Dopaminergic Receptors: Potential Therapeutic

Applications

Clinical Pharmacology Program,Vargas Medical School,

Central University of Venezuela.Caracas, Venezuela

Manuel Velasco, MD, FRCP Edin

Introduction

• Brief story of dopamine.• Vasodilatory effect of dopamine in

experimental animals treated with and adrenergic blocking drugs.

• Hypotensive effect induced by L-dopa in patients with Parkinson disease.

• Animal experiments undertaken by Goldberg et al showing that vasodilatory effect of dopamine on renal arterioles, coronary arteries and mesenteric arteries are antagonized by DA, dopaminergic blockers.

Introduction

• Human experiments undertaken by Goldberg et al showing:

– Increase of cardiac contractibility through -adrenergic receptors stimulation.

– Increase of renal blood flow through stimulation of DA, dopaminergic receptors.

– Use of dopamine in congestive heart failure and cardiogenic shock.

  D1-like D2-like

Receptor name D1 D5 D2S D2L D3 D4

Number of Amino acids

446 477 414 443 400 387

Chromosomal localization

5q 35.1 4p 15.1–16.1

11q 22–23 3q 13.3 11p 15.5

LocalizationHypothalamus,

striatum,,neocortex, kidneys (renal tubules,

juxtaglomerular cells) and blood vessels.

CNS, Postganglionic sympathetic nerves, Sympathetic ganglia, kidney (zona glomerulosa cells, renal tubules,

juxtaglomerular cells) and blood vessels.

Cardio-vascular action

Vasodilatation, Natriuresis, diuresis,

Renin release

Vasodilatation, bradycardia, Inhibition of aldosterone and renin release, antinatriuresis.

 Note: The D1C and D1D receptors had not been included in this table since they are not yet described in humans.

Characteristics and distribution of dopamine receptors in humans.

IUPHAR. 2004

Dopaminergic Receptors

Dopamine Receptor Structure:D1-like receptor.

COOH

NH 2

Missale C et al. Physiol Rev. 1998;78(1):189-225

Biochemical Mechanisms Involved in the Activation of Dopaminergic Receptors

1. Adenyl Cyclase:

The D1-like receptors increase the production of cAMP through the interaction with Gs proteins (s=stimulant), stimulating adenyl cyclase. The D2-like receptors, in turn, interact with Gi proteins (i=inhibitory), inhibiting adenyl cyclase and decreasing production of cAMP.

Biochemical Mechanisms Involved in the Activation of Dopaminergic Receptors

2. Calcium Channels:

The D1-like receptors, modulate intracellular calcium levels by:

- Stimulation of phosphatidylinositol (PI) hydrolisis by phospholipase C (PLC), resulting in the production of inositol 1,4,5-triphosphate, which mobilizes intracellular calcium stores.

Biochemical Mechanisms Involved in the Activation of Dopaminergic Receptors

2. Calcium Channels (continued):

- Activate protein kinase A (pKa), which by means of increasing cAMP levels, stimulate release of intracellular calcium stores.

- Affects the activity of calcium channels, increasing calcium currents by the pKa mediated blockade of a phosphatase which dephosphorylates the channels leading to their inactivation.

Lin C et al. Mol Pharmacol. 1995;47:131-139

Phospholipase C Adenyl Cyclase

DA

D1-like

G s G q

IP3DAGcAM P

Protein Kinase A Protein Kinase C

Na, H - Exchanger Na, K - ATPase

Increase in RenalSodium Excretion

Hussain T, Lokhandwala M. Hypertension. 1998;32:187-197

Biochemical Mechanisms Involved in the Activation of Dopaminergic Receptors

2. Calcium Channels (continued):

The D2-like receptors, can modulate intracellular calcium levels by:

- Decrease in intracellular calcium levels by inhibition of inward calcium currents:

a) By means of alterations in membrane potential by activation of potasium currents.

b) Activation of G proteins that directly inhibit some calcium channels.

Biochemical Mechanisms Involved in the Activation of Dopaminergic Receptors

3. Potassium Channels:

D2-like receptors increase outward potassium currents, leading to cell hyperpolarization.

This appears to be modulated by G protein mechanisms

Biochemical Mechanisms Involved in the Activation of Dopaminergic Receptors

4. Na+/H+ Exchanger:

It is inhibited by the paracrine effect of DA on the proximal tube (nephron) resulting from D1 receptors activation, which stimulate adenyl cyclase.(1)

It can be also inhibited by a mechanism independent of the cAMP generation, directly through the G protein interaction. (2)

1. Felder CC et al . Am J Physiol. 1990;259(28):F297-F303 2. Felder CC et al. Am J Physiol. 1993;264(33):F1032-F1037

Biochemical Mechanisms Involved in the Activation of Dopaminergic Receptors

4. Na+/H+ Exchanger (continued):

In spontaneously hypertensive rats (SHR), the lower expression and activity of the Gs protein and a higher expression and activity of Giprotein has demonstrated to have an effect in the efficacy of DA to lower the AHT(1).

The PLC mediated activation, is also cAMP independent (2).

1. Xu J, Li XX et al. Hypertension. 2000;36:395-3992. Yu PY, Eisner GM et al. J Biol Chem. 1996;271(32):19503-19508

Biochemical Mechanisms Involved in the Activation of Dopaminergic Receptors

Na+/K+/ATPase pump:

DA reversibly inhibits this pump, mainly through type 1 receptors, which results in an increase in natriuresis (1)

Second messengers(2) : PLA2

PLCNa +

K +K +

Na +

Na +

1. Missale C et al. Physiol Rev. 1998;78(1):189-225 2. Aperia AC et al. Ann Rev Physiol. 2000;62:621-647

Biochemical Mechanisms Involved in the Activation of Dopaminergic Receptors

Na+/K+/ATPase pump (continued):

In the gastrointestinal tract also influences the Na+ balance. A recent study showed that a high Na+-content diet inhibits this pump by increasing DA synthesis, which would aid hydroelectrolitic tolerance of the body during a sodium overload (1). This is observed in animals after ablation of one of their kidneys, suggesting a connection between kidney and intestine (2).

1. Lucas-Texeira V et al. . Acta Physiol Scand. 2000;168:225-231 2. Vieira-Coelho MA et al. Am J Physiol (Renal Physiol). 2000;279:F1033-F1044

Pharmacological Actions of DopamineTissue Receptor Function

Blood Vessels   

Adventitia D2-like Inhibition of NE release Media D1-like Vasodilatation Intima D2-like UnknownHeart D4 UnknownAdrenal Gland  

 

Glomerulosa D1-like UnknownD2-like Inhibition of aldosterone secretion

Medulla D1-like Stimulation of E / NE releaseD2-like Inhibition of E / NE release

Kidney   

Glomerulus D1-like Increase of filtration rate Juxtaglomerular apparatus D1-like Stimulation of renin secretion D2-like Inhibition of renin secretion Proximal tubule D1-like Inhibition of Na+ reabsorption Ascending limb of loop of Henle D1-like Inhibition of Na+ reabsorption

D1-like Inhibition of Na+ reabsorptionD2-like Inhibition of vasopressin action

Sympathetic ganglia/endings D2-like Inhibition of NE release

Cortical collecting duct

Pharmacological Actions of Dopamine

CNS:

Motor activity:Forward Locomotion: It is primarily controlled by the ventral striatum, through activation of D1,D2 and D3 receptors. Whereas activation of postsynaptic D2 receptors increases locomotion and D1 has little or no effect on locomotor activity.

D3 receptor, mainly postsynaptically located in the nucleus accumbens, plays an inhibitory effect on locomotion.

Missale C et al. Physiol Rev. 1998;78(1):189-225

Pharmacological Actions of Dopamine

CNS (continued):

Behavioral effects:Reinforcement mechanisms: It has been demonstrated that a sinergistic action of both receptors (D1 and D2 like) controls the reward and reinforcements mechanisms in mesolimbic areas, demonstrated in drug abuse and obesity.

Wang G et al. Lancet. 2001;357:354-357

Low doses of dopamine activate the two diferent types of receptors (D1 and D2) in the cardiovascular system and the kidney, each is able to decrease blood pressure in normotensive and hypertensive subjects.

D1-like Receptor:

– At the vascular smooth muscle it mediates vasodilation and in renal tubular cells, it modulates sodium excretion.

– It is postsynaptic.

Pharmacological Actions of Dopamine Cardiovascular actions of Dopamine

D1-like Receptor (continued):

– It is distributed in arterioles (renal, mesenteric, hepatic, splenic, coronary , pulmonary, cerebral, etc.) and in yuxtaglomerular apparatus.

– Its mechanism of action involves adenylcyclase activity stimulation and cAMP formation; IP3 stimulation and increase of Ca 2+ conductance.

Pharmacological Actions of Dopamine Cardiovascular actions of Dopamine

D2-like Receptor:

– At the presynaptic endings it inhibits noradrenaline release by which reduces blood pressure.

– It is distributed at the presynaptic sympathetic endings (renal, mesenteric and coronary resistance vessels) and the sympathetic ganglia.

Pharmacological Actions of Dopamine Cardiovascular actions of Dopamine

D2-like Receptor (continued):

– Its mechanism of action involves adenylcyclase activity inhibition and decrease of cAMP formation.

– It increases K+ conductance and decreases Ca++ conductance, promoting phosphoinositide hydrolysis and arachidonic acid release.

Pharmacological Actions of Dopamine Cardiovascular actions of Dopamine

Pharmacological Actions of Dopamine Cardiovascular actions of Dopamine

Dosis dependent effect (i.v.):

• 0.5-1 ug/kg/min: decreases blood pressure (DA1 and DA2 dopaminergic receptor stimulation).

• 2-4 ug/kg/min: increases cardiac contractility (ß1 adrenergic receptor stimulation and noradrenaline myocardial release).

• Greater than 5- ug/kg/min: increases blood pressure (α1 adrenergic vascular receptors and ß1 cardiac adrenergic receptors).

Pharmacological Actions of Dopamine:Dopamine in the kidney

• Dopaminergic nerves are located at the vascular pole of yuxtaglomerular apparatus of renal cortical glomerulus.

• Dopamine is formed within the renal tubular epithelium.

• Proximal tubules are the major sites of dopamine synthesis derived of L-dopa.

• L-dopa uptake is Na+ dependent.• MAO oxidation / inactivation of dopamine is

inhibited by high [Na+] in tissue renal slices.

Pharmacological Actions of Dopamine:Dopamine in the kidney

• It induces renal arteriolar vasodilation and increases blood flow.

• It causes sodium and water reabsortion inhibition at the tubular level and provokes natriuresis and diuresis.

• Dopamine released from intrarenal nerves regulates glomerular filtration.

• It produces beneficial effects on acute and chronic renal insufficiency.

Pharmacological Actions of Dopamine:Dopamine in the kidney

Glomerulus:

• At low doses, dopamine increases blood flow by afferent and efferent arteriolar relaxation. This effect is exerted by Fenoldopam and blocked by DA1 blockers .

• Within the yuxtaglomerular apparatus DA1 receptor stimulation induces renin release.

• Dopamine regulates release of NE through DA2 receptors located at the nerve endings.

Pharmacological Actions of Dopamine:Dopamine in the kidney

Renal tubules

• Dopamine inhibits Na+/H+ antiporter by activation of D1 receptors.

• In proximal tubules dopamine inhibits Na+-K+-ATPasa by mechanisms that require activation of both DA1-like and DA2 - like receptors.

• DA1 receptors in the medullary thick ascending limb of the loop of Henle inhibits Na+-K+-ATPasa by a cAMP regulated phosphorilation mechanism.

• Dopamine blocks the action of Vasopresin possible by DA2-.like receptors stimulation.

Pharmacological Actions of Dopamine:Hormonal Actions

• D2 receptors stimulation increases serum insulin release.

• D2 receptor stimulation decreases plasma prolactine concentration.

• D1 receptors stimulation increases renin release within yuxtaglomerular apparatus.

• Activation of D2 receptors inhibits angiotensin induced aldosterone secretion in adrenal glomerulosa cells.

Pharmacological Actions of Dopamine:Bronchial actions

• Low doses of inhaled dopamine (0.5ug/kg/min) induces bronchodilation when the bronchial tone is already increased by acute asthma attack but did not modify the resting bronchial tone in normal subjects or in ashmatics without acute bronchospasm.

• DA2 blockade with metoclopramide did not modify resting bronchial tone.

• There is a potential use of inhaled dopamine and its agonists in the treatment of hypertensive patients with bronchial asthma

Selective Agonists and Antagonists

SelectiveAgonists

Selective Antagonists

D1Fenoldopam(SKF82623)SKF38393A68930Piribedil

SCH23390SKF83566

D2SKF38393Fenoldopan (SKF82623)

SCH23390SKF83566

D3N-437BromocriptinePergolideLergotrileLisurideCarmoxirole

Spiperone(-)SulpirideDomperidoneMetoclopramideHaloperidol

D4Quimpirole

Spiperone(+)UH232

D5 n/a

SpiperoneClozapine

Uses in Medicine

Hypertension:

The distribution and function of receptors for DA within the cardiovascular system is such that DA agonists, by acting at differents levels, may induce changes that synergistically operate to reduce blood pressure, thus making them a target for a new class of antihypertensive drugs.

Uses in Medicine:Hypertension

• In Parkinson disease patients , L-dopa causes arterial hypotension.

• In hypertensive patients, Bromocriptine (DA2 agonist) reduces blood pressure with a decrease in heart rate and plasma aldosterone.

• In labetalol pre-treated patients, dopamine induces arterial hypotension which is blocked by the use of metoclopramide (DA2 blocker).

Uses in Medicine:Hypertension

• In hypertensive patients, Piribidil (DA1 agonist) reduces blood pressure with a modest increase in heart rate, plasma renin activity, and plasma aldosterone, and an important increment in creatinine and urea clearences.

• In severe hypertensive patients, Fenoldopam (DA1 agonist) increases glomerular filtration, diuresis and natriuresis and decreases systolic and diastolic blood pressure.

Uses in Medicine:Hypertension

“…Fenoldopam mesylate is a selective dopamine (D1) receptor agonist approved for use in hypertensive emergencies. Dopamine receptor activation in the kidney leads to vasodilation and increased renal blood flow. Several randomized studies have shown a lower incidence of contrast nephropathy with fenoldopam and hydration when compared with historical controls (50% risk reduction)…”

Singri N, Ahya SN Levin ML. JAMA.2003:289(6);747-51

Uses in Medicine:Hypertension

Wood AJJ. NEJM. 2001;345(21):1548-1557

Uses in Medicine:Hypertension

Wood AJJ. NEJM. 2001;345(21):1548-1557

Uses in Medicine:Low Dose Dopamine

CHEST 2003; 123:1266–1275

Uses in Medicine: Low Dose Dopamine (< 5 g/kg/min)

1. The renal-dose of dopamine is not predictable in critically ill humans

2. Dopaminergic receptor downregulation and hysteresis to the effect of low-dose dopamine occurs

3. Renin-angiotensin system activation in patients with critical illnesses negates the effects of dopaminergic stimulation.

4. Renal medullary dysoxia appears to be a demand-side problem, not a supply problem, and dopamine may increase medullary oxygen demand.

5. The predominant effect of dopamine in critically ill humans appears to be diuresis, which is contraindicated in patients who are in most oliguric states that are associated with critical illness.

6. There is compelling evidence that dopamine is harmful to the GI, endocrine, immunologic, and respiratory systems in patients with critical illnesses.

CHEST 2003; 123:1266–1275

Conclusions

The distribution and function of dopaminergic receptors within the cardiovascular system is such that agonists, by acting at different levels may induce changes that synergistally operate to reduce blood pressure. In particular:

• Dilation of splachnic vascular beds.• Reduction of circulating catecholamines.• Inhibition of stimulated aldosterone secretion.• Increase of sodium excretion.

Conclusions (continued):

• The multiple antihypertensive dopaminergic receptors mechanisms, make them a potential target for a new class of antihypertensive drugs.

• The different dopaminergic receptors which have been so far described, represent a novel mechanism of antihypertensive action.

• If a new dopaminergic compound posseses DA1 and DA2 dopaminergic agonist properties combined, it would not be necessary to add diuretics and/or beta blockers in the treatment of hypertensive patients.

• The widespread use of DA at low doses in UCI Units, must be abandoned.

• DA exhibits potential use as a bronchodilator, different experiments in animals and in humans exerted by Cabezas et al are under progress.

• The research with other metabolic diseases such as diabetes , hypertension and obesity, is

also done at the endothelium level(endothelin, nitric oxide and angiotensin II)

Conclusions (continued):