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Physiology

Cardiovascular

Question 72 of 180

Digoxin exhibits its positive inotropic effect by which of the following mechanisms:

Answer:

Cardiac glycosides (e.g. digoxin) slow the removal of Ca2+ from the cell by inhibiting the membrane Na+ pump (Na+/K+ ATPase) which generates the Na+ gradient required for driving the export of Ca2+ by Na+/Ca2+ exchange; consequently the removal of Ca2+ from the myocyte is slowed and more Ca2+ is available for the next contraction.

Inotropic Effects on Heart

Physiology / Cardiovascular / Cardiac Cycle

Last Updated: 28th February 2019

Factors that affect intracellular [Ca2+] and hence cardiac contractility are called inotropes.

Sympathetic stimulation increases cardiac muscle contractility because it causes release of noradrenaline. Noradrenaline is a positive inotrope; it binds to β1-adrenoceptors on the membrane and causes increased Ca2+ entry via L-type channels during the AP and thus increases Ca2+ release from the SR. Noradrenaline also increases Ca2+ sequestration into the SR and thus more Ca2+ is available for the next contraction.

Cardiac glycosides (e.g. digoxin) slow the removal of Ca2+ from the cell by inhibiting the membrane Na+ pump which generates the Na+ gradient required for driving the export of Ca2+; consequently the removal of Ca2+ from the myocyte is slowed and more Ca2+ is available inside the myocyte for the next contraction.

Acidosis is negatively inotropic, largely because H+ competes for Ca2+ binding sites.

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  • Biochemistry
  • Blood Gases
  • Haematology
Biochemistry Normal Value
Sodium 135 – 145 mmol/l
Potassium 3.0 – 4.5 mmol/l
Urea 2.5 – 7.5 mmol/l
Glucose 3.5 – 5.0 mmol/l
Creatinine 35 – 135 μmol/l
Alanine Aminotransferase (ALT) 5 – 35 U/l
Gamma-glutamyl Transferase (GGT) < 65 U/l
Alkaline Phosphatase (ALP) 30 – 135 U/l
Aspartate Aminotransferase (AST) < 40 U/l
Total Protein 60 – 80 g/l
Albumin 35 – 50 g/l
Globulin 2.4 – 3.5 g/dl
Amylase < 70 U/l
Total Bilirubin 3 – 17 μmol/l
Calcium 2.1 – 2.5 mmol/l
Chloride 95 – 105 mmol/l
Phosphate 0.8 – 1.4 mmol/l
Haematology Normal Value
Haemoglobin 11.5 – 16.6 g/dl
White Blood Cells 4.0 – 11.0 x 109/l
Platelets 150 – 450 x 109/l
MCV 80 – 96 fl
MCHC 32 – 36 g/dl
Neutrophils 2.0 – 7.5 x 109/l
Lymphocytes 1.5 – 4.0 x 109/l
Monocytes 0.3 – 1.0 x 109/l
Eosinophils 0.1 – 0.5 x 109/l
Basophils < 0.2 x 109/l
Reticulocytes < 2%
Haematocrit 0.35 – 0.49
Red Cell Distribution Width 11 – 15%
Blood Gases Normal Value
pH 7.35 – 7.45
pO2 11 – 14 kPa
pCO2 4.5 – 6.0 kPa
Base Excess -2 – +2 mmol/l
Bicarbonate 24 – 30 mmol/l
Lactate < 2 mmol/l

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