Tachycardia-Induced Heart Failure Does Not Alter Myocardial P-glycoprotein Expression

Cardiomyocytes of Chronically Ischemic Pig Hearts Express the MDR-1 Gene-encoded P-glycoprotein

The multidrug-resistant (MDR)-1 gene-encoded P-glycoprotein (Pgp-170) is not normally present in the cardiomyocyte. Given that in other tissues Pgp-170 is not found under normoxic conditions but is expressed during hypoxia, we searched for Pgp-170 in chronically ischemic porcine cardiomyocytes. Pgp-170 was detected and localized via immunohistochemistry in ischemic and nonischemic cardiomyocytes of eight adult pigs 8 weeks after placement of an Ameroid constrictor at the origin of the left circumflex artery (Cx). Regional myocardial ischemia in the Cx bed was documented with nuclear perfusion scans. Pgp-170 mass was quantified using Western blot analysis. In all pigs, Pgp-170 was consistently present in the sarcolemma and T invaginations of the cardiomyocytes of the ischemic zone. Pgp-170 expression decreased toward the border of the ischemic zone and was negative in nonischemic regions as well as in the myocardium of sham-operated animals. Western blot analysis yielded significantly higher Pgp-170 mass in ischemic than in nonischemic areas. We conclude that Pgp-170 is consistently expressed in the cardiomyocytes of chronically ischemic porcine myocardium. Its role in the ischemic heart as well as in conditions such as myocardial hibernation, stunning, and preconditioning may have potentially relevant clinical implications and merits further investigation.

The ATP-binding cassette transporters and their implication in drug disposition: a special look at the heart

The passage of drugs across cell membranes dictates their absorption, distribution, metabolism, and excretion. This process is determined by several factors including the molecular weight of the compounds, their shape, degree of ionization, and binding to proteins. Accumulation of xenobiotics into tissues does not depend only on their ability to enter cells, but also on their ability to leave them. For instance, the role of efflux transporters such as ATP-binding cassette (ABC) proteins in the disposition of drugs is now well recognized. Actually, ABC transporters act in synergy with drug-metabolizing enzymes to protect the organism from toxic compounds. The most studied transporter from the ABC transporter superfamily, P-glycoprotein, was found to be overexpressed in tumor cells and associated with an acquired resistance to several anticancer drugs. P-glycoprotein, thought at first to be confined to tumor cells, was subsequently recognized to be expressed in normal tissues such as the liver, kidney, intestine, and heart. Even though information remains rather limited on the functional role of ABC transporters in the myocardium, it is hypothesized that they may modulate efficacy and toxicity of cardioactive agents. This review addresses recent progress on knowledge about the ABC transporters in drug disposition and more precisely their role in drug distribution to the heart.