Cardiac Muscle Contractions

Cardiac Muscle Contractions


CARDIAC MUSCLE CONTRACTIONS

  • Cardiac muscle contra  ction initiated by Ca2+

Mechanism of contraction:

  • Increase in cytoplasmic/sarcoplasmic Ca2+—-> Causes contraction.
Steps:

On action potential development,

Ca2+ influxes within cardiac cell.

  • Via L-type Ca2+ channels in sarcolemma.

Influx triggers Ca2+ release from sarcoplasmic reticulum to sarcoplasm.

  • Via ryanodine receptors.

Increases intracellular Ca2+

Causing cardiac muscle contraction.

Mechanism during relaxation:

  • Decreased cytoplasmic/sarcoplasmic Ca2+ –> Cardiac muslce relaxation. 
  • Ca2+ conc. reduced by 2 mechanisms,
  • Pumping back Ca2+ into SR.
  • Done by Sarcoplasmic reticulum calcium pump/SR-Ca2+ATPase/SRCA.
  • Also by, Ca2+-3Na+ antiport in sarcolemma.

PHOSPHOLAMBAN: 

  • Regulatory protein in cardiac muscles.
  • Attaches & inactivates SRCA in its dephosphorylated state.
  • Normally SRCA transports Ca2+ into SR lumen.
  • Causing muscle relaxation.

Phosphorylation of phospholamban:

  • Activated by β-adrenergic stimulation.
  • Enzyme involved – Protein kinase ‘A’.

Effects of phospholamban phosphorylation:

  • Causes cardiac muscle relaxation
  • By relieving inhibitory effect of phospholamban on SRCA.
  • Resulting in increased Ca2+ uptake into sarcoplasmic reticulum from sarcoplasm.
  • This decreases sarcoplasmic Ca2+concentration
  • Hence accelerates relaxation.
  • Reduces cardiac cycle duration.
  • Positive chronotropic effect:
  • Increases heart rate.

Effects of phospholamban dephosphorylation:

  • Produces two effects – 
  • Direct: 
  • Positive chronotropic effect:
  • Acceleration of relaxation causing increased heart rate.

Indirect:

  • Positive inotropic effect:
  • Increased contraction.
  • Resultant of increased Ca2+ sequestration in SR.
  • Hence, phospholamban “Critical Repressor of Myocardial Contractility”

In response to β-adrenergic/sympathetic stimulation:

  • Phospholamban “brake” is released.
  • Rapid increase in both contraction & relaxation.
  • As in “Fight or Flight” response.

Note:

  • Type of calcium channels in skeletal, smooth & cardiac muscles – L-type channels.
Exam Question
 

CARDIAC MUSCLE CONTRACTIONS

  • Cardiac muscle contraction are initiated by Ca2+.
  • Increased cytoplasmic/sarcoplasmic Ca2+ cause contraction.
  • AP generation causes Ca2+ influx within cardiac cell via L-type of Ca2+ channels in sarcolemma.
  • Ca2+ influx triggers Ca2+ release from sarcoplasmic reticulum to sarcoplasm via ryanodine receptors.
  • Phospholamban is the regulatory protein present in cardiac muscles.
  • Normally sarcoplasmic reticulum calcium pump/SR Ca2+- ATPase/SRCA, transports Ca2+ into SR lumen causing muscle relaxation.
  • Phosphorylation of phospholamban is activated by β-adrenergic stimulation with help of protein kinase ‘A’.
  • On phospholamban phosphorylation, its inhibitory effect on SRCA is relieved.
  • This causes increased Ca2+ uptake into sarcoplasmic reticulum from sarcoplasm of cardiac muscles.
  • Also produces positive chronotropic effect – Increased heart rate. 
  • Phospholamban is called “Critical Repressor of Myocardial Contractility”.
  • Because of positive inotropic effect.
  • Type of calcium channels in skeletal, smooth & cardiac muscles – L-type channels.
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