Updated: 8/5/2019

Cardiac Myocyte Physiology

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Snapshot
  • A 64-year-old man with chronic obstructive pulmonary disease presents with palpitations and lightheadedness. In his chart, there are documented allergies to calcium channel blockers. He is tachycardic (160/min). An electrocardiogram reveals an irregular supraventricular rhythm with polymorphic P waves. A medication that increases contractility is given. (Digoxin)
Introduction
  • Generation of action potential
    • initiated by the sinoatrial (SA) node
    • delayed by the atrioventricular (AV) node
      • allows for ventricular filling prior to contraction
  • Excitability and refractory periods
    • excitability
      • myocytes are able to generate an action potential in response to depolarization
      • cells must reach the threshold potential to achieve an action potential
      • note, myocytes are electrically coupled via gap junctions that allow propagation of the action potential
    • refractory period
      • occurs after depolarization
      • prevents further depolarization as the inactivation gates on the Na+ channels are closed and no current can flow through them
      • once a cell is repolarized, the inactivation gates on the Na+ channels re-open
        • a cell is once again excitable
Excitation-Contraction Coupling
  • The action potential causes a rush of Ca2+ into the cell
    • via L-type voltage-gated Ca2+ channels 
      • indirectly blocked by β-blockers via ↓ cAMP
      • directly blocked by non-dihydropyridine Ca2+ channel blockers
  • This triggers the release of more Ca2+ from the sarcoplasmic reticulum
    • via ryanodine receptors
      • blocked by the ryanodine toxin
  • Ca2+ binds to troponin C
    • tropomyosin is moved out of the way
  • Actin and myosin can now bind
    • cross-bridges are formed and broken, and thin and thick filaments slide past each other creating tension
  • Contraction occurs
Contractility
  • Contractility, or level of tension generated, is determined by intracellular Ca2+
    • positive inotropic agents increase intracellular Ca2+
    • digoxin
      • inhibits extracellular K+ binding site of the Na+-K+ ATPase on myocyte cell membrane
        • when the ATPase is inhibited, ↑ Na+ intracellular concentration
        • Ca2+-Na+ exchanger decreases the amount of Ca2+ it pumps out of the cell
        • ↑ Ca2+ intracellular concentration
Relaxation
  • Relaxation and repolarization of the cell membrane occurs when ↓ intracellular Ca2+ concentration back to resting levels 
    • Ca2+ ATPase on sarcoplasmic reticulum membrane
      • Ca2+ returns to sarcoplasmic reticulum
    • Ca2+ ATPase on cell membrane
      • Ca2+ extruded out of the cell
    • Ca2+-Na+ exchange on cell membrane
      • exchanges 1 Ca2+ out of the cell for 3 Na+ into the cell
 

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Lab Values
Blood, Plasma, Serum Reference Range
ALT 8-20 U/L
Amylase, serum 25-125 U/L
AST 8-20 U/L
Bilirubin, serum (adult) Total // Direct 0.1-1.0 mg/dL // 0.0-0.3 mg/dL
Calcium, serum (Ca2+) 8.4-10.2 mg/dL
Cholesterol, serum Rec: < 200 mg/dL
Cortisol, serum 0800 h: 5-23 μg/dL //1600 h:
3-15 μg/dL
2000 h: ≤ 50% of 0800 h
Creatine kinase, serum Male: 25-90 U/L
Female: 10-70 U/L
Creatinine, serum 0.6-1.2 mg/dL
Electrolytes, serum  
Sodium (Na+) 136-145 mEq/L
Chloride (Cl-) 95-105 mEq/L
Potassium (K+) 3.5-5.0 mEq/L
Bicarbonate (HCO3-) 22-28 mEq/L
Magnesium (Mg2+) 1.5-2.0 mEq/L
Estriol, total, serum (in pregnancy)  
24-28 wks // 32-36 wks 30-170 ng/mL // 60-280 ng/mL
28-32 wk // 36-40 wks 40-220 ng/mL // 80-350 ng/mL
Ferritin, serum Male: 15-200 ng/mL
Female: 12-150 ng/mL
Follicle-stimulating hormone, serum/plasma Male: 4-25 mIU/mL
Female: premenopause: 4-30 mIU/mL
midcycle peak: 10-90 mIU/mL
postmenopause: 40-250
pH 7.35-7.45
PCO2 33-45 mmHg
PO2 75-105 mmHg
Glucose, serum Fasting: 70-110 mg/dL
2-h postprandial:<120 mg/dL
Growth hormone - arginine stimulation Fasting: <5 ng/mL
Provocative stimuli: > 7ng/mL
Immunoglobulins, serum  
IgA 76-390 mg/dL
IgE 0-380 IU/mL
IgG 650-1500 mg/dL
IgM 40-345 mg/dL
Iron 50-170 μg/dL
Lactate dehydrogenase, serum 45-90 U/L
Luteinizing hormone, serum/plasma Male: 6-23 mIU/mL
Female: follicular phase: 5-30 mIU/mL
midcycle: 75-150 mIU/mL
postmenopause 30-200 mIU/mL
Osmolality, serum 275-295 mOsmol/kd H2O
Parathyroid hormone, serume, N-terminal 230-630 pg/mL
Phosphatase (alkaline), serum (p-NPP at 30° C) 20-70 U/L
Phosphorus (inorganic), serum 3.0-4.5 mg/dL
Prolactin, serum (hPRL) < 20 ng/mL
Proteins, serum  
Total (recumbent) 6.0-7.8 g/dL
Albumin 3.5-5.5 g/dL
Globulin 2.3-3.5 g/dL
Thyroid-stimulating hormone, serum or plasma .5-5.0 μU/mL
Thyroidal iodine (123I) uptake 8%-30% of administered dose/24h
Thyroxine (T4), serum 5-12 μg/dL
Triglycerides, serum 35-160 mg/dL
Triiodothyronine (T3), serum (RIA) 115-190 ng/dL
Triiodothyronine (T3) resin uptake 25%-35%
Urea nitrogen, serum 7-18 mg/dL
Uric acid, serum 3.0-8.2 mg/dL
Hematologic Reference Range
Bleeding time 2-7 minutes
Erythrocyte count Male: 4.3-5.9 million/mm3
Female: 3.5-5.5 million mm3
Erythrocyte sedimentation rate (Westergren) Male: 0-15 mm/h
Female: 0-20 mm/h
Hematocrit Male: 41%-53%
Female: 36%-46%
Hemoglobin A1c ≤ 6 %
Hemoglobin, blood Male: 13.5-17.5 g/dL
Female: 12.0-16.0 g/dL
Hemoglobin, plasma 1-4 mg/dL
Leukocyte count and differential  
Leukocyte count 4,500-11,000/mm3
Segmented neutrophils 54%-62%
Bands 3%-5%
Eosinophils 1%-3%
Basophils 0%-0.75%
Lymphocytes 25%-33%
Monocytes 3%-7%
Mean corpuscular hemoglobin 25.4-34.6 pg/cell
Mean corpuscular hemoglobin concentration 31%-36% Hb/cell
Mean corpuscular volume 80-100 μm3
Partial thromboplastin time (activated) 25-40 seconds
Platelet count 150,000-400,000/mm3
Prothrombin time 11-15 seconds
Reticulocyte count 0.5%-1.5% of red cells
Thrombin time < 2 seconds deviation from control
Volume  
Plasma Male: 25-43 mL/kg
Female: 28-45 mL/kg
Red cell Male: 20-36 mL/kg
Female: 19-31 mL/kg
Cerebrospinal Fluid Reference Range
Cell count 0-5/mm3
Chloride 118-132 mEq/L
Gamma globulin 3%-12% total proteins
Glucose 40-70 mg/dL
Pressure 70-180 mm H2O
Proteins, total < 40 mg/dL
Sweat Reference Range
Chloride 0-35 mmol/L
Urine  
Calcium 100-300 mg/24 h
Chloride Varies with intake
Creatinine clearance Male: 97-137 mL/min
Female: 88-128 mL/min
Estriol, total (in pregnancy)  
30 wks 6-18 mg/24 h
35 wks 9-28 mg/24 h
40 wks 13-42 mg/24 h
17-Hydroxycorticosteroids Male: 3.0-10.0 mg/24 h
Female: 2.0-8.0 mg/24 h
17-Ketosteroids, total Male: 8-20 mg/24 h
Female: 6-15 mg/24 h
Osmolality 50-1400 mOsmol/kg H2O
Oxalate 8-40 μg/mL
Potassium Varies with diet
Proteins, total < 150 mg/24 h
Sodium Varies with diet
Uric acid Varies with diet
Body Mass Index (BMI) Adult: 19-25 kg/m2
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(M1.CV.13.101) A scientist observes a myocyte beating in cell culture. Which step is the most direct necessary component of relaxation for this cell? Review Topic | Tested Concept

QID: 100617
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Influx of sodium ions

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Efflux of potassium ions

50%

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Influx of calcium ions from the sacroplasmic reticulum

4%

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Influx of calcium ions from outside the myocyte

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Efflux of calcium ions

38%

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