The mechanism of heart contraction and relaxation (mechanical restitution) is the basic of cardiac functioning. This mechanical restitution is driven by calcium movement and concentrations. An increase in cystolic calcium concentration causes contraction in myocardium.
Calcium from the extracellular fluid enters the cardiomyocyte through the L-type Ca2+ channels after being propagated by the cardiac action potential which depolarises the cell. This flood of calcium triggers more calcium release from the sarcoplasmic reticulum via Calcium-Induced Calcium Release (CICR). Calcium binds to the ryanodine receptor on the sarcoplasmic reticulum which then causes vast release of more calcium. This calcium then forms cross-bridges by binding to the troponin. Calcium binds to tropinin-C to induce a conformational change which moves tropinin-I away from actin/tropomyosin. Tropomyosin then moves to clear the myosin binding site on the actin thus allowing cross bridge attachment. Hence, contraction is critically dependent on intracellular calcium concentrations. The process of cross-bridges formation and the shortening of the individual cardiomyocyte require ATP (phosphate group). The result of the shortening is contraction when the myosin heads swivel along the actin filaments of the cardiomyocytes.
The greater the cystolic calcium concentration the greater the force of contraction as the calcium can bind to more tropinin-C. In cardiac muscle, the amount of calcium that is normally released from the sarcoplasmic reticulum alone is not enough to saturate all of the troponin sites. Hence, an increase in intracellular calcium means that more calcium will bind to more tropinin-C and more cross-bridges will be formed, causing an increase in contractile force.
Based on this background, I hypothesise that an increase in the extracellular calcium concentration will lead to an increase in left ventricular pressure due to greater amount of cross-bridges, and an increase in coronary blood flow due to elevated nutrients level to cater for the increase in contractility. However, the heart rate will not be affected by this increase in extracellular calcium concentration as heart rate is independent from it.
Langendorff Apparatus. Purified Krebs Henseleit Buffer (KHB) solution perfused the heart by travelling in the Langendorff apparatus. The Langendorff apparatus was made up of two similar perfusion systems, each containing a perfusate reservoir (in which the KHB solution was initially put and also where the KHB solutions was bubbled with carbogen (95% O2, 5% CO2) to oxygenate and maintain a pH of 7.4), tubing and a bubble trap before reaching the end stage of canulla from which the heart was suspended to. The Langendorff apparatus is thermo-regulated at which the KHB solutions are kept at a constant temperature of 37°C.
Each perfusion reservoir was filled with different KHB solution differing in the amount of calcium concentrations. To the left reservoir, KHB solution with calcium concentration of 2.5mM (from here on is referred to as the standard KHB solution) was added and to the right was KHB solution with calcium concentration of 0.625mM (KHB [Ca2+] = 0.625). The standard KHB solution was allowed to run through the apparatus while the heart was being fixed to the end stage (canulla) of the Langendorff apparatus. The right hand side perfusion system which was filled with the KHB [Ca2+] = 0.625 was not allowed to run through the apparatus as this was one of the variables being changed and observed. Constant refilling of the reservoirs with the KHB solutions was done throughout the experiment.
Experimental Protocol. A rat was anaesthetised with sodium pentobarbitone (50mg/kg). The chest of the rat was cut open and the heart was removed with the lungs attached. The heart was then arrested in ice-cold KHB solution.
The rate of the standard KHB solution running through the Langendorff apparatus was reduced while the...
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