Voltage-gated L-type calcium channels (Cav1.2) are the primary route of calcium entry into vascular smooth muscle cells, cardiac myocytes, SA node pacemaker cells, and AV node conduction cells.¹¹ In vascular smooth muscle: membrane depolarization leads to L-type channel opening, Ca2+ influx, calmodulin activation, myosin light chain kinase (MLCK) phosphorylation, myosin-actin cross-bridge cycling, and contraction (vasoconstriction). CCB blockade reduces Ca2+ entry, reduces MLCK activity, causes smooth muscle relaxation, and produces vasodilation.¹¹ In cardiac pacemaker and conduction tissue: Ca2+ entry drives phase 0 depolarization in SA and AV nodal cells (unlike ventricular myocytes where phase 0 is driven by fast Na+ channels). CCB blockade slows SA node automaticity (negative chronotropy), slows AV conduction (negative dromotropy), and reduces ventricular rate.¹¹ In ventricular myocardium: Ca2+ entry triggers calcium-induced calcium release from the sarcoplasmic reticulum (CICR) and contraction. CCB blockade reduces the Ca2+ transient and reduces contractility (negative inotropy).¹¹

All CCBs block L-type channels, but they bind to different regions of the channel alpha-1 subunit with markedly different affinities for vascular versus cardiac tissue.¹¹ Dihydropyridines (DHPs) have a vascular:cardiac selectivity ratio of approximately 10:1 to 30:1 (highly vascular-selective).¹¹ Their primary effect is vascular smooth muscle relaxation producing peripheral vasodilation (afterload reduction) and coronary vasodilation. Cardiac effects are minimal at therapeutic doses (negligible chronotropy, dromotropy, or inotropy). The dominant clinical liability when used without beta-blockers is baroreceptor-mediated reflex tachycardia and sympathetic activation in response to the BP fall from peripheral vasodilation.³·⁴ Non-dihydropyridines (non-DHPs): verapamil (phenylalkylamine) has a vascular:cardiac selectivity ratio of approximately 1:1 (equal vascular and cardiac effects). Diltiazem (benzothiazepine) has a ratio of approximately 3:1 (intermediate; somewhat more vascular than cardiac).¹¹ Primary effects of both include vasodilation AND cardiac rate/conduction slowing. Vasodilation tends to be offset by direct negative chronotropy; HR does not rise with non-DHPs and often HR falls.¹¹

Classification: phenylalkylamine; L-type CCB. Selectivity: approximately equal cardiac and vascular effects (1:1 ratio). Half-life: 6-8 hours (IR); ~12 hours (ER). Bioavailability: ~20-35% with significant first-pass hepatic metabolism via CYP3A4. Active metabolite: norverapamil (~20% activity of parent). Dose (angina): 80-120 mg three times daily (IR); 120-480 mg once daily (SR/ER).¹¹ Hemodynamic profile: coronary and peripheral vasodilation; negative chronotropy (reduces HR 15-20% at therapeutic doses); negative dromotropy (slows AV nodal conduction, prolongs PR interval); negative inotropy (clinically significant; most pronounced in pre-existing LV dysfunction). Net result: no reflex tachycardia; HR falls or is unchanged; BP reduces modestly.¹¹

Verapamil drug interactions: quantified clinical reference: Verapamil is both a CYP3A4 inhibitor and a P-glycoprotein inhibitor, producing clinically significant interactions across multiple drug classes.¹¹ Digoxin: verapamil inhibits P-glycoprotein-mediated renal tubular secretion of digoxin and reduces its non-renal clearance, increasing digoxin plasma levels by 70-80%. The digoxin dose should be reduced by 30-50% when verapamil is introduced and levels re-checked within 7-14 days. Additive negative chronotropy and dromotropy further increase the risk of bradycardia and AV block at any given digoxin concentration.¹¹ Statins: CYP3A4 (cytochrome P450 3A4) inhibition by verapamil substantially increases plasma concentrations of simvastatin (by approximately 2-3 fold) and lovastatin, raising myopathy and rhabdomyolysis risk. The simvastatin dose should be limited to 20 mg/day with concomitant verapamil or the statin switched to rosuvastatin, pravastatin, or fluvastatin, which are not significantly metabolized by CYP3A4.¹¹ Beta-blockers: the combination of verapamil with any beta-blocker is CONTRAINDICATED due to additive and potentially synergistic depression of SA node automaticity and AV nodal conduction, risking severe bradycardia, complete AV block, and hemodynamic compromise. This contraindication extends to ophthalmic beta-blocker preparations (timolol eye drops) which have systemic absorption.³·⁴ Colchicine: verapamil inhibits both CYP3A4 and P-glycoprotein, the two primary elimination pathways for colchicine, and can increase colchicine levels to toxic ranges producing myopathy, bone marrow suppression, and multi-organ failure; particularly dangerous in renal impairment where colchicine accumulation is already elevated.¹¹ Cyclosporine: verapamil substantially increases cyclosporine plasma levels via CYP3A4 (cytochrome P450 3A4) inhibition, risking nephrotoxicity; transplant patients on cyclosporine require careful monitoring if verapamil is added. Dofetilide: verapamil inhibits renal organic cation transporter 2 (OCT2)-mediated secretion of dofetilide, raising dofetilide plasma levels and QTc; this combination is CONTRAINDICATED (included in the dofetilide prescribing information as an absolute contraindication).¹¹ Ranolazine: verapamil moderately inhibits CYP3A4, increasing ranolazine levels by approximately 1.5-2.5 fold; ranolazine should be limited to 500 mg twice daily when prescribed with verapamil, and baseline and follow-up QTc monitoring is required.³ Constipation affects approximately 30% of patients on verapamil and is the most common reason for discontinuation, arising from inhibition of Ca2+-dependent smooth muscle contraction in the gut.¹¹ Manage with dietary fiber and stool softeners; dose reduction if severe.

Classification: benzothiazepine; L-type CCB. Selectivity: intermediate (3:1 vascular:cardiac), more vascular-selective than verapamil but with clinically meaningful cardiac rate-limiting effects. Half-life: 3-4 hours (IR); 5-7 hours (SR); once-daily ER formulations achieve 12-24 hour coverage. Bioavailability: ~40% with hepatic first-pass via CYP3A4. Dose (angina): 120-360 mg daily (divided or ER once daily).¹¹ Hemodynamic profile: peripheral and coronary vasodilation; moderate negative chronotropy (less than verapamil); AV nodal slowing (prolongs PR interval, less than verapamil); mild negative inotropy (less pronounced than verapamil; generally safe in mild-moderate LV dysfunction with caution). Net result: HR reduction + vasodilation without reflex tachycardia.¹¹ Clinical advantage: combines vasodilation AND HR reduction in a single agent, useful when a patient needs both effects but cannot tolerate beta-blockers.³·⁴ Better tolerated than verapamil in terms of constipation and negative inotropy.

Diltiazem drug interactions: quantified clinical reference: Diltiazem is a moderate CYP3A4 inhibitor and a moderate P-glycoprotein inhibitor, producing a drug interaction profile qualitatively similar to verapamil but generally of lower magnitude.¹¹ Digoxin: diltiazem increases digoxin levels by approximately 20-40% via combined effects on P-glycoprotein and renal clearance, less than verapamil but still clinically significant, particularly given the narrow therapeutic window of digoxin. Digoxin levels should be rechecked 7-14 days after adding diltiazem, and additive nodal effects monitored by ECG.¹¹ Ranolazine: diltiazem's moderate CYP3A4 inhibition increases ranolazine plasma concentrations by approximately 1.5-2.5 fold. The maximum recommended ranolazine dose when co-administered with diltiazem is 500 mg twice daily (not the standard maximum of 1000 mg twice daily). QTc must be monitored at baseline and after uptitration.³ This is one of the most clinically relevant interactions in the angina context because ranolazine is frequently added to patients already on diltiazem as their rate-controlling agent. Ivabradine: both diltiazem and ivabradine reduce heart rate by distinct mechanisms (AV nodal calcium channel blockade versus sinus node If inhibition). Their combination produces additive and potentially unpredictable HR reduction. Diltiazem also inhibits CYP3A4, raising ivabradine plasma levels. If this combination is used, the resting HR must be monitored closely and doses adjusted to maintain HR above 50 bpm.³ Beta-blockers: CONTRAINDICATED in combination with diltiazem for the same reasons as verapamil; additive SA and AV nodal depression risks severe bradycardia and AV block. The relative contraindication is slightly less absolute than with verapamil because diltiazem's negative dromotropy is less pronounced, but the combination remains contraindicated in clinical practice.³·⁴ Statins: diltiazem increases simvastatin levels by approximately 2-4 fold via CYP3A4 inhibition, similar in magnitude to verapamil. Limit simvastatin to 20 mg/day or substitute a non-CYP3A4 statin.¹¹ Cyclosporine: as with verapamil, diltiazem increases cyclosporine levels; transplant patients require monitoring. Carbamazepine: diltiazem increases carbamazepine levels substantially, risking neurotoxicity; this combination requires careful monitoring of carbamazepine plasma concentrations.¹¹

Vasospastic angina is a disorder of abnormal coronary smooth muscle Ca2+-mediated reactivity.⁵ CCBs directly address this pathophysiology by blocking L-type Ca2+ channels in coronary smooth muscle, preventing and reversing spasm regardless of the vasoconstrictive trigger (endothelin, serotonin, alpha-adrenergic stimulation, cold, hyperventilation).⁵ Both DHP and non-DHP CCBs are effective (ESC 2019 Class I for CCBs in vasospastic angina).⁵ The effect is not dependent on endothelial function.⁵

Higher CCB doses are typically required than for stable exertional angina:⁵ amlodipine 5-10 mg once daily (up to maximum); nifedipine GITS 60-90 mg once daily; diltiazem ER 240-360 mg once daily; verapamil ER 240-480 mg once daily. If attacks persist on maximal CCB monotherapy: add a long-acting nitrate (isosorbide mononitrate extended-release (ISMN-ER) or NTG patch); review and eliminate triggers; and reinforce smoking cessation (most important modifiable risk factor).⁵ Refractory vasospastic angina requires specialist referral; dual CCB therapy under supervision; and PCI for focal coronary spasm at stenosis sites in selected cases.⁵

Vasospastic angina may remit spontaneously over months to years, particularly after smoking cessation.⁵ Annual reassessment with consideration for slow CCB dose reduction if attack-free for 6-12 months is reasonable; abrupt discontinuation risks rebound spasm.⁵

Peripheral edema (ankle/pedal edema) is the most common DHP adverse effect, affecting 10-30% of patients at standard doses and up to 50% with amlodipine 10 mg.¹¹ The mechanism is arteriolar dilation without proportionate venodilation, increasing capillary hydrostatic pressure and causing fluid transudation into the interstitium. It is NOT due to sodium retention or cardiac failure.¹¹ Furosemide will worsen it by activating the renin-angiotensin-aldosterone system (RAAS). Management options include dose reduction; adding an ACE inhibitor/ARB (venodilation balances arteriolar dilation); the ACCOMPLISH trial showed amlodipine + benazepril had less edema than amlodipine alone);⁸ leg elevation; and switching to lercanidipine where available. Reflex tachycardia is more pronounced with short-acting DHPs (nifedipine IR); minimized with amlodipine (slow onset/offset); and abolished by concurrent beta-blocker.³·⁴ Flushing and headache are vasodilatory symptoms, more common at initiation, usually resolving within 1-2 weeks, and more common with nifedipine than amlodipine.¹¹ Gingival hyperplasia is a class effect of all CCBs, more common with nifedipine and verapamil than amlodipine, with risk increased with concurrent cyclosporine or phenytoin; manage with meticulous oral hygiene; may require gingivectomy in severe cases.¹¹

Bradycardia from SA node suppression may be profound at high doses or in combination with digoxin or beta-blockers. Monitor resting HR; target 55-65 bpm for angina.¹¹ AV block: both verapamil and diltiazem prolong the PR interval. Risk of 2nd or 3rd degree AV block exists in combination with other AV-nodal agents or with pre-existing conduction disease. Baseline ECG is required and PR interval monitoring ongoing.¹¹ Negative inotropy and heart failure risk: both verapamil and diltiazem are CONTRAINDICATED in HFrEF (EF <40%).⁶·¹¹ The mechanism is reduced Ca2+ transient impairing contractility in a myocardium already relying on sympathetic upregulation. Amlodipine is safe in HFrEF (PRAISE-1; PREVENT).⁶·³ Constipation affects ~30% of verapamil patients due to inhibition of intestinal smooth muscle Ca2+-mediated contraction;¹¹ diltiazem causes much less constipation. Wolff-Parkinson-White (WPW) syndrome + verapamil is CONTRAINDICATED: verapamil blocks the AV node but not the accessory pathway, potentially accelerating ventricular rate via the accessory pathway during AF and causing VF risk.¹¹ CYP3A4 drug interactions (both non-DHPs) include digoxin toxicity, statin myopathy (avoid simvastatin >20 mg; use rosuvastatin or pravastatin), cyclosporine toxicity, colchicine toxicity, and interactions with midazolam/triazolam.¹¹

In stable exertional angina: first-line monotherapy when BB is contraindicated: amlodipine 5-10 mg once daily;³·⁴ first-line combination is amlodipine + beta-blocker;³·⁴ if rate control is also needed and BB is contraindicated: diltiazem ER or verapamil ER.³·⁴ In vasospastic angina: first-line (all subtypes) is any long-acting CCB;⁵ AVOID all beta-blockers.⁵ In microvascular angina: first-line is beta-blocker or CCB (amlodipine preferred);⁴ add-on options are ranolazine and ACE inhibitors.⁴ In angina with HFrEF (EF <40%): safe CCB is AMLODIPINE ONLY;⁶·¹¹ AVOID verapamil and diltiazem.⁶·¹¹ In angina with COPD/asthma when BB is contraindicated: preferred is amlodipine or diltiazem ER (if HR is also high).³·⁴ In angina with AF when rate control is needed: preferred is diltiazem ER or verapamil ER;⁴ AVOID combination with beta-blocker.³·⁴