Functional Relationship between a 65-kD mdr1 P-Glycoprotein and ATP-Activated Chloride Conductance in Pancreatic Zymogen Granule Membranes

Abstract

Zymogen granule (ZG) membranes contain a regulated Cl– conductance which may contribute to secretagogue-induced enzyme, salt and fluid secretion. ATP and nonhydrolyzable ATP analogs, such as adenosine 5’-[βγ-methylene]triphosphate (AMP-PCP) increase ZG membrane (ZGM) Cl– conductance. Here we demonstrate that monoclonal antibodies (mAbs) against multidrug resistance (MDR1) P-glycoprotein inhibit AMP-PCP-activated rat ZG Ch conductance. Moreover, in ZG from mdrla knockout mice, AMP-PCP-activated Cl– conductance is reduced by about 50%. Immunocytochemistry of rat pancreatic tissue using mAbs against MDR1 reveals labeling of the surface of acinar ZG and of apical membranes of pancreatic ducts. A ∼ 180-kD protein is detected in pancreatic duct cell lysates, as expected for full-length P-glycoprotein, and a ∼ 65-kD protein is labeled in ZGM. A high-affinity probe for dihydropyridine-binding domains on MDR1, a benzoyldihydrocinnamic acid derivative of dihydropyridine ((-)-[3H]BZDC-DHP), binds to ZGM with high affinity (Kd ∼ 8 nM), which resembles the binding affinity of full-length MDR1 P-glycoprotein. However, the profile of pharmacological modulation of (-)-[3H]BZDC-DHP binding differs from that of full-length MDR1. The data suggest that ZGM express a marl gene product distinct from the known mdr1 proteins, that may be functionally associated with AMP-PCP-activated ZG Cl- conductance.