Metoprolol Succinate is a beta blocker which works specifically on the heart to reduce heart rate and make the heart more efficient at pumping blood throughout the body. Hydrochlorothiazide is a diuretic which lowers blood pressure by removing extra water and certain electrolytes from the body. Over time it also relaxes blood vessels and improves blood flow. Metoprolol is a cardioselective β-blocker medication that acts on the β-1 adrenergic receptors in the heart and reduces the effects of catecholamines (such as adrenaline and noradrenaline) on cardiac function. The mechanism of action of metoprolol can be explained in the following steps: Metoprolol binds to the β-1 adrenergic receptors on the surface of the cardiac cells, especially in the sinoatrial node, the atrioventricular node, and the ventricular myocardium. Metoprolol blocks catecholamine receptor activation by competing for the same receptors; this reduces the intracellular signaling pathways mediated by cyclic adenosine monophosphate and protein kinase A (PKA). Metoprolol decreases the slope of phase 4 in the nodal action potential, which is the spontaneous depolarization phase that determines the heart rate. By reducing the sodium influx during this phase, metoprolol slows down the firing rate of the pacemaker cells and lowers the heart rate.