23515
During repetitive stimulation of skeletal muscle , extracellular ATP levels raise , activating purinergic receptors , increasing Ca ( 2 + ) influx , and enhancing contractile force , a response called potentiation .
 Chemical: ATP; Chemical: Ca ( 2 + )
Please list all entity words in the text that fit the category.Output format is "type1: word1; type2: word2". 
Option: Chemical 
Text: {0} 
Answer:
 Chemical: ATP; Chemical: Ca ( 2 + )