Physiology
Cardiovascular
Regarding cardiac excitation-contraction coupling, which of the following statements is CORRECT:
Answer:
Although Ca2+ entry during the action potential (AP) is essential for contraction, it only accounts for about 25% of the rise in intracellular Ca2+. The rest is released from Ca2+ stores in the sarcoplasmic reticulum (SR). In relaxation, about 80% of Ca2+ is rapidly pumped back into the SR (sequestered) by Ca2+ ATPase pumps. The Ca2+ that entered the cell during the AP is transported out of the cell primarily by the Na+/Ca2+ exchanger in the membrane. When more action potentials occur per unit time, more Ca2+ enters the cell during the AP plateau, more Ca2+ is stored in the SR, more Ca2+ is released from the SR and thus more Ca2+ is left inside the cell and greater tension is produced during contraction. Increased heart rate increases the force of contraction in a stepwise fashion as intracellular [Ca2+] increases cumulatively over several beats; this is the Treppe effect. Cardiac glycosides such as digoxin have a positive inotropic effect.Cardiac Excitation-Contraction Coupling
Physiology / Cardiovascular / Cardiac Cycle
Last Updated: 29th July 2024
Cardiac muscle contracts when intracellular Ca2+ rises (> 100 nmol/L).
Contraction
Although Ca2+ entry during the action potential (AP) is essential for contraction, it only accounts for about 25% of the rise in intracellular Ca2+. The rest is released from Ca2+ stores in the sarcoplasmic reticulum (SR).
APs travel down invaginations of the sarcolemma called T-tubules, which are close to, but do not touch, the terminal cisternae of the SR. During the AP plateau, Ca2+ enters the cell and activates Ca2+ sensitive Ca2+ release channels in the sarcoplasmic reticulum allowing stored Ca2+ to flood into the cytosol; this is called Ca2+-induced Ca2+ release. The amount of Ca2+ released is dependent on how much is stored, and on the size of the initial Ca2+ influx during the AP.
Excitation-Contraction Coupling. (Image by OpenStax [CC BY 4.0 , via Wikimedia Commons)
Relaxation
In relaxation, about 80% of Ca2+ is rapidly pumped back into the SR (sequestered) by Ca2+ ATPase pumps. The Ca2+ that entered the cell during the AP is transported out of the cell primarily by the Na+/Ca2+ exchanger in the membrane which pumps one Ca2+ ion out in exchange for three Na+ ions in, using the Na+ electrochemical gradient as an energy source. This is relatively slow and continues during diastole.
Treppe Effect
When more action potentials occur per unit time, more Ca2+ enters the cell during the AP plateau, more Ca2+ is stored in the SR, more Ca2+ is released from the SR and thus more Ca2+ is left inside the cell and greater tension is produced during contraction. Increased heart rate increases the force of contraction in a stepwise fashion as intracellular [Ca2+] increases cumulatively over several beats.
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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 |
