in the blood level of ANF. The release of ANF from the heart was also measured in human patients with valvular disease and expansion of blood volume who were undergoing cardiac catheterization, a procedure that allows sampling of blood in the arteries and heart chambers. The plasma level of ANF was from two to eight times as high in venous blood from the coronary sinus , which drains the heart=s atria , as it was in blood circulating in the arteries or peripheral veins. This confirms that ANF is released primarily, if not exclusively, by cells in the atria..In both the rats subjected to stress and the catheterized patients the atrial cardiocytes are stretched, and the stretching is the signal for the release of ANF. An increase in blood volume can also cause the atrial cardiocytes to stretch and release ANF, as was shown when the blood volume of rats was experimentally increased by the infusion of a salt solution. Once the cardiocytes respond to stretching by releasing ANF, the peptide travels through the arteries to targets in the kidneys, the adrenal glands, the brain and various other tissues. In general what ANF does is to modify the activity of a complicated homoeostatic feedback loop regulating blood pressure, blood volume and sodium retention: the renin-angiotensin system, which links certain functions of the brain, heart, arteries, adrenals, kidneys and other organs . One of the key substances in the system is the enzyme renin. It is secreted by cells in the arteries that lead to the glomeruli, which are saclike strictures at the entrance to the kidney. These juxtaglomerular cells secrete renin into the bloodstream whenever the level of sodium is low in the distal tubules of the kidney or when the local pressure in the kidney is low . Circulating renin cleaves a polypeptide called angiotensinogen to produce angiotensin I, which in turn is converted into angiotensin II. This small peptide is a powerful constrictor of vascular smooth muscle. Angiotensin II also has a feedback effect, partially suppressing rening secretion from the juxtaglomerular cells. Finally, it stimulates the adrenal gland to secrete the hormone aldosterone, which travels to the kidney and the posterior pituitary to inhibit the excretion of sodium and water. ANF affects the renin-angiotensin system by somehow inhibiting the secretion of renin and also by directly inhibiting the adrenal secretion of aldosterone. The relation between ANF aldosterone was elucidated in our laboratory by a series of experiments with cultured bovine and rat adrenal cells. ANF inhibited the normal production of adrenal aldosterone by 20 percent; it reduced by from 40 to 70 percent the stepped-up production that ordinarily follows stimulation of the adrenal cells with angiotensin II or the pituitary hormone ACTH.. Similar significant decreases in plasma aldosterone levels have been noted in rats and dogs after the injection of ANF. In order to determine whether the decreases in aldosterone levels were caused by the presence of specific binding sites for ANF on the surface of adrenal cells, we introduced radioactively labeled ANF info adrenal-cell cultures and then added Acold@, or unlabeled , ANF. We observed that the concentration of the labeled ANF decreased significantly after introduction of the cold ANF, a sign that the cold ANF was displacing that the labeled peptide at many sites on the surface of the cells. We determined that ANF binds at very specific sites on the cell by introducing ACTH , angiotensin II and other active peptides.