RESPONSE OF SUPERFICIAL PROXIMAL CONVOLUTED TUBULE TO DECREASED AND INCREASED RENAL PERFUSION-PRESSURE - INVIVO MICROPERFUSION STUDY IN RATS

Although urinary sodium excretion is positively influenced by acute changes in renal perfusion pressure, micropuncture studies show highly conflicting results concerning the response of superficial proximal tubule sodium reabsorption to changes in renal perfusion pressure. In the present study, the changes of superficial proximal reabsorption to decreased and increased renal perfusion pressure were determined in rats by an in vivo microperfusion method. In vivo microperfusion was selected as the method to determine the proximal sodium reabsorption because this method made it possible to deliver a constant fluid and electrolyte load to the proximal tubule without the influence of possible changes of glomerular filtration rate. Renal perfusion pressure was decreased from normal pressure by inflating a suprarenal aortic cuff and was increased from the normal level by the occlusion of celiac and mesenteric arteries and the infrarenal aorta. Although fractional excretion of sodium (FE(Na)) in the urine was decreased from 1.24 ± 0.23% to 0.45 ± 0.11% (n = 7, p < 0.01) when renal perfusion pressure was decreased from 125 ± 6 to 99 ± 3 mm Hg, absolute tubular reabsorption by the superficial proximal convoluted tubules was not increased (from 4.4 ± 0.5 to 4.2 ± 0.3 nl/min/mm, n = 22). When the renal perfusion pressure was elevated from 126 ± 4 to 149 ± 4 mm Hg, tubular reabsorption by the superficial proximal tubules was decreased from 4.1 ± 0.3 to 2.5 ± 0.3 nl/min/mm (n = 36, p < 0.01) with an accompanying increase in FE(Na) (from 1.28 ± 0.24% to 2.29 ± 0.37%, n = 9, p < 0.05). In summary, superficial proximal tubule reabsorption is decreased by increases in renal perfusion pressure but is not affected by decreases in renal perfusion pressure.