Vascular Pharmacology 2022 Notes PDF

Vascular Pharmacology Rodrigo Andrade, 3108 Scott Hall. October 2022 Objectives To understand the mechanisms underlying contraction in vascular smooth muscle and its pharmacological regulation through α-adrenergic receptors. To understand the Renin-Angiotensin system and its ph...

Vascular Pharmacology Rodrigo Andrade, 3108 Scott Hall. October 2022 Objectives To understand the mechanisms underlying contraction in vascular smooth muscle and its pharmacological regulation through α-adrenergic receptors. To understand the Renin-Angiotensin system and its pharmacological regulation. To understand endothelin signaling and its pharmacological regulation in the context of vascular muscle contraction. To understand the mechanisms underlying vascular smooth muscle relaxation To learn the pharmacology of different classes of vasodilators Outline Introduction (00:00) I. Vascular contraction (00:18) II. Pharmacological control of vascular contraction via α1-adrenergic receptors (03:08) III. Centrally acting sympatholytic Drugs (05:28) IV. The Renin-Angiotensin system (06:51) V. Endothelin (12:29) VI. Vascular relaxation (14;52) VII. Pharmacological regulation of NO-cGMP signaling (18:29) End (24:18) Introduction. In previous lectures you have heard about the contraction of skeletal and, most recently, cardiac muscle. Today we will address the mechanisms that control the contractility of the vasculature and explore pharmacological avenues for regulating it. I. Vascular contraction Smooth muscle contractions tend to be tonic rather than phasic, and this is reflected by the mechanism by which calcium triggers the contraction. Specifically, in smooth muscle calcium binds calmodulin to activate the enzyme myosin light chain kinase which then phosphorylates myosin light chain to initiate contraction. The phosphorylation of myosin light chain is reversed by myosin light chain phosphatase to terminate contraction (Fig. 1). Contractility in vascular smooth muscle, especially the arteries and arterioles, is also self-regulating such that the vessel contracts in response to an increase in transmural pressure (myogenic response). This involves a class of cation channels, collectively known as stretch receptors, which open in response to membrane stretch. In arteries and arterioles this results in a membrane depolarization, the opening of L type calcium channels, calcium influx into the cell and of course muscle contraction. No wonder dihydropyridines are so effective in reducing blood pressure! In smooth muscle, calcium influx from the extracellular environment is not the sole mechanism for elevating intracellular calcium and multiple extracellular signaling molecules trigger smooth muscle contraction by acting on G protein coupled receptors to activate heterotrimeric G proteins of the Gq family (Fig. 1). Gαq activates PLC𝛽 to break down PtdIns(4,5)P2, release IP3 and trigger the release of calcium from intracellular calcium stores. In parallel Gαq also binds RhoGEFs to activate RhoA/RhoA kinase signaling which phosphorylates and inhibits myosin light chain phosphatase thus potentiating the actions of myosin light chain kinase (Calcium sensitization). Thus smooth muscle contraction can be triggered by calcium originating from two different sources. Figure 1. Mechanisms underlying smooth muscle contraction II. Pharmacological control of vascular contraction via α1-adrenergic receptors Sympathetic activation leads to the contraction of many vascular beds (e.g. skin, kidneys) through the activation of α1-adrenergic receptors. These receptors activate Gq and elicit vessel contraction through the mechanism outlined above. Thus targeting α1-adrenergic receptors offers an effective strategy for regulating blood pressure. Nonselective sympathomimetic agents such as epinephrine, norepinephrine or even high dose dopamine, can be used to elicit a pressor response during septic shock (epinephrine, norepinephrine) or heart failure (dopamine). Alternatively, a selective α1-adrenergic agonist such as phenylephrine or oxymetazoline can be used to trigger a pressor response while avoiding β-adrenoceptor activation. In contrast, selective α1-adrenergic antagonists such as Prazosin can be used as antihypertensive agents, as can drugs such as Carvedilol and Labetalol that combine α1-adrenergic and β-adrenergic antagonist activity. Given the essential role of sympathetic output in baroreceptor reflexes one anticipated adverse effect of these drugs on first administration (or after a dose increase) is orthostatic hypotension. III. Centrally acting sympatholytic Drugs An alternative approach to reduce sympathetic output is to target the CNS. Methyldopa, which is used to treat hypertension during pregnancy, is a prodrug metabolized in the brain to -methyl- norepinephrine which is thought to act on 2-adrenergic receptors in the brainstem to reduce sympathetic outflow. A similar strategy can be implemented using the selective 2-adrenergic agonists Clonidine and Monoxidine. IV. The Renin-Angiotensin system The renin–angiotensin hormone system plays an important role linking kidney function and the cardiovascular system. Renin is an aspartyl protease that is released into the circulation from the juxtaglomerular (JG) cells of the kidney. In the bloodstream Renin cleaves the prohormone Angiotensinogen to generate Angiotensin I, which in turn is cleaved by a second protease, angiotensin converting enzyme (ACE) to generate Angiotensin II (AngII). AngII plays an essential role in the regulation of vascular volume and you will learn much more about this hormone, and this system, in the renal physiology lectures. Most importantly from the standpoint of the current lecture, AngII also plays a key role in the control of vascular resistance. AngII exerts its biological effects through two different receptors, both heptahelical G protein coupled receptors, known as AT1 and AT2. Most effects in the cardiovascular system are mediated by the AT1 receptor which couples predominantly to Gq but appears capable of also activating other G proteins, at least in some cells. In vascular smooth muscle AngII activation of AT1 receptors activates Gq, to release intracellular calcium and activate RhoA kinase thus producing a pressor response. Figure 2 The Renin-Angiotensin system This hormonal system offers multiple points for pharmacological regulation (Fig. 3). The most widely used agents targeting this hormonal system are the ACE inhibitors. Drugs such as Captopril (Capoten) and Benazepril (a prodrug, Lotensin) which inhibit the formation of AngII by inhibiting ACE. Cough is a common adverse effect for these drugs. The most serious side effect of these drugs is angioedema, which in some serious cases can affect the tongue and larynx and lead to airway obstruction. An alternative to the ACE inhibitors are the Angiotensin Receptor Inhibitors (ARBs) which include drugs such as Losartan (Cozaar) or Irbesartan (Avapro) and directly inhibit the AT1 receptors. The incidence of cough and angioedema with ARBs is much less than with ACE inhibitors. ACE inhibitors and ARBs are among the most widely used drugs for the treatment of hypertension and, as we saw in the previous class, are front line drugs for the treatment of heart failure. Renin inhibition offers a second effective approach for targeting the renin-angiotensin system. Aliskiren (Tekturna, Rasilez) is a highly selective Renin inhibitor approved by the FDA for the treatment of hypertension. Figure 3. Pharmacological targeting of the Renin-Angiotensin system V. Endothelin Endothelial cells are important regulators of vascular smooth muscle contractility that can signal dilation or contractions. Their pressor response is signaled by the release of endothelins, three closely related peptides thought to contribute to the maintenance of muscle tone in the vasculature. Endothelins are synthesized as pre-pro hormones and processes through a two-step enzymatic process to generate endothelin 1, 2 and 3 (ET-1, ET-2 and ET-3) and act through two heptahelical G protein coupled receptors known as ETA and ETB (Fig. 4). ETA is preferentially located on vascular smooth muscle cells and vasculature and ETB on endothelial cells. ET1 and its receptors appear to be especially important in the control of the pulmonary vasculature and ET receptor antagonists such as bosentan (Tracleer) and Macitentan (Opsumit) have been approved for the treatment of pulmonary arterial hypertension (PAH). These drugs however carry considerable liver toxicity and are Figure 4. Pharmacological regulation of endothelin signaling contraindicated during pregnancy. VI. Vascular relaxation In smooth muscle vascular relaxation requires a return of intracellular calcium to resting levels and dephosphorylation of myosin light chain. This process is actively regulated through multiple mechanisms. One important mechanism involves the release of nitric oxide (NO) from endothelial cells or nerve terminals. NO is a membrane permeable gas formed from arginine by a group of enzymes known as Nitric Oxide Synthetases (NOS). Upon formation NO diffuses across the cell membrane and directly activates soluble guanylate cyclase. The resulting increase in intracellular cGMP is thought to activate Protein kinase G to counter the calcium-induced contraction of the vascular smooth muscle cells. cGMP is hydrolyzed by the cGMP-specific phosphodiesterase type 5 PDE5 thus terminating this response (Fig. 5). Figure 5. cGMP signals relaxation of vascular smooth muscle cells A second pathway involves atrial natriuretic peptide (ANP), a hormone released by the atria in response to stretch. Circulating ANP binds it receptors, Natriuretic Peptide Receptor A and B (NRPA and NRPB) which are intrinsic membrane guanylate cyclases (aka particulate guanylate cyclase) to increase cGMP and signal membrane relaxation. A third pathway involves 2-adrenergic receptor activation leading to the formation of cAMP and the activation of protein kinase A leading to relaxation. This last pathway is especially important in pulmonary pharmacology and will be discussed by Dr. Mattingly in a subsequent lecture.. VII. Pharmacological regulation of NO-cGMP signaling. Organic nitrates such as nitroglycerin and isosorbide dinitrate (nitrovasodilators) are prodrugs that release NO upon reduction and consequently elicit vascular smooth muscle relaxation and vasodilation. One useful feature of these drugs is that, for reasons not well understood, they preferentially dilate large veins and spares medium and small arterioles. This results in a reduction in venous return (preload), leading to reduced oxygen consumption in the heart without large changes in blood pressure. These drugs are important agents in the control of angina. Of course given that NO-cGMP signaling is widespread in smooth muscle, the nitrovasodilators also relax smooth muscle in the lungs, biliary tract, etc. Repeated use of these drugs leads to tolerance and a decrease in angina threshold. Sodium nitroprusside, a NO prodrug belonging to a different chemical class, acts along the same mechanism but its repeated use does not lead to tolerance. It is generally used in the treatment of acute hypertensive crises. An alternative pharmacological approach to increasing cGMP is to inhibit PDE5. This is the target for drugs such as sildenafil (Viagra), which is used for the treatment of erectile dysfunction and also pulmonary arterial hypertension. Given the important role for PDE5 in the regulation of cGMP levels there is the potential for a significant interaction of these drugs with NO releasing prodrugs. Figure 6. Pharmacological control of cGMP signaled relaxations in vascular smooth muscle ANP is released by the atria onto the circulation where it is degraded by the metalloprotease neprilysin. Sacubitril is a prodrug that generates the neprilysin blocker sacubitrilat, a drug that when formulated with the ARB valsartan has been shown highly effective in the treatment of heart faiure. An additional and important vasodilator is Hydralazine. This drug is thought to selectively dilate arterioles and thus reduce blood pressure. Its mechanism of action is not well understood but appears to involve directly or indirectly a reduction in intracellular calcium. An advantage of hydralazine is that, because of its selective effect on arterioles, its administration is not associated with significant postural hypotension. Hydralazine is used in the treatment of hypertension.