Final answer:
Isoproterenol hydrochloride is a cardiac drug that increases myocardial contractility by mimicking catecholamines and increasing intracellular calcium levels, thus enhancing the strength of heart muscle contractions.
Step-by-step explanation:
The cardiac drug that increases myocardial contractility is Isoproterenol hydrochloride. Isoproterenol, a synthetic drug, mimics the effects of epinephrine and norepinephrine, leading to an increased influx of calcium ions from the extracellular fluid into the cardiomyocytes. This action heightens the concentration of intracellular calcium ions, resulting in a stronger myocardial contraction. Drugs such as digitalis also enhance myocardial contractility by preventing the sequestration of calcium ions into the sarcoplasmic reticulum, thereby increasing intracellular calcium levels.
Hormones like thyroid hormones and glucagon similarly exert positive inotropic effects on the heart. Conversely, certain drugs, including beta blockers and calcium channel blockers, act as negative inotropic agents, reducing myocardial contractility by blocking the entry of calcium into the heart cells. Understanding these mechanisms is crucial in the clinical management of various cardiac conditions, where modulating the force of myocardial contractions can be therapeutic. This ranges from increasing contractility in heart failure to decreasing it in conditions where less cardiac workload is desired.