Principle of myotropic action and hemodynamics
Peripheral vascular tone is one of the three key factors determining blood pressure levels. Drugs in this group exert a direct spasmolytic effect on vascular smooth muscle. As a result of this action, blood vessels dilate, total peripheral resistance decreases, which naturally leads to a drop in systemic blood pressure.
However, a prolonged or rapid therapeutic effect triggers the body's compensatory mechanisms:
- Reflex tachycardia: develops in response to a drop in blood pressure via baroreceptors of the aortic arch and carotid sinus.
- Secondary RAAS activation: a drop in renal perfusion stimulates juxtaglomerular cells, increasing renin secretion.
- Fluid retention: with long-term use, sodium and water accumulate.
Physiological regulation of vascular tone
Normally, vessel lumen diameter is maintained through a balance of neurohumoral factors:
- Neuronal regulation
- Mediated by the sympathetic nervous system.
- The mechanism involves stimulation of postsynaptic alpha-1 adrenergic receptors.
- The net effect is increased tone and vasoconstriction.
- Humoral regulation
- Includes circulating substances and local factors.
- Divided into vasopressor systems, which increase tone, and vasodilator systems, which decrease it.
Endogenous vasoactive substances
To understand pharmacotherapy, it is important to consider natural endogenous regulators of the vascular lumen:
- Vasoconstrictors:
- Monoamines: adrenaline, noradrenaline.
- Peptides and hormones: angiotensin II, vasopressin, endothelin, neuropeptide Y.
- Arachidonic acid derivatives: thromboxane, prostaglandin F2-alpha.
- Vasodilators:
- Endothelial factors: endothelium-derived relaxing factor (nitric oxide, NO), prostacyclin.
- Peptides: bradykinin, adrenomedullin, natriuretic peptides (types A, B, C).
- Mediators and metabolites: acetylcholine, histamine, adenosine, ATP.
Classification of myotropic drugs
Drugs that reduce vascular tone through a direct effect on muscle are divided into 4 groups according to their mechanism of action:
- Calcium channel blockers (calcium antagonists).
- Potassium channel openers.
- Nitric oxide (NO) donors.
- Other agents.
Briefly: answers to topical questions
Which drugs belong to calcium channel blockers?
Calcium channel blockers (calcium antagonists) include amlodipine, nifedipine, cinnarizine, and nimodipine.
- Amlodipine is a direct-acting myotropic agent targeting vascular smooth muscle.
- Nifedipine is a calcium channel blocker; classified as a myotropic agent.
- Cinnarizine is a representative of the calcium channel blocker group affecting cerebral circulation.
- Nimodipine is a specific drug with tropism for cerebral vessels.
Which drugs are included in the potassium channel opener group?
The potassium channel opener group includes minoxidil and diazoxide.
- Minoxidil is an antihypertensive agent that lowers blood pressure.
- Diazoxide is a drug that induces the opening of potassium channels in vascular smooth muscle membranes.
Both drugs promote the efflux of potassium ions from the cell, membrane hyperpolarization, and subsequent relaxation of the vascular wall.
What are the nitric oxide donors?
Nitric oxide (NO) donors include sodium nitroprusside and nitroglycerin.
- Sodium nitroprusside (Sodium nitroprusside) releases NO, activates cytosolic guanylate cyclase, increases cGMP levels, decreases intracellular calcium concentration, and dilates both resistance and capacitance vessels.
- Nitroglycerin acts as a donor of nitric oxide (NO), an endothelium-derived relaxing factor; via guanylate cyclase activation and cGMP accumulation, it leads to the relaxation of vascular smooth muscle cells.
Which agents are included in the group of other myotropic drugs?
The group of other myotropic vasodilators includes direct-acting drugs, as well as specific and newer drug classes.
- Bendazol is most commonly used as part of combination therapy.
- Magnesium sulfate is used intramuscularly for hypertensive crises.
- Nesiritide is a recombinant human B-type natriuretic peptide.
Additionally, promising directions and specific agents for the treatment of acute heart failure include natriuretic peptide analogues, endothelin antagonists, and vasopressin antagonists.
Direct relaxation of vascular smooth muscle reduces total peripheral resistance, but the body activates defense mechanisms: tachycardia, the RAAS, and fluid retention.
Class and exam questions
What is the main hemodynamic result of direct-acting vasodilators?
A direct spasmolytic effect on vascular smooth muscle dilates blood vessels, leading to a decrease in total peripheral resistance (TPR) and a drop in systemic blood pressure.
Why does tachycardia occur when taking direct-acting vasodilators?
Reflex tachycardia develops in response to a drop in blood pressure via the activation of baroreceptors located in the aortic arch and carotid sinus.
What are the main groups distinguished in the classification of direct-acting antihypertensive drugs?
According to their mechanism of action, they are divided into 4 groups: calcium channel blockers, potassium channel openers, nitric oxide (NO) donors, and other agents.
What else to study on this topic
- Detailed analysis of the role of aortic arch and carotid sinus baroreceptors
- Mechanisms of renal juxtaglomerular cell stimulation and RAAS activation
- The influence of the sympathetic nervous system and alpha-1 adrenergic receptors on vasoconstriction
- The role of endothelial factors and arachidonic acid derivatives in humoral regulation
- Pharmacological properties of nitric oxide donors and potassium channel openers
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