2.2. Regulation of ANP secretion
2.2.1. Mechanical factors
The major determinant of ANP secretion is atrial wall stretch (Lang et al. 1985, Ruskoaho et al. 1986a), but several other factors, such as rate of contraction, hormones and vasoactive peptides influence the release of ANP (for review see Ruskoaho 1992, de Bold et al. 1996) (Fig. 1). In vitro atrial stretch and in vivo volume expansion increase ANP release and it has been shown that wall stretch and not pressure per se is a direct stimulator of ANP release from the atria (Lang et al. 1985, Ruskoaho et al. 1986a, Edwards et al. 1988). The predominant stimulus controlling the release of BNP from the atria and ventricles as well as ANP release from the ventricles also appears to be the myocyte stretch (Mäntymaa et al. 1993, Kinnunen et al. 1993). The primary sensors for stretch-dependent natriuretic peptide secretion may be cardiac myocytes or other cell types, including endothelial and endocardial cells, and fibroblasts. However, it has not been established whether wall stretch acts directly or via local factors such as ET-1, nitric oxide (NO), and Ang II liberated in response to distension.
Figure 1. Some factors that have been shown to affect plasma ANP levels in vivo or stimulate ANP secretion in vitro.
An increase in heart rate and contractility are effective stimuli for ANP release both in vitro and in vivo (for review see Ruskoaho 1992). In isolated rat atria and in perfused rat hearts an increase in beating rate enhances ANP release (Schiebinger & Linden 1986, Doubell 1989). In humans both supraventricular (Tikkanen et al. 1985, Roy et al. 1987) and ventricular tachycardia (Crozier et al. 1987) increases plasma ANP concentrations. Tachycardia seems to increase ANP release by mechanisms associated with hemodynamic changes, such as increased mean atrial pressure.
ANP secretion is also stimulated by hypoxia both in vivo and in vitro (Baertschi et al. 1986, Lew & Baertschi 1989a). Atrial stretch, tachycardia, increased sympathetic activity and metabolic changes may be the factors which mediate hypoxia-induced increase in ANP release. In isolated perfused rat hearts reduction of coronary flow produces an increase in ANP release and this response shows a positive correlation with the lactate to pyruvate ratio and a negative correlation with phosphorylation potential (Uusimaa et al. 1992a). Thus, ANP release may be modulated by changes in myocardial energy metabolism.
2.2.2. Neurohumoral factors and pressor hormones
Although increase in atrial wall stretch appears to be the major signal for the release of ANP, a variety of humoral factors have been implicated in the control of ANP secretion. Stimulation of both α- and β -adrenoreceptors by adrenaline or noradrenaline has been reported to increase ANP secretion from the isolated perfused heart (Currie & Newman 1986, Toth et al. 1986), dispersed myocytes (Gibbs 1987) and cultured neonatal myocytes (Matsubara et al. 1988), although conflicting results exist (for review see Ruskoaho 1992). In addition, cholinergic stimulation by acetylcholine increases ANP release from the isolated perfused heart and cultured myocytes (Toth et al. 1986, Ruskoaho 1992).
Pressor hormones such as vasopressin and Ang II have also been shown to modulate ANP secretion. Infusion of vasopressin or Ang II in vivo increases plasma ANP levels in several species (Katsube et al. 1985, Edwards et al. 1986, Uehlinger et al. 1986, Inoue et al. 1988, Cases et al. 1992). The stimulatory effect of vasopressin on ANP release in vitro has been demonstrated only in isolated atria (Sonnenberg & Veres 1984), whereas no effect has been seen in other experimental models. Similarly, Ang II had no effect on ANP release in rat heart-lung preparation (Dietz 1988) or in cultured atrial myocytes (Glembotski et al. 1991). On the contrary, in cardiac myocyte and nonmyocyte coculture Ang II has been shown to increase ANP and BNP production (Harada et al. 1997). However, this effect was not seen in myocyte culture alone, suggesting that it depends on the existence of nonmyocytes (Harada et al. 1997). Therefore, hemodynamic effects of pressor hormones may mediate the positive effects on ANP secretion (Steward et al. 1990, Ruskoaho 1992).
2.2.3. The endothelial factors, endothelin-1 and nitric oxide
Endothelial cells line the inner surface of blood vessels and the much-trabeculated cavity of the cardiac chambers. Endothelial cells sense the transmural pressure at the endothelial surface, and release of endothelium-derived vasoconstrictor and relaxant factors has been observed in response to mechanical stretch and rise in transmural pressure. Endothelins (ETs) are a family of three (ET-1, ET-2 and ET-3) regulatory peptides produced by endothelial cells (Rubanyi & Polokoff 1994, Levin 1995). ET-1 was the firstly identified ET-peptide and it was found to be the most potent vasoconstrictor substance yet identified (Yanagisawa et al. 1988). ET-1 has been shown to be a potent ANP secretagogue in cultured rat atrial myocytes (Fukuda et al. 1988, Sei & Glembotski 1990, Lew & Baertschi 1989b, 1992, Gardner et al. 1991, Uusimaa et al. 1992b, Muir et al. 1993), isolated atria (Hu et al. 1988, Stasch et al. 1989,Winquist et al. 1989, Schiebinger & Gomez-Sanchez 1990) and isolated perfused rat heart (Mäntymaa et al. 1990, Pitkänen et al. 1991) as well as in vivo (Stasch et al. 1989, Garcia et al. 1990, Kohno et al. 1990). On the contrary, low doses of ET-1 have been reported to inhibit ANP secretion both in perfused rat hearts and in conscious unrestrained rats (Shirakami et al. 1993). The inhibitory effect of ET-1 on ANP secretion was almost abolished by simultaneous administration of indomethacin but not by methylene blue, suggesting that prostanoids, not NO, are involved in the ET-1-induced inhibition of ANP secretion (Shirakami et al. 1993).
Currently, two major ET receptor subtypes have been cloned, the ETA and ETB receptors, which are G-protein coupled (Rubanyi & Polokoff 1994, Levin 1995). Vasoconstriction induced by ET-1 is mainly mediated by ETA receptors but in some blood vessels also by ETB receptors (Clozel et al. 1992). In the heart, both ETA and ETB mRNAs are found throughout the myocardium of the atria and ventricles (Hori et al. 1992). However, ETA receptors constitute approximately 91% of the total population of ET receptors in human right atrial myocytes (Molenaar et al. 1993). It has been shown that ET-stimulated secretion of natriuretic peptides in cultured myocytes is mediated by ETA receptors (Irons et al. 1993, Leite et al. 1994, Thibault et al. 1994).
In addition to ET, vascular endothelium has been shown to produce factors which can induce relaxation of vascular smooth muscle cells (Furchgott & Zawadzki 1980). This endothelium-derived relaxing factor, which was discovered to be nitric oxide (NO)(Palmer et al. 1987), is synthesized in endothelial cells from L-arginine by NO synthase (NOS)(Palmer et al. 1988, Palmer & Moncada 1989, for reviews see Moncada et al. 1991, Änggård 1994). Three different NOSs have been identified, all of which are also present in the heart. Endothelial cells continuously release small amounts of NO, producing a basal level of vascular smooth muscle relaxation (Moncada et al. 1991, Calver et al. 1993, Änggård 1994). In addition, endocardial cells and both vascular and cardiac myocytes can synthesize NO (Schulz et al. 1991, Schulz et al. 1992, Schulz & Triggle 1994). The most important physiological stimulus for the release of NO in the vasculature is the increase in shear stress (for review see Davies 1995), but also some physiological agents such as acetylcholine, bradykinin, thrombin and adenosine diphosphate regulate NO release (Nathan 1992). NO stimulates soluble guanylyl cyclase and increases vascular and cardiac muscle cGMP formation and exerts a negative inotropic effect on cardiac muscle cells similar to its relaxation of smooth muscle cells (Smith et al. 1991). Thus, by releasing NO endocardial cells may have a physiological role in the regulation of the function of the heart.
NO has been suggested to be involved in the regulation of ANP release. Coculture of aortic endothelial cells with atrial myocytes stimulates ANP release, and this release is inhibited by acetylcholine, known to evoke the release of NO (Lew & Baertschi 1989b). Removal of endocardium by saponin or infusion of inhibitors of NO significantly increases the release of ANP from isolated rat atria (Sanchez-Ferrer et al. 1990). Furthermore, in the isolated rat heart competitive inhibition of NO synthesis with L-NMMA overcame the inhibitory effect of acetylcholine on ANP secretion, showing that NO release from endothelium negatively modulates ANP secretion (Melo & Sonnenberg 1996). In addition, in conscious rats basal plasma ANP concentrations are dose-dependently increased by L-NAME, an inhibitor of NOS (Leskinen et al. 1995). These results suggest that NO released from the endothelium may tonically inhibit the secretion of ANP from cardiac myocytes.