Calcium Channel Blockers

October 3, 2007

Frank F. Vincenzi

Ca signalling and Ca channel blockers: The fundamental information

Ca 2+ Ca10 M 10 M2+-7 -3==

intracellular extracellular

Calcium

• The FIRST second messengerfundamental intracellular messenger fora wide variety of physiological responsesin essentially all cell types (muscle tension,

neurotransmitter/hormone release, production of inflammatory mediators)

• The LAST second messengeraccumulation of intracellular Ca2+ is a finalcommon pathway in both apoptotic andnecrotic cell death

Ca signalling: Two fundamental mechanisms

• Influx of Ca across the plasma membrane

– voltage-operated Ca channels (VOCs)

– receptor-operated Ca channels (ROCs)

• Internal release of Ca from intracellular stores (sarcoplasmic/endoplasmic reticulum) (SER)

– Ca-induced Ca release (‘trigger Ca’)

– Inositol tris-phosphate( IP3) induced Ca release

• Combinations of the above mechanisms

Intracellular Calcium Release

• Inositol tris-phosphate (IP3) – Present in almost all cells. IP3 acts on

sarcoplasmic/endoplasmic reticulum receptors (IP3Rs)– IP3 may sensitize Ca-induced Ca release

• Calcium-induced calcium release– Mainly in striated muscle– Ryanodine receptors (RyRs)

• RyR1 (predominantly in skeletal muscle)– [Mutation associated with malignant hyperthermia]

• RyR2 (predominantly in cardiac muscle)• RyR3 (predominantly in brain & non-muscle)

Ca2+

Ca2+

Ca2+

Ca2+

Ca2+Ca2+ Na

+

Ca2+

Ca2+

Ca2+

sarcolemma

RyR

SR

Ca2+

ATP

Ca2+

AgonistROC

ROC

Receptor operatedCa channels mediatesome agonist actions

Ca2+

Ca2+

Ca2+

Ca2+

Ca signalling: Three examples• Histamine on H-1 receptor activates phospholipase C and

releases inositol trisphosphate (IP3), IP3 acts on SR/ER IP3 receptors causing Ca release (CHEMICAL)

• Norepinephrine on beta-1 receptors in heart couple to G protein and activate adenylyl cyclase; increase cAMP, activate protein kinase A (PKA), phophorylate a cytoplasmic domain of the Ca channel, increase Ca current; PKA also phosphorylates phospholamban, an SR protein and increases the activity of the SERCA pump; increase Ca storage and release (CHEMICAL/ELECTRICAL)

• Cell depolarization activates voltage operated Ca channels (VOCs) in the plasma membrane, promoting influx of Ca into the cell cytoplasm, may be amplified by Ca induced Ca release (ELECTRICAL)

Ca2+

Ca2+

Ca2+

Ca2+

Ca2+Ca2+ Na

+

Ca2+

Ca2+

sarcolemma

RyR

SR

Ca2+

ATP

Ca2+

histamine

IP R3

IP3

PLC DAG

Phospholipase C andinositol tris phosphatemediate some agonistactions by promotingintracellular Ca release

Ca2+

GAC

2+Ca

Ca2+

Ca2+

Ca2+

RyR

2+Ca

2+Ca

2+Ca

2+Ca2+Ca

2+Ca

2+Ca

2+Ca

2+Ca

Ca2+Ca2+ Na

+ Ca2+

sarcolemma

Ca2+

ATP

Ca2+

Beta-1 ReceptorVOC

VOC

norepinephrine

GAC

cAMP

PKA

P

2+Ca

Ca2+Ca2+

Ca2+

Ca2+

RyR

P

2+Ca

2+Ca

2+Ca

2+Ca2+Ca

2+Ca

2+Ca

2+Ca

2+Ca

Ca2+Ca2+ Na

+ Ca2+

sarcolemma

Ca2+

ATP

Ca2+

Beta-1 Receptor

cAMP and proteinkinase A mediate someof the positive inotropiceffects of catecholaminesby increasing Ca influxand intracellular Ca storageand release

VOC

VOC

Ca regulation of muscle tension:different in different types of muscle

• Reversible ionic binding of Ca to the Ca binding protein, troponin C(Ca/TNC disinhibits actin/myosin ATPase)

– skeletal myo (force varied by recruitment)

– cardiac myo (force varied by cellular regulation)

• Covalent modification of myosin light chain (Ca binds to the Ca binding protein, calmodulin

CaM, CaM binds to MLCK, MLCK phosphorylates MLC, activates ATPase - tension) – smooth myo (force varied by level of MLC phosphorylation)

The calcium cycle in HEART muscle

Ca2+ Ca2+

Ca2+

Ca2+

Ca2+

Ca2+Ca2+ Na

+

Ca2+

Ca2+

Ca2+ Ca2+

sarcolemma Ca channel, DHPR

RyR

SR

Ca2+

ATP

Ca2+

VOC

VOC

The calcium cycle in

SMOOTH muscle

The calcium cycle in

SKELETAL muscle

Ca2+ Ca2+

Ca2+Ca2+Ca2+

Ca2+Ca2+ Ca2+

Ca2+ Ca2+

sarcolemma Ca “channel”, DHPR

RyR

SR

Ca2+

Ca2+

Ca2+

Ca2+

T-tubule

Ca regulation in muscle: variations on a few themes

SR Ca2+

actinmyosin mitochondria

VOC

PMCA

Na/Caexchange

Na/Kpump

Na+

K+

ROCROC

Muscle control: differential sensitivity to Ca channel blockers (CEBs)

Overview of Plasma Membrane Ca Channels

• Voltage operated channels (VOCs)L-type, N-type, T-type….etc.

• Receptor operated channels (ROCs)coupled to diverse receptors in the same cell

• Mechanically operated channels (MOCs)not widely appreciated (yet)

Voltage operated Ca Channels

• L-type (‘long’, important in heart & smooth muscle)

• N-type(‘nerve’, important in conduction & synaptic

transmission)

• T-type(‘transient’, heart & smooth muscle)

Calcium Channel Blockers

• Currently marketed Ca channel blockers all block L-type channels.

• Other terminology for these drugs:Calcium entry blockers (CEBs)Slow channel blockers“Calcium antagonists” (ugh!)

(why NOT call them calcium antagonists?)

Chemical classifications of Ca channel blockers

• dihydropyridinese.g., nifedipine (Procardia®)

(have given the name to voltage operated Ca signaling proteins - ‘the dihydropyridine receptor’

• benzothiazepinese.g., diltiazem (Cardizem®)

• phenylalkylaminese.g., verapamil (Isoptin®, Calan®)

In General Calcium Channel Blockers Are:

• Lipid soluble

• Rapidly absorbed

• Largely protein bound

• Of low bioavailability

• Metabolized by the liver

Calcium Channel Blockers: Site and Mechanism of Action

• L-type voltage operated channels (VOCs) and receptor operated channels (ROCs)

• Binding to Ca channel protein (slightly different site for each class):– inhibits ionophoric activity of affected channels (all

agents)

– delays recovery of the affected voltage operated channels (verapamil > diltiazem >> nifedipine)

Calcium Channel Blockers:Frequency Dependence

• Verapamil (and to some extent, diltiazem) selectively inhibits rapid cardiac rhythms by delaying recovery of calcium channels

• Dihydropyridines, such as nifedipine, have little or no effect on cardiac Ca channels in vivo

Calcium channel blockers: potencies for different tissues compared

• Smooth muscle dihydropyridines (e.g., nifedipine) >>

verapamil ~ diltiazem

• Heart– SA & AV nodes

verapamil ~ diltiazem >> nifedipine

– myocardium verapamil > diltiazem >> nifedipine

Clinical Effects of Ca Channel Blockers

• Relax arteriolar smooth muscle, little or no venodilation• Verapamil & diltiazem also depress SA and AV nodes

and depress myocardial contractility to some extent.• Dihydropyridines do not affect heart in vivo in normal

doses

• Note: little or no effect on nervous system or skeletal muscle (no block of N-type Ca channels, and internal cycling of Ca, respectively)

Calcium Channel Blockers: Indications

• Angina (multiple mechanisms)

• Atrial arrhythmias (AV node conduction depends on ‘Ca action potentials’ - some Ca channel blockers increase AV node refractory period)

• Hypertension (decrease vascular resistance)( ± nifedipine for malignant hypertension)(sodium nitroprusside is a drug of choice here)

(Raynaud’s syndrome?, asthma?, atherosclerosis?, reperfusion injury?)

nifedipine: indications

• Angina, variant angina• Hypertension

• Off labelaortic regurgitation, diabetic nephropathy, hiccups, hypertensive emergency/urgency, prophylaxis of migraine, premature labor (FDA-use-in-pregnancy C)

nifedipine: selected adverse reactions

• Hypotension• Peripheral edema• Flushing, headache

diltiazem: indications

• Angina, variant angina• Atrial flutter, atrial fibrillation• Paroxysmal supraventricular tachycardia• Hypertension - slow release formulations only

• Off labelCardiomyopathy, diabetic nephropathy, proteinuria, prophylaxis of migraine

diltiazem: selected adverse reactions

• Depression of cardiac muscle• AV nodal blockade• Hypotension• Peripheral edema• Flushing, headache

verapamil: indications

• Angina, unstable angina, variant angina• Atrial flutter, atrial fibrillation• Paroxysmal atrial tachycardia (prevent and Tx)• Hypertension

• Off labelclaudication, prophylaxis of migraine, mania

verapamil: selected adverse reactions

• Depression of cardiac muscle• AV nodal blockade• Hypotension• Peripheral edema• Flushing, headache• Constipation

Psaty et al., (1995)* Use of short acting (emphasis

added) Ca channel blockers, especially in high doses,

and particularly when combined with diuretics, was associated with an

increased risk of myocardial infarction.

(None of the therapies monitored decreased MI

risk significantly)

Cochrane, different result with long acting CEBs

*JAMA 274: 620-625, 1995

Scarlett O’Brian

• Scarlett O’Brian is a 38 year old female who was diagnosed with mild hypertension. She was given a prescription for verapamil (80 mg, TID, #100).

• Two days later she called her boyfriend (a prominent [married] business executive) to an urgent meeting to ‘sort out their relationship’.

• The boyfriend later testified that they had a ‘couple of drinks’ while he explained that he was not going to leave his wife.

Scarlett O’Brian collapses• As they continued to talk, Scarlett began to feel

poorly. She fainted, but appeared to be breathing adequately. Over the course of the next hour, while lying down, she became worse with a pounding headache, profound ‘weakness’ and inability to stand.

• The boyfriend delayed calling 911 for fear of making their relationship public. When she became unconscious, he finally called. By the time EMTs arrived she was in complete heart block. Efforts to restore an effective cardiac rhythm failed.

Scarlett O’Brian, what happened?

• Did she die of some underlying disease?

• Did she die of alcohol poisoning?

• Did she die because of verapamil?

Scarlett O’Brian, Autopsy

• Body is that of a well developed, well nourished female (60 kg).

• No structural cause of death could be identified.• Gastric contents contained partially disintegrated

tablets (yellow)• Toxicology (blood) identified:

caffeine - traceethanol - 350 mg/l (0.035 BAC)verapamil - 23 mg/L

If taken as prescribed, what was the predicted average steady state concentration of

verapamil in Scarlett?

• CL verapamil = 15 mL/kg/min = 54 L/h in 60 kg pt.

• Rx: 80 mg TID = 240 mg/24 h = 10 mg/h

• Css = (10 mg/h) * (0.22) / (54 L/h) = 0.04 mg/L

(equivalent to 40 ng/mL)

Verapamil, some useful numbers

• NORMALLY, a large first-pass effect, 22% bioavailability(metabolized in the liver to norverapamil)

• Urinary excretion < 3%• Plasma binding = 90%• CL = ‘15 mL/min/kg’ (15 mL*min-1*kg-1)

• Vd = 5 L/kg• Half life = 4 hours• Effective concs. = 120 ng/mL• Toxic concs. ?

Scarlett, more questions• Was Scarlett a poor metabolizer?• Suppose she had metabolized none of the verapamil, how

much should be in her body?3 days * 240 mg/day = 720 mg

• How much verapamil was in the prescription?• 100 tablets * 80 mg/tab = 8000 mg

• Even if Scarlett had taken all 100 tablets at once, if the drug was absorbed and metabolized normally only about 1600 mg should have been present.

Scarlett, the numbers tell the story

• How much verapamil should she have taken? (three days time 240 mg/day = 720 mg)

• Theoretical Vd of verapamil in Scarlett’s body? (60 kg * 5 L/kg = 300 L total Vd)*

• Blood conc at autopsy = 23 mg/L• IF distributed in total Vd, then the total verapamil

in Scarlett’s body:• 300 L * 23 mg/L = 6900 mg.

Scarlett O’Brian: A suicide gesture gone bad

• Scarlett probably took most or all of the prescription of verapamil shortly before her boyfriend arrived.

• The drug was rapidly absorbed and saturated first pass mechanisms (bioavailability approached 1).

• Hypotension and heart block were produced as the concentration of verapamil increased.

• The delay in calling 911 probably turned a suicide gesture into a suicide.