The long-term goal of this proposed study is to identify the mechanisms of pressure natriuresis in proximal tubules and its relationship with hypertension. Pressure natriuresis is an increase of Na+ excretion caused by an increase in arterial pressure in the absence of changes in renal blood flow and glomerular filtration rate, which enables the kidney to use arterial pressure as an input signal to regulate the extracellular fluid volume. However, pressure natriuresis is set to higher blood pressure levels in all form of hypertension. An acute blood pressure increase inhibits fluid and NaC1 reabsorption in rate proximal tubules, and this response is associated with a rapid internalization of apical Na+/H+ exchangers from the brush border into subapical intracellular stores. This response probably contributes a large fraction of the natriuresis and diuresis that occurs when blood pressure rises. The objective of this proposal is to test whether trafficking of apical Na+/H+ exchangers induced by acute hypertension inhibits apical Na+/H+ exchangers activity and fluid reabsorption in proximal tubules in vivo, to delineate the mechanisms of this protein-trafficking process, and to probe the possible changers in hypertension. The rate of change in intracellular pH (dpHi/dt) upon luminal removal of Na+ in perfused proximal tubules in situ is used as a index of the activities of Na+/H+ exchangers. The intracellular pH is monitored with a dual pulse-lasers system designed to measure intracellular pH with minimum bleaching of BCECF, a pH sensitive fluorescence dye commonly-used to measure intracellular pH. Proximal fluid reabsorption is measured continuously with a non-obstructing optical technique developed in this laboratory. Confocal fluorescent microscopy and immunohistochemistry are employed to investigate the protein trafficking process triggered by acute hypertension. Spontaneously hypertensive rates (SHR) are used as an animal model of chronic hypertension. This proposed study will provide new information to understand the cellular mechanism of pressure natriuresis in proximal tubules, and the altered relationship between renal perfusion pressure and urinary Na+ excretion in hypertension. These may lead to a better understanding of the role of the kidney in hypertension, and possibly to new treatment principles.