A DECREASE IN RENAL MEDULLARY TONICITY STIMULATES ANION TRANSPORT IN HENLES LOOP OF RAT KIDNEYS

To investigate the effect of reduction in renal medulla osmolality on loop of Henle (LOH) net bicarbonate reabsorption, clearance and microp erfusion experiments were performed on Sprague-Dawley rats. The decrea se of renal medulla osmolality was induced by intravenous infusion of either a large dose of mannitol (mannitol protocol) or a hypotonic sol ution (hypotonic protocol) delivered at a rate to match the sodium and bicarbonate load of the control period. During the mannitol protocol, clearance data demonstrated a rise in glomerular filtration rate (GFR ), renal plasma flow, urine pH, and fractional bicarbonate excretion. On the contrary, microperfusion experiments, performed in the absence of mannitol in the tubular perfusate, revealed a significant increase both in the absolute and fractional LOH bicarbonate transport. During the hypotonic protocol, there was a decrease in GFR, associated with a n increase in fractional excretion of bicarbonate. In the microperfusi on experiments, hypotonic saline, similar to mannitol, stimulated abso lute and fractional LOH bicarbonate transport. Net reabsorption of chl oride, measured under the same experimental conditions, was also found to be activated. Therefore, the intravenous infusion of hypotonic sol ution affected the LOH transepithelial net reabsorption of both bicarb onate and chloride. We hypothesize that the increase in the transport rate of these two anions, along the same segment and in similar experi mental conditions, may be mediated, at least in part, by decreased med ullary tonicity, which is one factor common both to hypertonic mannito l and hypotonic saline infusion.