A nucleotide-regulated Cl-/OH- anion exchanger in endoplasmic reticulum-enriched pig pancreatic microsomes

The anion conductive pathways in preparations of endoplasmic reticulum (ER)-enriched microsomes from pig pancreas were investigated. The rate of swelling induced by cation ionophores (nigericin (nig) and/or valinomycin (val)) was measured in iso-osmotic solutions by light scattering, in the presence or absence of an inward Cl- and/or pH gradients. The rate of swelling in the presence of the inward Cl- gradient and ionophores was faster than that of controls. Low pH did not change the swelling rate in the presence of valinomycin, but it increased it in the presence of nigericin. When the Cl- gradient was abolished, valinomycin plus the pH gradient increased the rate of swelling, and this was further enhanced by nigericin. Anion transport inhibitors reduced the swelling rate. The nigericin-induced swelling was stimulated by ATP and GTP. The non-hydrolysable analogues, adenosine 5'-[beta,gamma-methylene]triphosphate, guanosine 5'-[beta-thio]triphosphate and guanosine 5'-[beta-thio]diphosphate, increased the rate of swelling, whereas adenosine 5'-[gamma-thio]triphosphate inhibited it. ADP, CTP and UTP had no effect. These data suggest the presence of a Cl-/OH- exchanger and a Cl- conductance in microsomes. They indicate that nucleotides may regulate the Cl-/OH- exchanger. Nucleotides do not need to be hydrolyzed but phosphorylation may occur to counter-balance the nucleotide-induced stimulation.