Guanylyl cyclase catalyzes the synthesis of cGMP from GTP. Guanylyl cyclase exists in both a membrane-bound and soluble form. The membrane-bound form is a plasma membrane receptor, while soluble forms of guanylyl cyclase undergo activation by nitric oxide. Nitric oxide then functions as a second messenger, amplifying the signal intracellularly. In this review, we discuss the biochemistry, function, and clinical relevance of guanylyl cyclase.
Hormones that use the guanylyl cyclase signaling pathway include atrial natriuretic peptide (ANP) and nitric oxide (endothelium-derived relaxing factor, or EDRF). These hormones either bind to their receptors on the cell membrane or their soluble receptor in the cytoplasm. The receptor has intrinsic guanylate cyclase activity, and the guanylyl cyclase converts GTP to cGMP. The cGMP then activates protein kinase G. The activated protein kinase G then causes vasodilation of smooth muscle.
The role of guanylyl cyclase is prominent in congestive heart failure, as ANP is elevated. The ventricles and the atria release atrial natriuretic peptide and B-type brain natriuretic peptide in response to increased stretch. The guanylyl cyclase also mediates penile erection through binding of nitric oxide to soluble guanylyl cyclase, as the soluble guanylyl cyclase catalyzes the conversion of guanosine 5’ triphosphate (GTP) to cyclic guanosine 3’, 5’ monophosphate (cGMP).
Nitric oxide binds to its intracellular receptor soluble guanylyl cyclase (sGP) and increases cGMP levels. Nitric oxide binds to a heme prosthetic group on the receptor. Once NO binds, this triggers a conformational change that increases cGMP synthesis. Nitric oxide (NO) is a freely diffusible intercellular signaling molecule that mediates vasodilation. The activation of soluble guanylyl cyclase leads to increased cGMP concentration, and this leads to soluble guanylyl cyclase transmitting a NO signal to downstream proteins in the signaling cascade: cGMP dependent protein kinase, cGMP gated cation channels, and cGMP regulated phosphodiesterase.
Nitric oxide also binds to the transmembrane receptor guanylyl cyclase; this converts GTP to cGMP, which acts as a second messenger to activate protein kinase G. Activation of protein kinase G causes smooth muscle relaxation and vasodilation.
Soluble guanylate cyclase is a heterodimeric protein with an alpha and beta subunit. It is a hemoprotein, meaning NO binds to soluble guanylate cyclase heme, and this binding activates the enzyme.
Membrane-bound guanylyl cyclase contains an extracellular domain which binds to hormones such as ANP and BNP, a transmembrane domain, kinase homology domain, hinge region, and guanylyl cyclase domain, which converts GTP to cGMP 
Membrane-bound guanylyl cyclase has seven different isozymes GC-A through GC-G. GC-A mediates ANP and BNP and controls blood pressure, along with regulating cellular growth in the brain and kidney, angiogenesis, liver regeneration, and lipolysis in adipose tissue. GC-B stimulates proliferation of chondrocytes by cGMP activation of protein kinase 2 at the growth plate.
GC-A is also called NPR-A and expresses throughout the cardiovascular system, in vascular smooth muscle, vascular endothelium, and heart, as well as the kidney. GC-B, also called NPR-B, is also highly expressed in vascular endothelium and smooth muscle, though its presence in cardiac tissue predominantly localize to the non-myocyte population and mostly in fibroblasts.
Mammalian single membrane-spanning guanylyl cyclases exist in at least seven varieties, including GC-A, B, C, D, E, F, and G. Guanylyl cyclases catalyze the conversion of GTP to cGMP and pyrophosphate. Nitric oxide, bicarbonate, and natriuretic peptides such as ANP and BNP, activate the guanylyl cyclase enzymes, which then form the cyclic GMP. Other activators of guanylyl cyclase enzymes include uroguanylin, guanylin, and guanylyl cyclase activating proteins. The cyclic GMP then serves as an intracellular second messenger that controls blood pressure, cardiac hypertrophy, sexual arousal, gut peristalsis, platelet aggregation, neurotransmission, long bone growth, lipolysis, intestinal fluid secretion, phototransduction, and oocyte maturation.
PDE3 is an enzyme that regulates vascular smooth muscle contractility as well as cardiac myocyte contractility. PDE 3 also regulates the phenotypic switch in vascular smooth muscle and also regulates the response to stress. PDE 3 interacts with L-type calcium channels and the cardiac sarcoplasmic reticulum calcium pump (SERCA2) to modulate cardiac myocyte contractility and relaxation.
PDE 5 is an enzyme found in the smooth muscle of the corpus cavernosum that cleaves and degrades cGMP to 5’GMP. PDE 5 inhibitors and cGMP have similar structures, both bind competitively to PDE 5 and inhibit cGMP hydrolysis, thus enhancing the effects of NO. The erection then prolongs as a result of the increase in cGMP in smooth muscle cells. However, after PDE 5 inhibitor administration, adequate sexual stimulation is still necessary for the erection to occur as PDE 5 inhibitors do not directly affect corpus cavernosum smooth muscle relaxation.
cGMP mediates many physiologic processes and cell types in the cardiovascular system. Dysfunctional signaling at any step of the cascade, including cGMP synthesis, effector activation, or cGMP breakdown, have been present in many cardiovascular diseases such as hypertension and atherosclerosis as well as cardiac hypertrophy and heart failure.
Guanylyl cyclase plays an important role in cardiac muscle relaxation. ANP and BNP are produced and released by cardiac muscle when there is an increase in myocardial wall stretch and/or pressure, an increase in catecholamines, arginine vasopressin, angiotensin 2, endothelin, and glucocorticoids. ANP and BNP are natriuretic peptides and both bind to membrane-associated guanylyl cyclase receptors (also called NPRs). Of the seven mammalian membrane-associated guanylyl cyclases, the NPR A and NPR B participate in natriuretic peptide pathways. ANP and BNP activate NPR A to regulate body volume homeostasis, blood pressure, and local anti-hypertrophic effects in the myocardium.
Guanylyl cyclase plays a prominent role in the process of erection as well. During sexual arousal, nerve terminals and endothelial cells in the corpus cavernosum. NO activates guanylate cyclase, which then converts guanosine triphosphate (GTP) into cyclic guanosine triphosphate (cGTP), triggering a cGMP dependent cascade of events. Smooth muscle relaxation then occurs when cGMP accumulates in the corpus cavernosum to increase blood flow to the penis.
Nitrates can relieve angina by acting as a vasodilator. Nitrates reduce symptomatic and silent ischemic episodes in coronary heart disease patients with ST-segment alterations. These anti-ischemic effects can improve prognosis by infarct prevention and prevention of deterioration of left ventricular function due to chronic myocardial ischemia. Nitrates decrease the frequency of ischemic episodes and reduce the number of anginal attacks that produce clinical symptoms. Nitrates decrease preload, as vasodilation pools blood into distal extremities and decrease the blood returning to the heart.
Sildenafil, vardenafil, tadalafil, and avanafil are PDE 5 inhibitors that are first-line for management of erectile dysfunction in men. PDE 5 is an enzyme found in the smooth muscle of the corpus cavernosum that cleaves and degrades cGMP to 5’GMP. PDE 5 inhibitors bind and inhibit PDE 5, inhibiting cGMP hydrolysis, thus enhancing the effects of NO. The erection then prolongs as a result of the increase in cGMP in smooth muscle cells.
Nitric oxide-cGMP enhancers are also useful in treating primary pulmonary hypertension; these act by dilating the pulmonary arteries and bringing the mean pulmonary artery pressure down, thus reducing right ventricle afterload; this has shown functional class improvement, cardiac index improvement and improvement in 6-minute walking distance. There have been trials in which nitrates have been combined with other agents like endothelin receptor blocker and prostaglandins to show improvements. But only combination therapy which is FDA approved combines ambrisentan (endothelin receptor blocker) and tadalafil (PDE5 inhibitor). There have been trials in which nitrates and sildenafil have been used concomitantly for treating pulmonary hypertension in congestive heart failure.
Nitrates can cause excessive hypotension. Vasodilation increases blood flow to distal extremities, decreasing blood flow returning to the brain, which can cause symptomatic hypotension and headaches, in which case, nitrate therapy should be discontinued. Hypertensive crisis can occur if nitrates are used in patients with acute inferior myocardial infarction associated with right ventricular dysfunction or infarction, or with concurrent use of PDE 5 inhibitor or N-acetylcysteine.
Nitrate tolerance can occur with long term continuous administration as it decreases vasodilatory effects. To prevent tolerance from long-term nitrate therapy, providers should advise a 10 to 12-hour nitrate-free interval; thus nitrates would be administered only for a portion of each day. Also, during the nitrate-free periods, some patients might develop an increase in angina and will require sublingual nitroglycerin for short term therapeutic relief.