Ramayana

Numerous pharmacological agents can alter cardiac rate and rhythm.   Describe the mechanisms of action and the general effects on heart rate and rhythm of the following drugs/drug types: (2 marks each; 10 marks)

(A)   Beta blockers -

Beta-Blockers are drugs that are used to bind beta-adrenoceptors (b1 and b2)which block the binding of epinephrine and nor epinephrine. By blocking these chemicals it inhibits the normal sympathetic effects on these receptors. Some beta-blockers are partial agonists which may activate the receptor however prevents the binding of nor epinephrine on to the receptor.
In Cardiac tissue, beta-blockers bind to these beta- adrenoceptors in the conducting system and the muscle cells on the two beta receptors. These receptors bind nor epinephrine that is released from the sympathetic adrenergic nerves as well as norepinephrine and epinephrine found in the circulating blood. The Beta-blockers prevent these chemicals from binding to the receptors by competing for the binding site. Thus   the result is the reduction of the sympathetic tone of heart, the decrease in the heart rate, contractility as well as electrical conduction velocity and relaxation.

(B)   Calcium channel blockers
Calcium Channel blockers, bind to calcium channels located on the smooth muscle cells, cardiac muscle cells and the cardiac nodal tissue. These channels regulate the influx of calcium into the muscle cells. The influx of calcium stimulates the contraction of smooth muscle and cardiac   Myocytes. Especially in the Cardiac nodal tissue, calcium channels are responsible for the pacemaker currents and the first phase of action potentials. By blocking the entry of calcium, the smooth muscles begin to relax, and decreases the heart rate, conduction velocity and the myocardial force.

(C)   Nicotine
Nicotine is an organic compound and it is found in the tobacco plant. Nicotine is composed of carbon, nitrogen, hydrogen and oxygen and belongs to a group of...