Cholera and pertussis toxins increase acidification of endocytic vesicles without altering ion conductances

Acidification of endocytic vesicles, driven by the vacuolar H+ pump, is affected by parallel ion transporters. Because adenosine 3',5'-cyclic monophosphate (cAMP) and heterotrimeric G proteins may alter ion transporters, I tested whether cholera and pertussis toxins affected acidification of rat liver endosomes. Fluorescein-labeled dextran-loaded "10-min" endosomes from cholera toxin-treated rats exhibited ATP-dependent rates of acidification in the presence and absence of Cl- or K+ that were approximately 60-120% (P < 0.05) faster than rates from control endosomes. This increase was greater for "older" "20-min" endosomes and less for 'early" "2-min" endosomes. Ion transport functions of 10-min and 20-min toxin-exposed endosomes were similar to those of 2-min control endosomes. Cholera toxin also increased ATP-dependent steady-state intravesicular H+ concentration by 38-218% (P < 0.05). Pertussis toxin increased endosome acidification rates by 20-54% (P < 0.05). Both toxins increased liver cAMP content, and endosomes prepared from perfused livers exposed to 0.75 mM dibutyryl cAMP exhibited similar increases in acidification rates. These studies indicate that both cholera and pertussis toxins markedly alter the function of rat liver endosomes. The mechanism is unlikely to reflect major changes in vesicle ion transporters but rather may indicate either an increase in the number of H+ pumps per endosome and/or changes in fusion, remodeling, and maturation of early endocytic vesicles in response to cAMP.