Differences in the Expression of Multi‐Drug Resistant Protein 5 (MRP5) and Regulation of cGMP Levels in Phasic and Tonic Smooth Muscles

Abstract

Previous studies have identified differences in the expression of contractile proteins and signaling pathways that regulate myosin light chain phosphorylation and contraction in phasic and tonic smooth muscle. cGMP plays a critical role in smooth muscle relaxation and the intracellular levels of cGMP are regulated by its hydrolysis via phosphodiesterase 5 (PDE5) and efflux via MRP5. AIM To determine the role of MRP5 in the regulation of cGMP levels in proximal versus distal stomach. METHODS Expression of MRP5 was analyzed by quantitative RT-PCR and western blot. Increase in cGMP levels in response to sodium nitroprusside (SNP) was measured in the presence of the PDE5 inhibitor, zaprinast. RESULTS Both mRNA and protein expression of MRP5 was 4-fold higher in muscle cells from proximal stomach than distal stomach. MRP5 was co-immunoprecipitated with caveolin-1, the main caveolar protein. In both proximal and distal stomach, SNP caused an increase in intra-and extracellular cGMP levels. The increase in extracellular cGMP was significantly higher than intracellular cGMP in muscle cells of proximal stomach, whereas extra- and intracellular cGMP levels were not significantly different in muscle cells of distal stomach. CONCLUSION Higher expression and function of MRP5, which provides a rapid mechanism for regulation of cGMP levels, may further contribute to the tonic contractile phenotype of proximal stomach.