Figure 5. Effect of ATP in the presence and absence of extracellular Ca²⁺.

Experimental procedures were as described for Fig. 1. A PCT was exposed to 100 μmol l⁻¹ ATP for the periods indicated by the horizontal filled bars successively in the presence (1.8 mmol l⁻¹ CaCl₂) and absence (100 μmol l⁻¹ EGTA, open bar) of external Ca²⁺. The break in the trace represents a 25 min period of superfusion with Ringer solution.