The sodium pump maintains the ionic gradients that generate resting potential in smooth muscles. Because the exchange of Na+ for K+ is non-symetrical, the pump is electrogenic and contributes to membrane potential. The sodium pump appears to be particularly important in colonic smooth muscle because up to 35 mV of the resting potential has been attributed to this transport protein. Previous experiments have demonstrated a 35 mV gradient in membrane potential across the circular layer in canine colonic muscles, which can be abolished by inhibition of the sodium pump. This gradient should greatly influence the generation and propagation of electrical slow waves in the circular muscle, because over the range of potentials the define the gradient there is significant voltage-dependent inactivation of the Ca2+ channels that participate in slow waves. A decrease in inward Ca2+ would decrease the ability of the cells to generate slow waves and would also tend to decrease the coupling between slow waves and contractions. Previous studies have suggested that the gradient in membrane potential is due to a gradient in the contribution of the sodium pump to resting potential. Several hypotheses might explain the heterogeneity in pump potential:i) The molecular from of the pump may differ between populations of cells, resting in different affinities of the pump for Na+ or K+.ii) Pump density (amount of pump protein/unit area of membrane) may differ through the circular layer.iii) Substrate concentration, such as [Na+]i, in cells from different regions of the circular layer may vary, perhaps due to differences in sodium permeability. This proposal will address these hypotheses with rigorous electrophysiological techniques in combination with modern molecular technology. First, the actual current generated by the sodium pump will be measured to determine in the electrogenic capability of the pump in various regions of colonic muscle will be compared. Molecular experiments will determine the distribution and levels of pump isoforms by measuring mRNA and polypeptide abundance in muscles from various regions of the circular layer. Experiments are also planned to explore some of the physiological factors which regulate genetic expression of the pump. The proposed experiments will be the first to use such powerful technology to explore the role of the sodium pump in smooth muscles, and the results should provide a model for future investigations of this protein.
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