Fig. 4. Schematic of NO-dependent, penicillin G activation of the H⁺, K⁺ ATPase in colonic crypts. 4a.) Maximum activation of the H⁺, K⁺ ATPase occurs when colonic crypts are co-perfused with both arginine and penG. 4b.) When L-NAME is added to the perfusate, nitric oxide synthesis is inhibited. penG still activates the H⁺, K⁺ ATPase, reflecting a NO-independent, penG-sensitive activation of the transporter.