Final answer:
Synthetic drugs such as dopamine and isoproterenol, thyroid hormones like thyroxin, and hypercalcemia cause dose-related increases in heart rate, blood pressure, and myocardial contractility by increasing intracellular calcium levels or altering gene activity in heart cells.
Step-by-step explanation:
The agents that cause a dose-related increase in heart rate, blood pressure, and myocardial contractility include synthetic drugs like dopamine and isoproterenol, thyroid hormones like thyroxine, and excess calcium leading to hypercalcemia. These substances act by increasing the influx of calcium ions into cells or by altering gene activity, which in turn increases the strength of heart muscle contractions.
Dopamine and isoproterenol work by mimicking the effects of epinephrine (adrenaline) and norepinephrine (NE), leading to higher intracellular calcium levels. While thyroid hormones, like the active form T3 or triiodothyronine, directly enter cardiomyocytes to alter activity at the genome level and increase cardiac rate and contractility. Hypercalcemia acts as a positive inotropic agent by increasing intracellular calcium, enhancing the strength of contractions. Conversely, beta-blockers are used to lower heart rate by blocking the beta-1 receptors which adrenaline binds to, addressing issues such as hypertension.