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Effects of atriopeptins on relaxation and cyclic GMP levels in human coronary artery in vitro
European Journal of Pharmacology, 124 (1986) 193-196
193
Elsevier
Short communication EFFECTS OF ATRIOPEPTINS ON RELAXATION A N D CYCLIC GMP LEVELS IN H U M A N C O R O N A R Y ARTERY IN VITRO i ROBERT M. RAPOPORT *, ROBERT GINSBURG, SCOTT A. WALDMAN and FERID MURAD
Departments of Medicine and Pharmacology, Stanford University and Veterans Administration Medical Center, Palo Alto, CA 94304, U.S.A. Received 10 March 1986, accepted 18 March 1986
R.M. RAPOPORT, R. GINSBURG, S.A. WALDMAN and F. MURAD, Effects of atriopeptins on relaxation and cyclic GMP levels in human coronary artery in vitro, European J. Pharmacol. 124 (1986) 193-196. The effects of atriopeptins on relaxation and cyclic GMP levels were examined in human coronary artery, a-Atrial natriuretic polypeptide and atriopeptins I, II and III all induced relaxation. Relaxations to atriopeptin I were of a smaller magnitude. The atriopeptins elevated cyclic GMP levels from 2- to 3-fold. These studies suggest that atriopeptins released from the heart may dilate the vasculature of this organ and increase coronary blood flow through the formation of cyclic GMP. a-Atrial natriuretic polypeptide
Human coronary artery
1. Introduction
The atriopeptins are a recently discovered class of hormonal peptides that appear to play an important role in the regulation of fluid and electrolyte balance and blood pressure (Needleman et al., 1985). Indeed, injections of atriopeptins have been shown to cause natriuretic, diuretic and hypotensive responses (Richards et al., 1985; Sugawara et al., 1985). Atriopeptins have also been shown to be released from the heart and circulate in the body in humans (Sugawara et al., 1985). One of the actions of the atriopeptins is a direct relaxant effect on isolated blood vessels (Geller et
1 This work was supported by Research Grants AM 30787 and HL 28474 from the National Institutes of Health, grants from the Veterans Administration, and the Council for Tobacco Research, U.S.A., Inc. The Cardiac Transplantation Program kindly provided human coronary vessels and is supported by Research Grant HL 13108 from the National Institutes of Health. * To whom all correspondence should be addressed: Department of Pharmacology and Cell Biophysics, University of Cincinnati College of Medicine, 231 Bethesda Avenue, M.L. No. 575, Cincinnati, OH 45267-0575, U.S.A. 0014-2999/86/$03.50 © 1986 Elsevier Science Publishers B.V.
Atriopeptins
cGMP
al., 1984; Winquist, 1985). However, the relaxant effects of the atriopeptins varies considerably among different blood vessels, and between species. In addition, the relaxant effects of the atriopeptins depends upon the peptide length of atriopeptin (Geller et al., 1984; Winquist, 1985; Rapoport et al., 1986). It was also hypothesized that the atriopeptins may induce relaxation through a mechanism similar to that of the nitrovasodilators, since the vasodilator profile of atriopeptin was similar to that of sodium nitroprusside (Winquist, 1985). Atriopeptin-induced relaxation was also demonstrated to be associated with the formation of cyclic G M P (Rapoport et al., 1985; 1986; Winquist, 1985). Similar correlations between cyclic G M P levels and vascular smooth muscle relaxation have been observed with the nitrovasodilators (Rapoport and Murad, 1983). Thus, the purpose of the present study was to investigate: (1) whether atriopeptins induce relaxation of human coronary artery; (2) the effect of peptide length on relaxant potency; and (3) whether the relaxation was associated with the formation of cyclic GMP.
194 2. Materials and methods
TABLE 1 Effects of atriopeptins on cyclic GMP levels in human coronary artery. Human coronary arteries were exposed to 40 mM KCI (see Methods) followed by 0.1 pM atripeptin I, II or a-atrial natriuretic polypeptide for 10 min. Tissues were then frozen, homogenized and assayed for cyclic GMP (see Methods). Shown are means-+ S.E. (N).
2.1. Relaxation studies Right, left a n d circumflex epicardial h u m a n c o r o n a r y arteries were dissected a n d p r e p a r e d as previously d e s c r i b e d ( G i n s b u r g et al., 1984). Briefly, coronaries were r e m o v e d from e x p l a n t e d h u m a n hearts p r o v i d e d b y the S t a n f o r d C a r d i a c T r a n s p l a n t a t i o n P r o g r a m with i n f o r m e d consent. T h e patients were n o t e x p o s e d to n i t r o v a s o d i l a t o r s for at least 72 h p r i o r to surgery. T h e arteries were free from gross atherosclerotic disease b y visual inspection. Vessels were cleaned of e x t r a n e o u s fatty and connective tissue, cut into 5 m m ring segments, a n d then h a d their e n d o t h e l i u m rem o v e d b y placing a w o o d e n stick through the l u m e n of the vessel a n d r u b b i n g the i n t i m a l surface. T h e effectiveness of this p r o c e d u r e at r e m o v i n g the e n d o t h e l i u m was c o n f i r m e d histologically a n d also t h r o u g h the i n a b i l i t y to relax to the C a 2÷ i o n o p h o r e A23187. R e l a x a t i o n of h u m a n c o r o n a r y artery to the C a 2÷ i o n o p h o r e A23187 has b e e n shown to b e d e p e n d e n t u p o n the presence of the e n d o t h e l i u m ( R a p o p o r t et al., 1985a). Segm e n t s were m o u n t e d in organ b a t h s c o n t a i n i n g K r e b s - R i n g e r b i c a r b o n a t e solution a n d p l a c e d at 1.2 g-force resting tension which was m a i n t a i n e d t h r o u g h o u t the e x p e r i m e n t ( G i n s b u r g et al., 1984).
2 .I0
b AP
I
Treatment
Cyclic GMP (pmol/mg protein)
KCI (40 raM) KC1 (40 mM)+ atriopeptin I (0.1 #M) KCI (40 mM) + atriopeptin III (0.1 #M) KC1 (40 mM) + a-atrial natriuretic polypeptide (0.1/~M)
0.10_+0.02 (13) *
AP
0.24 __0.04 (14) 0.29-+0.04 (14)
* Significantly less than tissue exposed to atriopeptin I, Ill or a-atrial natriuretic polypeptide. Tissues were allowed to equilibrate for at least 60 m i n p r i o r to a d d i t i o n of any agents. Segments were then c o n t r a c t e d with 40 m M KC1 and, d u r i n g the p l a t e a u contraction, various a t r i o p e p t i n s were a d d e d in single or in cumulative concentrations.
2.2. Cyclic G M P measurements H u m a n c o r o n a r y artery ring segments without e n d o t h e l i u m were p l a c e d in flasks c o n t a i n i n g
AP
c
a-hANP
0.20 _+0.03 (15)
, III
II
2 0
I
0
i
i
50
i
i
100
I
I
150
I
I
II
200 0
I
I
i
50
L
i
100
I
150
i
i
I
50
L
100
150
Time (min)
Fig. 1. Effects of atriopeptins on relaxation on human coronary artery. Coronaries were contracted with 40 mM KC1 (see Methods) and then exposed to 0.1 #M of the following peptides: atriopeptin I (AP-I), followed by a-atrial natriuretic polypeptide (a-hANP (a)); atriopeptin II (AP lI (b)); atriopeptin III (AP III (c)). A maximally effective relaxant concentration of nitroglycerin was then added (10 pM; NTG).
195 Krebs-Ringer bicarbonate solution and exposed to 40 mM KC1 as above. Atriopeptins were added for various times and concentrations and then frozen between clamps precooled in liquid nitrogen. Cyclic GMP levels were assayed as previously described (Rapoport et al., 1985). Briefly, frozen tissues were homogenized in 6% trichloroacetic acid and the homogenates centrifuged. The supernatant fractions were then extracted with ether, acetylated and radioimmunoassayed for cyclic GMP. Statistical significance was accepted at the 0.05 level of probability using Student's t-test. 2.3. Materials
Atriopeptin I (rat, 21 amino acids), II (rat, 23 amino acids) and III (rat, 24 amino acids) and a-atrial natriuretic polypeptide (human, 28 amino acids), were obtained from Peninsula Laboratories, and nitroglycerin (1:10, nitroglycerin: lactose) from ICI Americas, Inc. Other materials were obtained as previously described (Rapoport et al., 1985).
3. Results
Atriopeptins II, III and a-atrial natriuretic polypeptide (0.1 #M) all relaxed human coronary artery contracted with 40 mM KCI to approximately the same magnitude (fig. 1). Relaxations to 1, 10 and 100 nM atriopeptin III were 5.8 + 1.9, 14.6 __+2.4 and 38.0 + 4.8% of the maximal relaxation to nitroglycerin, respectively ( m e a n + S.E.; N = 7 in each case). In contrast, 0.1 #M atriopeptin I induced only a small amount of relaxation (fig. 1). Cyclic G M P levels were also elevated 2- to 3-fold by the atriopeptins (table 1).
4. Discussion
The present study demonstrates that the atriopeptins relax human coronary artery. This is the first observation, to our knowledge, that the atriopeptins will relax coronary vessels, and, in addition, have a direct relaxant effect on the human vasculature. Furthermore, the relaxant effects
are independent of the endothelium. The atriopeptins only partially relaxed human coronary arteries contracted with KC1. We and others have demonstrated that relaxations to the atriopeptins were greatly inhibited in blood vessels contracted with KC1 as compared to those contracted with agonists which act at receptors (Rapoport et al., 1985; Winquist, 1985). Isoproterenol induced maximal relaxation of approximately 20% of the contractile response to 40 mM KC1 (data not shown). It was necessary to contract the human coronary artery with KC1, since contractions induced by a variety of agonists, including histamine, serotonin, carbachol, angiotensin and phenylephrine, were not sustained (unpublished observation). Furthermore, many of these agonists induced phasic contractions. Lower concentrations of KC1 did not consistently induce maintained, plateau responses (data not shown). Atriopeptins II, III and a-natriuretic polypeptide relaxed the coronary artery to a greater extent than atriopeptin I. This relative potency is similar to that observed in rabbit aorta (GeUer et al., 1984; Winquist, 1985). In contrast, atriopeptin I was only slightly less potent than atriopeptin II at relaxing the rat aorta (Rapoport et al., 1986). Thus, the rabbit aorta atriopeptin receptor may be similar to that present in the human vasculature, although this awaits further investigation. Exposure of the human coronary artery to the atriopeptins also induced increased levels of cyclic GMP. Similar elevations of cyclic GMP levels have been observed in animal blood vessels (Rapoport et al., 1985 and unpublished observations; 1986; Winquist, 1985). Thus, the mechanism for atriopeptin-induced relaxation of the human vasculature may also be through the formation of cyclic GMP. The magnitudes of increased cyclic GMP levels, although significant, were relatively small. Thus, it was difficult to accurately compare the magnitude of increased cyclic GMP levels by the different atriopeptins. Furthermore, the relationship between magnitude of relaxation and cyclic GMP level could not be accurately determined due to the small elevation in cyclic G M P level. It has recently been demonstrated that the atriopeptins are released from the heart and cir-
196 c u l a t e i n the b o d y i n h u m a n s ( S u g a w a r a et al., 1985). F u r t h e r m o r e , i n j e c t i o n s o f a t r i o p e p t i n s h a v e b e e n s h o w n to c a u s e n a t r i u r e t i c , d i u r e t i c a n d hyp o t e n s i v e r e s p o n s e s in h u m a n s ( R i c h a r d s et al., 1985; S u g a w a r a et al., 1985). T h e p r e s e n t results f u r t h e r suggest t h a t the h e a r t m a y regulate its o w n b l o o d flow t h r o u g h the release of a t r i o p e p t i n s . T h e v a s o d i l a t o r y effects of the a t r i o p e p t i n s o n the c o r o n a r y arteries m a y b e m e d i a t e d t h r o u g h the f o r m a t i o n o f cyclic G M P .
Acknowledgements We thank Karen Schwartz and Pauline Zera for their technical assistance, Liz Wendelmoot, Robin Wright and Kathryn Siefert for help in preparation of the manuscript, and Gwen Kraft for the illustration.
References Geller, D.M., M.G. Currie, K. Wakitani, B.R. Cole, S.P. Adams, K.F. Fok, N.R. Siegel, S.R. Eubanks, G.R. Galluppi and P. Needleman, 1984, Atriopeptins: a family of potent biologically active peptides derived from mammalian atria, Biochem. Biophys. Res. Commun. 120, 333. Ginsburg, R., M.R. Bristow, K. Davis, A. Dibiase and M.E. Billingham, 1984, Quantitative pharmacologic responses of
normal and atherosclerotic isolated human epicardial coronary arteries, Circulation 69, 430. Needleman, P., S.P. Adams, B.R. Cole, M.G. Currie, D.M. Geller, M.L. Michener, C.B. Saper, D. Schwartz and D.G. Standaert, 1985, Atriopeptins as cardiac hormones, Hypertension 7, 469. Rapoport, R.M. and F. Murad, 1983, Endothelium-dependent and nitrovasodilator-induced relaxation of vascular smooth muscle: role of cyclic GMP, J. Cycl. Nucl. Prot. Phosphor. Res. 9, 281. Rapoport, R.M., S.A. Waldman, K. Schwartz, R.J. Winquist and F. Murad, 1985, Effects of atrial natriuretic factor, sodium nitroprusside, and acetylcholine on cyclic GMP levels and relaxation in rat aorta, European J. Pharmacol. 115, 219. Rapoport, R.M., R.J. Winquist, S.A. Waldman and F. Murad, 1986, Effects of atriopeptins on relaxation and cyclic GMP levels in rat and rabbit aortas, European J. Pharmacol. 120, 123. Richards, A.M., H. Ikram, T.G. Yandle, M.G. Nicholls, M.W.I. Webster and E.A. Espiner, 1985, Renal, haemodynamic, and hormonal effects of human alpha atrial natriuretic peptide in healthy volunteers, Lancet 1, 545. Sugawara, A., K. Nakao, N. Morii, M. Sakamoto, M. Suda, M. Shimokura, Y. Kiso, M. Kihara, Y. Yamori, K. Nishimura, J. Soneda, T. Ban and H. Imura, 1985, a-Human atrial natriuretic polypeptide is released from the heart and circulates in the body, Biochem. Biophys. Res. Commun. 129, 439. Winquist, R.J., 1985, The relaxant effects of atrial natriuretic factor on vascular smooth muscle, Life Sci. 37, 1081.
193
Elsevier
Short communication EFFECTS OF ATRIOPEPTINS ON RELAXATION A N D CYCLIC GMP LEVELS IN H U M A N C O R O N A R Y ARTERY IN VITRO i ROBERT M. RAPOPORT *, ROBERT GINSBURG, SCOTT A. WALDMAN and FERID MURAD
Departments of Medicine and Pharmacology, Stanford University and Veterans Administration Medical Center, Palo Alto, CA 94304, U.S.A. Received 10 March 1986, accepted 18 March 1986
R.M. RAPOPORT, R. GINSBURG, S.A. WALDMAN and F. MURAD, Effects of atriopeptins on relaxation and cyclic GMP levels in human coronary artery in vitro, European J. Pharmacol. 124 (1986) 193-196. The effects of atriopeptins on relaxation and cyclic GMP levels were examined in human coronary artery, a-Atrial natriuretic polypeptide and atriopeptins I, II and III all induced relaxation. Relaxations to atriopeptin I were of a smaller magnitude. The atriopeptins elevated cyclic GMP levels from 2- to 3-fold. These studies suggest that atriopeptins released from the heart may dilate the vasculature of this organ and increase coronary blood flow through the formation of cyclic GMP. a-Atrial natriuretic polypeptide
Human coronary artery
1. Introduction
The atriopeptins are a recently discovered class of hormonal peptides that appear to play an important role in the regulation of fluid and electrolyte balance and blood pressure (Needleman et al., 1985). Indeed, injections of atriopeptins have been shown to cause natriuretic, diuretic and hypotensive responses (Richards et al., 1985; Sugawara et al., 1985). Atriopeptins have also been shown to be released from the heart and circulate in the body in humans (Sugawara et al., 1985). One of the actions of the atriopeptins is a direct relaxant effect on isolated blood vessels (Geller et
1 This work was supported by Research Grants AM 30787 and HL 28474 from the National Institutes of Health, grants from the Veterans Administration, and the Council for Tobacco Research, U.S.A., Inc. The Cardiac Transplantation Program kindly provided human coronary vessels and is supported by Research Grant HL 13108 from the National Institutes of Health. * To whom all correspondence should be addressed: Department of Pharmacology and Cell Biophysics, University of Cincinnati College of Medicine, 231 Bethesda Avenue, M.L. No. 575, Cincinnati, OH 45267-0575, U.S.A. 0014-2999/86/$03.50 © 1986 Elsevier Science Publishers B.V.
Atriopeptins
cGMP
al., 1984; Winquist, 1985). However, the relaxant effects of the atriopeptins varies considerably among different blood vessels, and between species. In addition, the relaxant effects of the atriopeptins depends upon the peptide length of atriopeptin (Geller et al., 1984; Winquist, 1985; Rapoport et al., 1986). It was also hypothesized that the atriopeptins may induce relaxation through a mechanism similar to that of the nitrovasodilators, since the vasodilator profile of atriopeptin was similar to that of sodium nitroprusside (Winquist, 1985). Atriopeptin-induced relaxation was also demonstrated to be associated with the formation of cyclic G M P (Rapoport et al., 1985; 1986; Winquist, 1985). Similar correlations between cyclic G M P levels and vascular smooth muscle relaxation have been observed with the nitrovasodilators (Rapoport and Murad, 1983). Thus, the purpose of the present study was to investigate: (1) whether atriopeptins induce relaxation of human coronary artery; (2) the effect of peptide length on relaxant potency; and (3) whether the relaxation was associated with the formation of cyclic GMP.
194 2. Materials and methods
TABLE 1 Effects of atriopeptins on cyclic GMP levels in human coronary artery. Human coronary arteries were exposed to 40 mM KCI (see Methods) followed by 0.1 pM atripeptin I, II or a-atrial natriuretic polypeptide for 10 min. Tissues were then frozen, homogenized and assayed for cyclic GMP (see Methods). Shown are means-+ S.E. (N).
2.1. Relaxation studies Right, left a n d circumflex epicardial h u m a n c o r o n a r y arteries were dissected a n d p r e p a r e d as previously d e s c r i b e d ( G i n s b u r g et al., 1984). Briefly, coronaries were r e m o v e d from e x p l a n t e d h u m a n hearts p r o v i d e d b y the S t a n f o r d C a r d i a c T r a n s p l a n t a t i o n P r o g r a m with i n f o r m e d consent. T h e patients were n o t e x p o s e d to n i t r o v a s o d i l a t o r s for at least 72 h p r i o r to surgery. T h e arteries were free from gross atherosclerotic disease b y visual inspection. Vessels were cleaned of e x t r a n e o u s fatty and connective tissue, cut into 5 m m ring segments, a n d then h a d their e n d o t h e l i u m rem o v e d b y placing a w o o d e n stick through the l u m e n of the vessel a n d r u b b i n g the i n t i m a l surface. T h e effectiveness of this p r o c e d u r e at r e m o v i n g the e n d o t h e l i u m was c o n f i r m e d histologically a n d also t h r o u g h the i n a b i l i t y to relax to the C a 2÷ i o n o p h o r e A23187. R e l a x a t i o n of h u m a n c o r o n a r y artery to the C a 2÷ i o n o p h o r e A23187 has b e e n shown to b e d e p e n d e n t u p o n the presence of the e n d o t h e l i u m ( R a p o p o r t et al., 1985a). Segm e n t s were m o u n t e d in organ b a t h s c o n t a i n i n g K r e b s - R i n g e r b i c a r b o n a t e solution a n d p l a c e d at 1.2 g-force resting tension which was m a i n t a i n e d t h r o u g h o u t the e x p e r i m e n t ( G i n s b u r g et al., 1984).
2 .I0
b AP
I
Treatment
Cyclic GMP (pmol/mg protein)
KCI (40 raM) KC1 (40 mM)+ atriopeptin I (0.1 #M) KCI (40 mM) + atriopeptin III (0.1 #M) KC1 (40 mM) + a-atrial natriuretic polypeptide (0.1/~M)
0.10_+0.02 (13) *
AP
0.24 __0.04 (14) 0.29-+0.04 (14)
* Significantly less than tissue exposed to atriopeptin I, Ill or a-atrial natriuretic polypeptide. Tissues were allowed to equilibrate for at least 60 m i n p r i o r to a d d i t i o n of any agents. Segments were then c o n t r a c t e d with 40 m M KC1 and, d u r i n g the p l a t e a u contraction, various a t r i o p e p t i n s were a d d e d in single or in cumulative concentrations.
2.2. Cyclic G M P measurements H u m a n c o r o n a r y artery ring segments without e n d o t h e l i u m were p l a c e d in flasks c o n t a i n i n g
AP
c
a-hANP
0.20 _+0.03 (15)
, III
II
2 0
I
0
i
i
50
i
i
100
I
I
150
I
I
II
200 0
I
I
i
50
L
i
100
I
150
i
i
I
50
L
100
150
Time (min)
Fig. 1. Effects of atriopeptins on relaxation on human coronary artery. Coronaries were contracted with 40 mM KC1 (see Methods) and then exposed to 0.1 #M of the following peptides: atriopeptin I (AP-I), followed by a-atrial natriuretic polypeptide (a-hANP (a)); atriopeptin II (AP lI (b)); atriopeptin III (AP III (c)). A maximally effective relaxant concentration of nitroglycerin was then added (10 pM; NTG).
195 Krebs-Ringer bicarbonate solution and exposed to 40 mM KC1 as above. Atriopeptins were added for various times and concentrations and then frozen between clamps precooled in liquid nitrogen. Cyclic GMP levels were assayed as previously described (Rapoport et al., 1985). Briefly, frozen tissues were homogenized in 6% trichloroacetic acid and the homogenates centrifuged. The supernatant fractions were then extracted with ether, acetylated and radioimmunoassayed for cyclic GMP. Statistical significance was accepted at the 0.05 level of probability using Student's t-test. 2.3. Materials
Atriopeptin I (rat, 21 amino acids), II (rat, 23 amino acids) and III (rat, 24 amino acids) and a-atrial natriuretic polypeptide (human, 28 amino acids), were obtained from Peninsula Laboratories, and nitroglycerin (1:10, nitroglycerin: lactose) from ICI Americas, Inc. Other materials were obtained as previously described (Rapoport et al., 1985).
3. Results
Atriopeptins II, III and a-atrial natriuretic polypeptide (0.1 #M) all relaxed human coronary artery contracted with 40 mM KCI to approximately the same magnitude (fig. 1). Relaxations to 1, 10 and 100 nM atriopeptin III were 5.8 + 1.9, 14.6 __+2.4 and 38.0 + 4.8% of the maximal relaxation to nitroglycerin, respectively ( m e a n + S.E.; N = 7 in each case). In contrast, 0.1 #M atriopeptin I induced only a small amount of relaxation (fig. 1). Cyclic G M P levels were also elevated 2- to 3-fold by the atriopeptins (table 1).
4. Discussion
The present study demonstrates that the atriopeptins relax human coronary artery. This is the first observation, to our knowledge, that the atriopeptins will relax coronary vessels, and, in addition, have a direct relaxant effect on the human vasculature. Furthermore, the relaxant effects
are independent of the endothelium. The atriopeptins only partially relaxed human coronary arteries contracted with KC1. We and others have demonstrated that relaxations to the atriopeptins were greatly inhibited in blood vessels contracted with KC1 as compared to those contracted with agonists which act at receptors (Rapoport et al., 1985; Winquist, 1985). Isoproterenol induced maximal relaxation of approximately 20% of the contractile response to 40 mM KC1 (data not shown). It was necessary to contract the human coronary artery with KC1, since contractions induced by a variety of agonists, including histamine, serotonin, carbachol, angiotensin and phenylephrine, were not sustained (unpublished observation). Furthermore, many of these agonists induced phasic contractions. Lower concentrations of KC1 did not consistently induce maintained, plateau responses (data not shown). Atriopeptins II, III and a-natriuretic polypeptide relaxed the coronary artery to a greater extent than atriopeptin I. This relative potency is similar to that observed in rabbit aorta (GeUer et al., 1984; Winquist, 1985). In contrast, atriopeptin I was only slightly less potent than atriopeptin II at relaxing the rat aorta (Rapoport et al., 1986). Thus, the rabbit aorta atriopeptin receptor may be similar to that present in the human vasculature, although this awaits further investigation. Exposure of the human coronary artery to the atriopeptins also induced increased levels of cyclic GMP. Similar elevations of cyclic GMP levels have been observed in animal blood vessels (Rapoport et al., 1985 and unpublished observations; 1986; Winquist, 1985). Thus, the mechanism for atriopeptin-induced relaxation of the human vasculature may also be through the formation of cyclic GMP. The magnitudes of increased cyclic GMP levels, although significant, were relatively small. Thus, it was difficult to accurately compare the magnitude of increased cyclic GMP levels by the different atriopeptins. Furthermore, the relationship between magnitude of relaxation and cyclic GMP level could not be accurately determined due to the small elevation in cyclic G M P level. It has recently been demonstrated that the atriopeptins are released from the heart and cir-
196 c u l a t e i n the b o d y i n h u m a n s ( S u g a w a r a et al., 1985). F u r t h e r m o r e , i n j e c t i o n s o f a t r i o p e p t i n s h a v e b e e n s h o w n to c a u s e n a t r i u r e t i c , d i u r e t i c a n d hyp o t e n s i v e r e s p o n s e s in h u m a n s ( R i c h a r d s et al., 1985; S u g a w a r a et al., 1985). T h e p r e s e n t results f u r t h e r suggest t h a t the h e a r t m a y regulate its o w n b l o o d flow t h r o u g h the release of a t r i o p e p t i n s . T h e v a s o d i l a t o r y effects of the a t r i o p e p t i n s o n the c o r o n a r y arteries m a y b e m e d i a t e d t h r o u g h the f o r m a t i o n o f cyclic G M P .
Acknowledgements We thank Karen Schwartz and Pauline Zera for their technical assistance, Liz Wendelmoot, Robin Wright and Kathryn Siefert for help in preparation of the manuscript, and Gwen Kraft for the illustration.
References Geller, D.M., M.G. Currie, K. Wakitani, B.R. Cole, S.P. Adams, K.F. Fok, N.R. Siegel, S.R. Eubanks, G.R. Galluppi and P. Needleman, 1984, Atriopeptins: a family of potent biologically active peptides derived from mammalian atria, Biochem. Biophys. Res. Commun. 120, 333. Ginsburg, R., M.R. Bristow, K. Davis, A. Dibiase and M.E. Billingham, 1984, Quantitative pharmacologic responses of
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