Which chemical agent will you use to modify the frog heart rate?

There are different chemical agents which are used to study the heart rate of frog. Generally Ringer's solution is used to study frog's heart simulation. At different temperature, frogs heart speeds up or slows down while using Ringer's solution at experiment. Ringer's solution is mixture of salt solution which comprises of NaCl, KCl, CaCl2 and Na2CO3. Sometimes other chemicals like MgCl or antibiotics are also used as addition in Ringer's solution. 
This solution is chiefly used to study in vitro experiments on organs and tissues like frog's heart.

Ringer's solution at 23 degree Celsius for normal heart rate
Ringer's solution at 32 degree Celsius, heart rate speed up and 
Ringer's solution at  5 degree Celsius, heart rate slows down.

other chemicals also have significant effect in heart rate,
For example, Calcium ion in excess will slow down the heart rate, Atropine increases heart rate and digitalis slows down the heart rate.

C. which metal is less dense