Circadian Relationships Between Circulating Atrial Natriuretic Peptides and Serum Calcium and Phosphate in Healthy Humans D.L. Vesely, R.B. Sothern, L.E. Scheving, F.W. Bremner, J.L.H.C. Third, J.B. McCormick, S. Dawson, S. Kahn, G. Augustine, M. Ryan, J. Greco, B.A. Nemchausky, P. Shirazi, and E.L. Kanabrocki Long-acting natriuretic peptide (LANP}, vessel dilator (VSDL}, and atrial natriuretic factor (ANF) consisting of amino acids (aa) 1 to 30, 31 to 67, and 99 to 126, respectively, of the 126-aa ANF prohormone circulate in humans. Among the biologic properties of these peptides is the ability of ANF to decrease intracellular calcium concentrations. To determine if atrial natriuretic peptides are directly related to serum calcium and/or phosphate in healthy normocalcemic humans, we examined 21 24-hour profiles of VSDL, LANP, ANF, and serum calcium and phosphate in 14 healthy humans. VSDL, LANP, and ANF each had significant (P < .001) circadian rhythms, with peak concentrations late during sleep (at 4:00 AM) being nearly twice the concentrations in the afternoon and evening. Serum calcium and phosphate also had significant circadian rhythms (P < .001} with troughs nearly opposite to those of the atrial natriuretic peptides, suggesting that atrial peptides may be important in the modulation of the circadian rhythms of calcium and phosphate. The nearly identical circadian rhythms of the atrial natriuretic peptides and of parathyroid hormone (PTH) reported by others, along with evidence that PTH may increase atrial peptide release, suggest that some of the effects attributed to PTH may be mediated by atrial natriuretic peptides. Copyright © 1996 by W.B. Saunders Company CALCIUM is to be of the of thought one regulators atrial natriuretic factor (ANF) synthesis and secre- tion. 1 When primary cultures of neonatal rat cardiocytes are exposed for 24 hours to 2 mmol/L CaCI2 in the culture media, ANF mRNA increases threefold. 1 Addition of calcium-channel-blocking agents to these cardiocytes re- sults in a 25% to 40% decrease in the synthesis and secretion of ANF. I Both oral and intravenous calcium have been reported to increase ANF release, z-4 Calcium infu- sions that increase serum calcium to within the normal physiologic range will also increase circulating ANF levels) Some of the effects of ANF, on the other hand. are calcium-dependent. 6,7 The calcium-dependent effects do not appear to be due to calcium uptake, since ANF does not increase 45Ca2+ uptake. 7 Rather, ANF can also decrease free Ca z+ within the cell 8,9 which, in part, appears to be due to its enhancing calcium extrusion from the cell. 7A° Stimulation of the ANF gene by calcium results in increased synthesis of the 126-amino acid (aa) ANF prohor- rnone. 1 This 126-aa prohormone is processed by proteases to produce several atrial natriuretic peptide hormones 11 (Fig 1). These peptides consisting of aa 1 to 30 (long-acting natriuretic peptide [LANP]), aa 31 to 67 (vessel dilator [VSDL]), and aa 99 to 126 (ANF) of the 126-aa ANF prohormone circulate 12,13 and have known biologic effects (blood pressure decrease, water, sodium, and/or potassium excretion) in animals 14,15 and humans. 16,17 These atrial natriuretic peptides (VSDL, LANP, and ANF) have a circadian rhythm in healthy diurnally active individuals, whose peak concentrations in the middle of the night I8,19 are near the 24-hour nadir of serum calcium. 2°,21 These circadian studies suggest that serum calcium may be regulated by atrial natriuretic peptides in healthy humans in an inverse relationship, whereby normal circadian in- creases in atrial natriuretic peptides may result in a de- crease in serum calcium. The present investigation was designed to determine at each time point throughout the day whether one or more of the atrial peptides have a temporal (circadian) and/or functional (overall) relation- ship to calcium and/or phosphate. SUBJECTS AND METHODS Subjects Ten clinically healthy men were studied in 1988 (mean age, 46.5 years; range, 41 to 60) and 11 men were evaluated in 1993 (mean age, 55.2 years; range, 46 to 72). Clinical characteristics and 24-hour mean blood values of these men are listed in Table 1. On each occasion, the volunteers were admitted to the clinical re- search unit of the Hines Veterans Affairs Hospital for one 24-hour period on May 15. 1988, and 5 years later on May 14, 1993. Each subject received a complete physical, and none were found to have any predisposing condition or were on any prescribed or over-the- counter medication. Nutritional assessments, including anthropo- metric measurements, were made for each subject, and all were found to have a good nutritional status. Meals prepared by the dietetic service of the hospital consisted of a general hospital diet totaling 2.500 cal and were served at 7:30 AM. 1:30 PM. and 4:30 PM. Mean calcium and phosphate intakes during this investigation were 952 mg (range, 737 to 1.0681 and 1.452 mg (range, 1,292 to 1,508), respectively. Calcium and phosphate intakes were not restricted before this investigation. Water intake was not restricted except for From the Department of Internal Medicine. University of South Florida Health Sciences Center. Tampa; James A. Haley Veterans Administration Hospital, Tampa, FL; The Rhythmometry Laboratory, St. Paul-Ramsey Medical Center, St. Paul." and the University of Minnesota, Minneapolis, MN; the University of Arkansas for Medical Sciences, Little Rock; and John L. McClellan Memorial Veterans Administration Hospital Little Rock, AR: the Cardiovascular Insti- tute, Chicago," and the Pathology Department. Foster G. McGraw Hospital, Loyola University Medical Center. Chicago; Pathology Medicine. Swedish Covenant Hospital, Chicago; and the Nursing Service. Nuclear Medicine Service. and Spinal Cord Injury Service, Edward J. Hines. Jr.. Veterans Affairs Hospital, Hines, IL. Submitted October31, 1995; accepted March 4, 1996. Supported in part by a Merit Review Grant from the US Department of Veterans Affairs (D. L. V. ); the American Heart Association, Florida Affiliate: the Medical Research Service of the Department of Veterans Affairs: and the Vascular Disease Research Foundation. Skokie, IL. Address reprint requests to D.L. Vesely, MD, PhD, Yames A. Haley Veterans Administration Hospital-151. 13000 Bruce B. Downs Blvd. Tampa, FL 33612. Copyright © 1996 by W.B. Saunders Company 0026-0495/96/4508-0019503.00/0 Metabolism, Vol 45, No 8 (August),1996:pp 1021-1028 1021