3803
The effects of varying the extracellular concentrations of Na and Ca ( [ Na ] o and [ Ca ] o ) on both , the spontaneous beating and the negative chronotropic action of verapamil , were studied in the isolated rat atria .
 Na, Ca, Na, Ca, verapamil
Please list all entity words in the text that fit the category. Output format is "word1, word2". 
Option: Disease, Chemical 
Text: {0} 
Answer:
 Na, Ca, Na, Ca, verapamil